drbd: get rid of drbd_queue_work_front

[deliverable/linux.git] / drivers / block / drbd / drbd_req.c

/*
   drbd_req.c

   This file is part of DRBD by Philipp Reisner and Lars Ellenberg.

   Copyright (C) 2001-2008, LINBIT Information Technologies GmbH.
   Copyright (C) 1999-2008, Philipp Reisner <philipp.reisner@linbit.com>.
   Copyright (C) 2002-2008, Lars Ellenberg <lars.ellenberg@linbit.com>.

   drbd is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   drbd is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with drbd; see the file COPYING.  If not, write to
   the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.

 */

#include <linux/module.h>

#include <linux/slab.h>
#include <linux/drbd.h>
#include "drbd_int.h"
#include "drbd_req.h"


static bool drbd_may_do_local_read(struct drbd_device *device, sector_t sector, int size);

/* Update disk stats at start of I/O request */
static void _drbd_start_io_acct(struct drbd_device *device, struct drbd_request *req)
{
        const int rw = bio_data_dir(req->master_bio);
        int cpu;
        cpu = part_stat_lock();
        part_round_stats(cpu, &device->vdisk->part0);
        part_stat_inc(cpu, &device->vdisk->part0, ios[rw]);
        part_stat_add(cpu, &device->vdisk->part0, sectors[rw], req->i.size >> 9);
        (void) cpu; /* The macro invocations above want the cpu argument, I do not like
                       the compiler warning about cpu only assigned but never used... */
        part_inc_in_flight(&device->vdisk->part0, rw);
        part_stat_unlock();
}

/* Update disk stats when completing request upwards */
static void _drbd_end_io_acct(struct drbd_device *device, struct drbd_request *req)
{
        int rw = bio_data_dir(req->master_bio);
        unsigned long duration = jiffies - req->start_time;
        int cpu;
        cpu = part_stat_lock();
        part_stat_add(cpu, &device->vdisk->part0, ticks[rw], duration);
        part_round_stats(cpu, &device->vdisk->part0);
        part_dec_in_flight(&device->vdisk->part0, rw);
        part_stat_unlock();
}

static struct drbd_request *drbd_req_new(struct drbd_device *device,
                                               struct bio *bio_src)
{
        struct drbd_request *req;

        req = mempool_alloc(drbd_request_mempool, GFP_NOIO);
        if (!req)
                return NULL;

        drbd_req_make_private_bio(req, bio_src);
        req->rq_state    = bio_data_dir(bio_src) == WRITE ? RQ_WRITE : 0;
        req->device   = device;
        req->master_bio  = bio_src;
        req->epoch       = 0;

        drbd_clear_interval(&req->i);
        req->i.sector     = bio_src->bi_iter.bi_sector;
        req->i.size      = bio_src->bi_iter.bi_size;
        req->i.local = true;
        req->i.waiting = false;

        INIT_LIST_HEAD(&req->tl_requests);
        INIT_LIST_HEAD(&req->w.list);

        /* one reference to be put by __drbd_make_request */
        atomic_set(&req->completion_ref, 1);
        /* one kref as long as completion_ref > 0 */
        kref_init(&req->kref);
        return req;
}

void drbd_req_destroy(struct kref *kref)
{
        struct drbd_request *req = container_of(kref, struct drbd_request, kref);
        struct drbd_device *device = req->device;
        const unsigned s = req->rq_state;

        if ((req->master_bio && !(s & RQ_POSTPONED)) ||
                atomic_read(&req->completion_ref) ||
                (s & RQ_LOCAL_PENDING) ||
                ((s & RQ_NET_MASK) && !(s & RQ_NET_DONE))) {
                drbd_err(device, "drbd_req_destroy: Logic BUG rq_state = 0x%x, completion_ref = %d\n",
                                s, atomic_read(&req->completion_ref));
                return;
        }

        /* remove it from the transfer log.
         * well, only if it had been there in the first
         * place... if it had not (local only or conflicting
         * and never sent), it should still be "empty" as
         * initialized in drbd_req_new(), so we can list_del() it
         * here unconditionally */
        list_del_init(&req->tl_requests);

        /* if it was a write, we may have to set the corresponding
         * bit(s) out-of-sync first. If it had a local part, we need to
         * release the reference to the activity log. */
        if (s & RQ_WRITE) {
                /* Set out-of-sync unless both OK flags are set
                 * (local only or remote failed).
                 * Other places where we set out-of-sync:
                 * READ with local io-error */

                /* There is a special case:
                 * we may notice late that IO was suspended,
                 * and postpone, or schedule for retry, a write,
                 * before it even was submitted or sent.
                 * In that case we do not want to touch the bitmap at all.
                 */
                if ((s & (RQ_POSTPONED|RQ_LOCAL_MASK|RQ_NET_MASK)) != RQ_POSTPONED) {
                        if (!(s & RQ_NET_OK) || !(s & RQ_LOCAL_OK))
                                drbd_set_out_of_sync(device, req->i.sector, req->i.size);

                        if ((s & RQ_NET_OK) && (s & RQ_LOCAL_OK) && (s & RQ_NET_SIS))
                                drbd_set_in_sync(device, req->i.sector, req->i.size);
                }

                /* one might be tempted to move the drbd_al_complete_io
                 * to the local io completion callback drbd_request_endio.
                 * but, if this was a mirror write, we may only
                 * drbd_al_complete_io after this is RQ_NET_DONE,
                 * otherwise the extent could be dropped from the al
                 * before it has actually been written on the peer.
                 * if we crash before our peer knows about the request,
                 * but after the extent has been dropped from the al,
                 * we would forget to resync the corresponding extent.
                 */
                if (s & RQ_IN_ACT_LOG) {
                        if (get_ldev_if_state(device, D_FAILED)) {
                                drbd_al_complete_io(device, &req->i);
                                put_ldev(device);
                        } else if (__ratelimit(&drbd_ratelimit_state)) {
                                drbd_warn(device, "Should have called drbd_al_complete_io(, %llu, %u), "
                                         "but my Disk seems to have failed :(\n",
                                         (unsigned long long) req->i.sector, req->i.size);
                        }
                }
        }

        mempool_free(req, drbd_request_mempool);
}

static void wake_all_senders(struct drbd_connection *connection)
{
        wake_up(&connection->sender_work.q_wait);
}

/* must hold resource->req_lock */
void start_new_tl_epoch(struct drbd_connection *connection)
{
        /* no point closing an epoch, if it is empty, anyways. */
        if (connection->current_tle_writes == 0)
                return;

        connection->current_tle_writes = 0;
        atomic_inc(&connection->current_tle_nr);
        wake_all_senders(connection);
}

void complete_master_bio(struct drbd_device *device,
                struct bio_and_error *m)
{
        bio_endio(m->bio, m->error);
        dec_ap_bio(device);
}


static void drbd_remove_request_interval(struct rb_root *root,
                                         struct drbd_request *req)
{
        struct drbd_device *device = req->device;
        struct drbd_interval *i = &req->i;

        drbd_remove_interval(root, i);

        /* Wake up any processes waiting for this request to complete.  */
        if (i->waiting)
                wake_up(&device->misc_wait);
}

/* Helper for __req_mod().
 * Set m->bio to the master bio, if it is fit to be completed,
 * or leave it alone (it is initialized to NULL in __req_mod),
 * if it has already been completed, or cannot be completed yet.
 * If m->bio is set, the error status to be returned is placed in m->error.
 */
static
void drbd_req_complete(struct drbd_request *req, struct bio_and_error *m)
{
        const unsigned s = req->rq_state;
        struct drbd_device *device = req->device;
        int rw;
        int error, ok;

        /* we must not complete the master bio, while it is
         *      still being processed by _drbd_send_zc_bio (drbd_send_dblock)
         *      not yet acknowledged by the peer
         *      not yet completed by the local io subsystem
         * these flags may get cleared in any order by
         *      the worker,
         *      the receiver,
         *      the bio_endio completion callbacks.
         */
        if ((s & RQ_LOCAL_PENDING && !(s & RQ_LOCAL_ABORTED)) ||
            (s & RQ_NET_QUEUED) || (s & RQ_NET_PENDING) ||
            (s & RQ_COMPLETION_SUSP)) {
                drbd_err(device, "drbd_req_complete: Logic BUG rq_state = 0x%x\n", s);
                return;
        }

        if (!req->master_bio) {
                drbd_err(device, "drbd_req_complete: Logic BUG, master_bio == NULL!\n");
                return;
        }

        rw = bio_rw(req->master_bio);

        /*
         * figure out whether to report success or failure.
         *
         * report success when at least one of the operations succeeded.
         * or, to put the other way,
         * only report failure, when both operations failed.
         *
         * what to do about the failures is handled elsewhere.
         * what we need to do here is just: complete the master_bio.
         *
         * local completion error, if any, has been stored as ERR_PTR
         * in private_bio within drbd_request_endio.
         */
        ok = (s & RQ_LOCAL_OK) || (s & RQ_NET_OK);
        error = PTR_ERR(req->private_bio);

        /* remove the request from the conflict detection
         * respective block_id verification hash */
        if (!drbd_interval_empty(&req->i)) {
                struct rb_root *root;

                if (rw == WRITE)
                        root = &device->write_requests;
                else
                        root = &device->read_requests;
                drbd_remove_request_interval(root, req);
        }

        /* Before we can signal completion to the upper layers,
         * we may need to close the current transfer log epoch.
         * We are within the request lock, so we can simply compare
         * the request epoch number with the current transfer log
         * epoch number.  If they match, increase the current_tle_nr,
         * and reset the transfer log epoch write_cnt.
         */
        if (rw == WRITE &&
            req->epoch == atomic_read(&first_peer_device(device)->connection->current_tle_nr))
                start_new_tl_epoch(first_peer_device(device)->connection);

        /* Update disk stats */
        _drbd_end_io_acct(device, req);

        /* If READ failed,
         * have it be pushed back to the retry work queue,
         * so it will re-enter __drbd_make_request(),
         * and be re-assigned to a suitable local or remote path,
         * or failed if we do not have access to good data anymore.
         *
         * Unless it was failed early by __drbd_make_request(),
         * because no path was available, in which case
         * it was not even added to the transfer_log.
         *
         * READA may fail, and will not be retried.
         *
         * WRITE should have used all available paths already.
         */
        if (!ok && rw == READ && !list_empty(&req->tl_requests))
                req->rq_state |= RQ_POSTPONED;

        if (!(req->rq_state & RQ_POSTPONED)) {
                m->error = ok ? 0 : (error ?: -EIO);
                m->bio = req->master_bio;
                req->master_bio = NULL;
        }
}

static int drbd_req_put_completion_ref(struct drbd_request *req, struct bio_and_error *m, int put)
{
        struct drbd_device *device = req->device;
        D_ASSERT(device, m || (req->rq_state & RQ_POSTPONED));

        if (!atomic_sub_and_test(put, &req->completion_ref))
                return 0;

        drbd_req_complete(req, m);

        if (req->rq_state & RQ_POSTPONED) {
                /* don't destroy the req object just yet,
                 * but queue it for retry */
                drbd_restart_request(req);
                return 0;
        }

        return 1;
}

/* I'd like this to be the only place that manipulates
 * req->completion_ref and req->kref. */
static void mod_rq_state(struct drbd_request *req, struct bio_and_error *m,
                int clear, int set)
{
        struct drbd_device *device = req->device;
        unsigned s = req->rq_state;
        int c_put = 0;
        int k_put = 0;

        if (drbd_suspended(device) && !((s | clear) & RQ_COMPLETION_SUSP))
                set |= RQ_COMPLETION_SUSP;

        /* apply */

        req->rq_state &= ~clear;
        req->rq_state |= set;

        /* no change? */
        if (req->rq_state == s)
                return;

        /* intent: get references */

        if (!(s & RQ_LOCAL_PENDING) && (set & RQ_LOCAL_PENDING))
                atomic_inc(&req->completion_ref);

        if (!(s & RQ_NET_PENDING) && (set & RQ_NET_PENDING)) {
                inc_ap_pending(device);
                atomic_inc(&req->completion_ref);
        }

        if (!(s & RQ_NET_QUEUED) && (set & RQ_NET_QUEUED))
                atomic_inc(&req->completion_ref);

        if (!(s & RQ_EXP_BARR_ACK) && (set & RQ_EXP_BARR_ACK))
                kref_get(&req->kref); /* wait for the DONE */

        if (!(s & RQ_NET_SENT) && (set & RQ_NET_SENT))
                atomic_add(req->i.size >> 9, &device->ap_in_flight);

        if (!(s & RQ_COMPLETION_SUSP) && (set & RQ_COMPLETION_SUSP))
                atomic_inc(&req->completion_ref);

        /* progress: put references */

        if ((s & RQ_COMPLETION_SUSP) && (clear & RQ_COMPLETION_SUSP))
                ++c_put;

        if (!(s & RQ_LOCAL_ABORTED) && (set & RQ_LOCAL_ABORTED)) {
                D_ASSERT(device, req->rq_state & RQ_LOCAL_PENDING);
                /* local completion may still come in later,
                 * we need to keep the req object around. */
                kref_get(&req->kref);
                ++c_put;
        }

        if ((s & RQ_LOCAL_PENDING) && (clear & RQ_LOCAL_PENDING)) {
                if (req->rq_state & RQ_LOCAL_ABORTED)
                        ++k_put;
                else
                        ++c_put;
        }

        if ((s & RQ_NET_PENDING) && (clear & RQ_NET_PENDING)) {
                dec_ap_pending(device);
                ++c_put;
        }

        if ((s & RQ_NET_QUEUED) && (clear & RQ_NET_QUEUED))
                ++c_put;

        if ((s & RQ_EXP_BARR_ACK) && !(s & RQ_NET_DONE) && (set & RQ_NET_DONE)) {
                if (req->rq_state & RQ_NET_SENT)
                        atomic_sub(req->i.size >> 9, &device->ap_in_flight);
                ++k_put;
        }

        /* potentially complete and destroy */

        if (k_put || c_put) {
                /* Completion does it's own kref_put.  If we are going to
                 * kref_sub below, we need req to be still around then. */
                int at_least = k_put + !!c_put;
                int refcount = atomic_read(&req->kref.refcount);
                if (refcount < at_least)
                        drbd_err(device,
                                "mod_rq_state: Logic BUG: %x -> %x: refcount = %d, should be >= %d\n",
                                s, req->rq_state, refcount, at_least);
        }

        /* If we made progress, retry conflicting peer requests, if any. */
        if (req->i.waiting)
                wake_up(&device->misc_wait);

        if (c_put)
                k_put += drbd_req_put_completion_ref(req, m, c_put);
        if (k_put)
                kref_sub(&req->kref, k_put, drbd_req_destroy);
}

static void drbd_report_io_error(struct drbd_device *device, struct drbd_request *req)
{
        char b[BDEVNAME_SIZE];

        if (!__ratelimit(&drbd_ratelimit_state))
                return;

        drbd_warn(device, "local %s IO error sector %llu+%u on %s\n",
                        (req->rq_state & RQ_WRITE) ? "WRITE" : "READ",
                        (unsigned long long)req->i.sector,
                        req->i.size >> 9,
                        bdevname(device->ldev->backing_bdev, b));
}

/* obviously this could be coded as many single functions
 * instead of one huge switch,
 * or by putting the code directly in the respective locations
 * (as it has been before).
 *
 * but having it this way
 *  enforces that it is all in this one place, where it is easier to audit,
 *  it makes it obvious that whatever "event" "happens" to a request should
 *  happen "atomically" within the req_lock,
 *  and it enforces that we have to think in a very structured manner
 *  about the "events" that may happen to a request during its life time ...
 */
int __req_mod(struct drbd_request *req, enum drbd_req_event what,
                struct bio_and_error *m)
{
        struct drbd_device *const device = req->device;
        struct drbd_peer_device *const peer_device = first_peer_device(device);
        struct drbd_connection *const connection = peer_device ? peer_device->connection : NULL;
        struct net_conf *nc;
        int p, rv = 0;

        if (m)
                m->bio = NULL;

        switch (what) {
        default:
                drbd_err(device, "LOGIC BUG in %s:%u\n", __FILE__ , __LINE__);
                break;

        /* does not happen...
         * initialization done in drbd_req_new
        case CREATED:
                break;
                */

        case TO_BE_SENT: /* via network */
                /* reached via __drbd_make_request
                 * and from w_read_retry_remote */
                D_ASSERT(device, !(req->rq_state & RQ_NET_MASK));
                rcu_read_lock();
                nc = rcu_dereference(connection->net_conf);
                p = nc->wire_protocol;
                rcu_read_unlock();
                req->rq_state |=
                        p == DRBD_PROT_C ? RQ_EXP_WRITE_ACK :
                        p == DRBD_PROT_B ? RQ_EXP_RECEIVE_ACK : 0;
                mod_rq_state(req, m, 0, RQ_NET_PENDING);
                break;

        case TO_BE_SUBMITTED: /* locally */
                /* reached via __drbd_make_request */
                D_ASSERT(device, !(req->rq_state & RQ_LOCAL_MASK));
                mod_rq_state(req, m, 0, RQ_LOCAL_PENDING);
                break;

        case COMPLETED_OK:
                if (req->rq_state & RQ_WRITE)
                        device->writ_cnt += req->i.size >> 9;
                else
                        device->read_cnt += req->i.size >> 9;

                mod_rq_state(req, m, RQ_LOCAL_PENDING,
                                RQ_LOCAL_COMPLETED|RQ_LOCAL_OK);
                break;

        case ABORT_DISK_IO:
                mod_rq_state(req, m, 0, RQ_LOCAL_ABORTED);
                break;

        case WRITE_COMPLETED_WITH_ERROR:
                drbd_report_io_error(device, req);
                __drbd_chk_io_error(device, DRBD_WRITE_ERROR);
                mod_rq_state(req, m, RQ_LOCAL_PENDING, RQ_LOCAL_COMPLETED);
                break;

        case READ_COMPLETED_WITH_ERROR:
                drbd_set_out_of_sync(device, req->i.sector, req->i.size);
                drbd_report_io_error(device, req);
                __drbd_chk_io_error(device, DRBD_READ_ERROR);
                /* fall through. */
        case READ_AHEAD_COMPLETED_WITH_ERROR:
                /* it is legal to fail READA, no __drbd_chk_io_error in that case. */
                mod_rq_state(req, m, RQ_LOCAL_PENDING, RQ_LOCAL_COMPLETED);
                break;

        case DISCARD_COMPLETED_NOTSUPP:
        case DISCARD_COMPLETED_WITH_ERROR:
                /* I'd rather not detach from local disk just because it
                 * failed a REQ_DISCARD. */
                mod_rq_state(req, m, RQ_LOCAL_PENDING, RQ_LOCAL_COMPLETED);
                break;

        case QUEUE_FOR_NET_READ:
                /* READ or READA, and
                 * no local disk,
                 * or target area marked as invalid,
                 * or just got an io-error. */
                /* from __drbd_make_request
                 * or from bio_endio during read io-error recovery */

                /* So we can verify the handle in the answer packet.
                 * Corresponding drbd_remove_request_interval is in
                 * drbd_req_complete() */
                D_ASSERT(device, drbd_interval_empty(&req->i));
                drbd_insert_interval(&device->read_requests, &req->i);

                set_bit(UNPLUG_REMOTE, &device->flags);

                D_ASSERT(device, req->rq_state & RQ_NET_PENDING);
                D_ASSERT(device, (req->rq_state & RQ_LOCAL_MASK) == 0);
                mod_rq_state(req, m, 0, RQ_NET_QUEUED);
                req->w.cb = w_send_read_req;
                drbd_queue_work(&connection->sender_work,
                                &req->w);
                break;

        case QUEUE_FOR_NET_WRITE:
                /* assert something? */
                /* from __drbd_make_request only */

                /* Corresponding drbd_remove_request_interval is in
                 * drbd_req_complete() */
                D_ASSERT(device, drbd_interval_empty(&req->i));
                drbd_insert_interval(&device->write_requests, &req->i);

                /* NOTE
                 * In case the req ended up on the transfer log before being
                 * queued on the worker, it could lead to this request being
                 * missed during cleanup after connection loss.
                 * So we have to do both operations here,
                 * within the same lock that protects the transfer log.
                 *
                 * _req_add_to_epoch(req); this has to be after the
                 * _maybe_start_new_epoch(req); which happened in
                 * __drbd_make_request, because we now may set the bit
                 * again ourselves to close the current epoch.
                 *
                 * Add req to the (now) current epoch (barrier). */

                /* otherwise we may lose an unplug, which may cause some remote
                 * io-scheduler timeout to expire, increasing maximum latency,
                 * hurting performance. */
                set_bit(UNPLUG_REMOTE, &device->flags);

                /* queue work item to send data */
                D_ASSERT(device, req->rq_state & RQ_NET_PENDING);
                mod_rq_state(req, m, 0, RQ_NET_QUEUED|RQ_EXP_BARR_ACK);
                req->w.cb =  w_send_dblock;
                drbd_queue_work(&connection->sender_work,
                                &req->w);

                /* close the epoch, in case it outgrew the limit */
                rcu_read_lock();
                nc = rcu_dereference(connection->net_conf);
                p = nc->max_epoch_size;
                rcu_read_unlock();
                if (connection->current_tle_writes >= p)
                        start_new_tl_epoch(connection);

                break;

        case QUEUE_FOR_SEND_OOS:
                mod_rq_state(req, m, 0, RQ_NET_QUEUED);
                req->w.cb =  w_send_out_of_sync;
                drbd_queue_work(&connection->sender_work,
                                &req->w);
                break;

        case READ_RETRY_REMOTE_CANCELED:
        case SEND_CANCELED:
        case SEND_FAILED:
                /* real cleanup will be done from tl_clear.  just update flags
                 * so it is no longer marked as on the worker queue */
                mod_rq_state(req, m, RQ_NET_QUEUED, 0);
                break;

        case HANDED_OVER_TO_NETWORK:
                /* assert something? */
                if (bio_data_dir(req->master_bio) == WRITE &&
                    !(req->rq_state & (RQ_EXP_RECEIVE_ACK | RQ_EXP_WRITE_ACK))) {
                        /* this is what is dangerous about protocol A:
                         * pretend it was successfully written on the peer. */
                        if (req->rq_state & RQ_NET_PENDING)
                                mod_rq_state(req, m, RQ_NET_PENDING, RQ_NET_OK);
                        /* else: neg-ack was faster... */
                        /* it is still not yet RQ_NET_DONE until the
                         * corresponding epoch barrier got acked as well,
                         * so we know what to dirty on connection loss */
                }
                mod_rq_state(req, m, RQ_NET_QUEUED, RQ_NET_SENT);
                break;

        case OOS_HANDED_TO_NETWORK:
                /* Was not set PENDING, no longer QUEUED, so is now DONE
                 * as far as this connection is concerned. */
                mod_rq_state(req, m, RQ_NET_QUEUED, RQ_NET_DONE);
                break;

        case CONNECTION_LOST_WHILE_PENDING:
                /* transfer log cleanup after connection loss */
                mod_rq_state(req, m,
                                RQ_NET_OK|RQ_NET_PENDING|RQ_COMPLETION_SUSP,
                                RQ_NET_DONE);
                break;

        case CONFLICT_RESOLVED:
                /* for superseded conflicting writes of multiple primaries,
                 * there is no need to keep anything in the tl, potential
                 * node crashes are covered by the activity log.
                 *
                 * If this request had been marked as RQ_POSTPONED before,
                 * it will actually not be completed, but "restarted",
                 * resubmitted from the retry worker context. */
                D_ASSERT(device, req->rq_state & RQ_NET_PENDING);
                D_ASSERT(device, req->rq_state & RQ_EXP_WRITE_ACK);
                mod_rq_state(req, m, RQ_NET_PENDING, RQ_NET_DONE|RQ_NET_OK);
                break;

        case WRITE_ACKED_BY_PEER_AND_SIS:
                req->rq_state |= RQ_NET_SIS;
        case WRITE_ACKED_BY_PEER:
                D_ASSERT(device, req->rq_state & RQ_EXP_WRITE_ACK);
                /* protocol C; successfully written on peer.
                 * Nothing more to do here.
                 * We want to keep the tl in place for all protocols, to cater
                 * for volatile write-back caches on lower level devices. */

                goto ack_common;
        case RECV_ACKED_BY_PEER:
                D_ASSERT(device, req->rq_state & RQ_EXP_RECEIVE_ACK);
                /* protocol B; pretends to be successfully written on peer.
                 * see also notes above in HANDED_OVER_TO_NETWORK about
                 * protocol != C */
        ack_common:
                D_ASSERT(device, req->rq_state & RQ_NET_PENDING);
                mod_rq_state(req, m, RQ_NET_PENDING, RQ_NET_OK);
                break;

        case POSTPONE_WRITE:
                D_ASSERT(device, req->rq_state & RQ_EXP_WRITE_ACK);
                /* If this node has already detected the write conflict, the
                 * worker will be waiting on misc_wait.  Wake it up once this
                 * request has completed locally.
                 */
                D_ASSERT(device, req->rq_state & RQ_NET_PENDING);
                req->rq_state |= RQ_POSTPONED;
                if (req->i.waiting)
                        wake_up(&device->misc_wait);
                /* Do not clear RQ_NET_PENDING. This request will make further
                 * progress via restart_conflicting_writes() or
                 * fail_postponed_requests(). Hopefully. */
                break;

        case NEG_ACKED:
                mod_rq_state(req, m, RQ_NET_OK|RQ_NET_PENDING, 0);
                break;

        case FAIL_FROZEN_DISK_IO:
                if (!(req->rq_state & RQ_LOCAL_COMPLETED))
                        break;
                mod_rq_state(req, m, RQ_COMPLETION_SUSP, 0);
                break;

        case RESTART_FROZEN_DISK_IO:
                if (!(req->rq_state & RQ_LOCAL_COMPLETED))
                        break;

                mod_rq_state(req, m,
                                RQ_COMPLETION_SUSP|RQ_LOCAL_COMPLETED,
                                RQ_LOCAL_PENDING);

                rv = MR_READ;
                if (bio_data_dir(req->master_bio) == WRITE)
                        rv = MR_WRITE;

                get_ldev(device); /* always succeeds in this call path */
                req->w.cb = w_restart_disk_io;
                drbd_queue_work(&connection->sender_work,
                                &req->w);
                break;

        case RESEND:
                /* Simply complete (local only) READs. */
                if (!(req->rq_state & RQ_WRITE) && !req->w.cb) {
                        mod_rq_state(req, m, RQ_COMPLETION_SUSP, 0);
                        break;
                }

                /* If RQ_NET_OK is already set, we got a P_WRITE_ACK or P_RECV_ACK
                   before the connection loss (B&C only); only P_BARRIER_ACK
                   (or the local completion?) was missing when we suspended.
                   Throwing them out of the TL here by pretending we got a BARRIER_ACK.
                   During connection handshake, we ensure that the peer was not rebooted. */
                if (!(req->rq_state & RQ_NET_OK)) {
                        /* FIXME could this possibly be a req->dw.cb == w_send_out_of_sync?
                         * in that case we must not set RQ_NET_PENDING. */

                        mod_rq_state(req, m, RQ_COMPLETION_SUSP, RQ_NET_QUEUED|RQ_NET_PENDING);
                        if (req->w.cb) {
                                /* w.cb expected to be w_send_dblock, or w_send_read_req */
                                drbd_queue_work(&connection->sender_work,
                                                &req->w);
                                rv = req->rq_state & RQ_WRITE ? MR_WRITE : MR_READ;
                        } /* else: FIXME can this happen? */
                        break;
                }
                /* else, fall through to BARRIER_ACKED */

        case BARRIER_ACKED:
                /* barrier ack for READ requests does not make sense */
                if (!(req->rq_state & RQ_WRITE))
                        break;

                if (req->rq_state & RQ_NET_PENDING) {
                        /* barrier came in before all requests were acked.
                         * this is bad, because if the connection is lost now,
                         * we won't be able to clean them up... */
                        drbd_err(device, "FIXME (BARRIER_ACKED but pending)\n");
                }
                /* Allowed to complete requests, even while suspended.
                 * As this is called for all requests within a matching epoch,
                 * we need to filter, and only set RQ_NET_DONE for those that
                 * have actually been on the wire. */
                mod_rq_state(req, m, RQ_COMPLETION_SUSP,
                                (req->rq_state & RQ_NET_MASK) ? RQ_NET_DONE : 0);
                break;

        case DATA_RECEIVED:
                D_ASSERT(device, req->rq_state & RQ_NET_PENDING);
                mod_rq_state(req, m, RQ_NET_PENDING, RQ_NET_OK|RQ_NET_DONE);
                break;

        case QUEUE_AS_DRBD_BARRIER:
                start_new_tl_epoch(connection);
                mod_rq_state(req, m, 0, RQ_NET_OK|RQ_NET_DONE);
                break;
        };

        return rv;
}

/* we may do a local read if:
 * - we are consistent (of course),
 * - or we are generally inconsistent,
 *   BUT we are still/already IN SYNC for this area.
 *   since size may be bigger than BM_BLOCK_SIZE,
 *   we may need to check several bits.
 */
static bool drbd_may_do_local_read(struct drbd_device *device, sector_t sector, int size)
{
        unsigned long sbnr, ebnr;
        sector_t esector, nr_sectors;

        if (device->state.disk == D_UP_TO_DATE)
                return true;
        if (device->state.disk != D_INCONSISTENT)
                return false;
        esector = sector + (size >> 9) - 1;
        nr_sectors = drbd_get_capacity(device->this_bdev);
        D_ASSERT(device, sector  < nr_sectors);
        D_ASSERT(device, esector < nr_sectors);

        sbnr = BM_SECT_TO_BIT(sector);
        ebnr = BM_SECT_TO_BIT(esector);

        return drbd_bm_count_bits(device, sbnr, ebnr) == 0;
}

static bool remote_due_to_read_balancing(struct drbd_device *device, sector_t sector,
                enum drbd_read_balancing rbm)
{
        struct backing_dev_info *bdi;
        int stripe_shift;

        switch (rbm) {
        case RB_CONGESTED_REMOTE:
                bdi = &device->ldev->backing_bdev->bd_disk->queue->backing_dev_info;
                return bdi_read_congested(bdi);
        case RB_LEAST_PENDING:
                return atomic_read(&device->local_cnt) >
                        atomic_read(&device->ap_pending_cnt) + atomic_read(&device->rs_pending_cnt);
        case RB_32K_STRIPING:  /* stripe_shift = 15 */
        case RB_64K_STRIPING:
        case RB_128K_STRIPING:
        case RB_256K_STRIPING:
        case RB_512K_STRIPING:
        case RB_1M_STRIPING:   /* stripe_shift = 20 */
                stripe_shift = (rbm - RB_32K_STRIPING + 15);
                return (sector >> (stripe_shift - 9)) & 1;
        case RB_ROUND_ROBIN:
                return test_and_change_bit(READ_BALANCE_RR, &device->flags);
        case RB_PREFER_REMOTE:
                return true;
        case RB_PREFER_LOCAL:
        default:
                return false;
        }
}

/*
 * complete_conflicting_writes  -  wait for any conflicting write requests
 *
 * The write_requests tree contains all active write requests which we
 * currently know about.  Wait for any requests to complete which conflict with
 * the new one.
 *
 * Only way out: remove the conflicting intervals from the tree.
 */
static void complete_conflicting_writes(struct drbd_request *req)
{
        DEFINE_WAIT(wait);
        struct drbd_device *device = req->device;
        struct drbd_interval *i;
        sector_t sector = req->i.sector;
        int size = req->i.size;

        i = drbd_find_overlap(&device->write_requests, sector, size);
        if (!i)
                return;

        for (;;) {
                prepare_to_wait(&device->misc_wait, &wait, TASK_UNINTERRUPTIBLE);
                i = drbd_find_overlap(&device->write_requests, sector, size);
                if (!i)
                        break;
                /* Indicate to wake up device->misc_wait on progress.  */
                i->waiting = true;
                spin_unlock_irq(&device->resource->req_lock);
                schedule();
                spin_lock_irq(&device->resource->req_lock);
        }
        finish_wait(&device->misc_wait, &wait);
}

/* called within req_lock and rcu_read_lock() */
static void maybe_pull_ahead(struct drbd_device *device)
{
        struct drbd_connection *connection = first_peer_device(device)->connection;
        struct net_conf *nc;
        bool congested = false;
        enum drbd_on_congestion on_congestion;

        rcu_read_lock();
        nc = rcu_dereference(connection->net_conf);
        on_congestion = nc ? nc->on_congestion : OC_BLOCK;
        rcu_read_unlock();
        if (on_congestion == OC_BLOCK ||
            connection->agreed_pro_version < 96)
                return;

        /* If I don't even have good local storage, we can not reasonably try
         * to pull ahead of the peer. We also need the local reference to make
         * sure device->act_log is there.
         */
        if (!get_ldev_if_state(device, D_UP_TO_DATE))
                return;

        if (nc->cong_fill &&
            atomic_read(&device->ap_in_flight) >= nc->cong_fill) {
                drbd_info(device, "Congestion-fill threshold reached\n");
                congested = true;
        }

        if (device->act_log->used >= nc->cong_extents) {
                drbd_info(device, "Congestion-extents threshold reached\n");
                congested = true;
        }

        if (congested) {
                /* start a new epoch for non-mirrored writes */
                start_new_tl_epoch(first_peer_device(device)->connection);

                if (on_congestion == OC_PULL_AHEAD)
                        _drbd_set_state(_NS(device, conn, C_AHEAD), 0, NULL);
                else  /*nc->on_congestion == OC_DISCONNECT */
                        _drbd_set_state(_NS(device, conn, C_DISCONNECTING), 0, NULL);
        }
        put_ldev(device);
}

/* If this returns false, and req->private_bio is still set,
 * this should be submitted locally.
 *
 * If it returns false, but req->private_bio is not set,
 * we do not have access to good data :(
 *
 * Otherwise, this destroys req->private_bio, if any,
 * and returns true.
 */
static bool do_remote_read(struct drbd_request *req)
{
        struct drbd_device *device = req->device;
        enum drbd_read_balancing rbm;

        if (req->private_bio) {
                if (!drbd_may_do_local_read(device,
                                        req->i.sector, req->i.size)) {
                        bio_put(req->private_bio);
                        req->private_bio = NULL;
                        put_ldev(device);
                }
        }

        if (device->state.pdsk != D_UP_TO_DATE)
                return false;

        if (req->private_bio == NULL)
                return true;

        /* TODO: improve read balancing decisions, take into account drbd
         * protocol, pending requests etc. */

        rcu_read_lock();
        rbm = rcu_dereference(device->ldev->disk_conf)->read_balancing;
        rcu_read_unlock();

        if (rbm == RB_PREFER_LOCAL && req->private_bio)
                return false; /* submit locally */

        if (remote_due_to_read_balancing(device, req->i.sector, rbm)) {
                if (req->private_bio) {
                        bio_put(req->private_bio);
                        req->private_bio = NULL;
                        put_ldev(device);
                }
                return true;
        }

        return false;
}

/* returns number of connections (== 1, for drbd 8.4)
 * expected to actually write this data,
 * which does NOT include those that we are L_AHEAD for. */
static int drbd_process_write_request(struct drbd_request *req)
{
        struct drbd_device *device = req->device;
        int remote, send_oos;

        remote = drbd_should_do_remote(device->state);
        send_oos = drbd_should_send_out_of_sync(device->state);

        /* Need to replicate writes.  Unless it is an empty flush,
         * which is better mapped to a DRBD P_BARRIER packet,
         * also for drbd wire protocol compatibility reasons.
         * If this was a flush, just start a new epoch.
         * Unless the current epoch was empty anyways, or we are not currently
         * replicating, in which case there is no point. */
        if (unlikely(req->i.size == 0)) {
                /* The only size==0 bios we expect are empty flushes. */
                D_ASSERT(device, req->master_bio->bi_rw & REQ_FLUSH);
                if (remote)
                        _req_mod(req, QUEUE_AS_DRBD_BARRIER);
                return remote;
        }

        if (!remote && !send_oos)
                return 0;

        D_ASSERT(device, !(remote && send_oos));

        if (remote) {
                _req_mod(req, TO_BE_SENT);
                _req_mod(req, QUEUE_FOR_NET_WRITE);
        } else if (drbd_set_out_of_sync(device, req->i.sector, req->i.size))
                _req_mod(req, QUEUE_FOR_SEND_OOS);

        return remote;
}

static void
drbd_submit_req_private_bio(struct drbd_request *req)
{
        struct drbd_device *device = req->device;
        struct bio *bio = req->private_bio;
        const int rw = bio_rw(bio);

        bio->bi_bdev = device->ldev->backing_bdev;

        /* State may have changed since we grabbed our reference on the
         * ->ldev member. Double check, and short-circuit to endio.
         * In case the last activity log transaction failed to get on
         * stable storage, and this is a WRITE, we may not even submit
         * this bio. */
        if (get_ldev(device)) {
                if (drbd_insert_fault(device,
                                      rw == WRITE ? DRBD_FAULT_DT_WR
                                    : rw == READ  ? DRBD_FAULT_DT_RD
                                    :               DRBD_FAULT_DT_RA))
                        bio_endio(bio, -EIO);
                else
                        generic_make_request(bio);
                put_ldev(device);
        } else
                bio_endio(bio, -EIO);
}

static void drbd_queue_write(struct drbd_device *device, struct drbd_request *req)
{
        spin_lock(&device->submit.lock);
        list_add_tail(&req->tl_requests, &device->submit.writes);
        spin_unlock(&device->submit.lock);
        queue_work(device->submit.wq, &device->submit.worker);
}

/* returns the new drbd_request pointer, if the caller is expected to
 * drbd_send_and_submit() it (to save latency), or NULL if we queued the
 * request on the submitter thread.
 * Returns ERR_PTR(-ENOMEM) if we cannot allocate a drbd_request.
 */
static struct drbd_request *
drbd_request_prepare(struct drbd_device *device, struct bio *bio, unsigned long start_time)
{
        const int rw = bio_data_dir(bio);
        struct drbd_request *req;

        /* allocate outside of all locks; */
        req = drbd_req_new(device, bio);
        if (!req) {
                dec_ap_bio(device);
                /* only pass the error to the upper layers.
                 * if user cannot handle io errors, that's not our business. */
                drbd_err(device, "could not kmalloc() req\n");
                bio_endio(bio, -ENOMEM);
                return ERR_PTR(-ENOMEM);
        }
        req->start_time = start_time;

        if (!get_ldev(device)) {
                bio_put(req->private_bio);
                req->private_bio = NULL;
        }

        /* Update disk stats */
        _drbd_start_io_acct(device, req);

        if (rw == WRITE && req->private_bio && req->i.size
        && !test_bit(AL_SUSPENDED, &device->flags)) {
                if (!drbd_al_begin_io_fastpath(device, &req->i)) {
                        drbd_queue_write(device, req);
                        return NULL;
                }
                req->rq_state |= RQ_IN_ACT_LOG;
        }

        return req;
}

static void drbd_send_and_submit(struct drbd_device *device, struct drbd_request *req)
{
        struct drbd_resource *resource = device->resource;
        const int rw = bio_rw(req->master_bio);
        struct bio_and_error m = { NULL, };
        bool no_remote = false;
        bool submit_private_bio = false;

        spin_lock_irq(&resource->req_lock);
        if (rw == WRITE) {
                /* This may temporarily give up the req_lock,
                 * but will re-aquire it before it returns here.
                 * Needs to be before the check on drbd_suspended() */
                complete_conflicting_writes(req);
                /* no more giving up req_lock from now on! */

                /* check for congestion, and potentially stop sending
                 * full data updates, but start sending "dirty bits" only. */
                maybe_pull_ahead(device);
        }


        if (drbd_suspended(device)) {
                /* push back and retry: */
                req->rq_state |= RQ_POSTPONED;
                if (req->private_bio) {
                        bio_put(req->private_bio);
                        req->private_bio = NULL;
                        put_ldev(device);
                }
                goto out;
        }

        /* We fail READ/READA early, if we can not serve it.
         * We must do this before req is registered on any lists.
         * Otherwise, drbd_req_complete() will queue failed READ for retry. */
        if (rw != WRITE) {
                if (!do_remote_read(req) && !req->private_bio)
                        goto nodata;
        }

        /* which transfer log epoch does this belong to? */
        req->epoch = atomic_read(&first_peer_device(device)->connection->current_tle_nr);

        /* no point in adding empty flushes to the transfer log,
         * they are mapped to drbd barriers already. */
        if (likely(req->i.size!=0)) {
                if (rw == WRITE)
                        first_peer_device(device)->connection->current_tle_writes++;

                list_add_tail(&req->tl_requests, &first_peer_device(device)->connection->transfer_log);
        }

        if (rw == WRITE) {
                if (!drbd_process_write_request(req))
                        no_remote = true;
        } else {
                /* We either have a private_bio, or we can read from remote.
                 * Otherwise we had done the goto nodata above. */
                if (req->private_bio == NULL) {
                        _req_mod(req, TO_BE_SENT);
                        _req_mod(req, QUEUE_FOR_NET_READ);
                } else
                        no_remote = true;
        }

        if (req->private_bio) {
                /* needs to be marked within the same spinlock */
                _req_mod(req, TO_BE_SUBMITTED);
                /* but we need to give up the spinlock to submit */
                submit_private_bio = true;
        } else if (no_remote) {
nodata:
                if (__ratelimit(&drbd_ratelimit_state))
                        drbd_err(device, "IO ERROR: neither local nor remote data, sector %llu+%u\n",
                                        (unsigned long long)req->i.sector, req->i.size >> 9);
                /* A write may have been queued for send_oos, however.
                 * So we can not simply free it, we must go through drbd_req_put_completion_ref() */
        }

out:
        if (drbd_req_put_completion_ref(req, &m, 1))
                kref_put(&req->kref, drbd_req_destroy);
        spin_unlock_irq(&resource->req_lock);

        /* Even though above is a kref_put(), this is safe.
         * As long as we still need to submit our private bio,
         * we hold a completion ref, and the request cannot disappear.
         * If however this request did not even have a private bio to submit
         * (e.g. remote read), req may already be invalid now.
         * That's why we cannot check on req->private_bio. */
        if (submit_private_bio)
                drbd_submit_req_private_bio(req);
        if (m.bio)
                complete_master_bio(device, &m);
}

void __drbd_make_request(struct drbd_device *device, struct bio *bio, unsigned long start_time)
{
        struct drbd_request *req = drbd_request_prepare(device, bio, start_time);
        if (IS_ERR_OR_NULL(req))
                return;
        drbd_send_and_submit(device, req);
}

static void submit_fast_path(struct drbd_device *device, struct list_head *incoming)
{
        struct drbd_request *req, *tmp;
        list_for_each_entry_safe(req, tmp, incoming, tl_requests) {
                const int rw = bio_data_dir(req->master_bio);

                if (rw == WRITE /* rw != WRITE should not even end up here! */
                && req->private_bio && req->i.size
                && !test_bit(AL_SUSPENDED, &device->flags)) {
                        if (!drbd_al_begin_io_fastpath(device, &req->i))
                                continue;

                        req->rq_state |= RQ_IN_ACT_LOG;
                }

                list_del_init(&req->tl_requests);
                drbd_send_and_submit(device, req);
        }
}

static bool prepare_al_transaction_nonblock(struct drbd_device *device,
                                            struct list_head *incoming,
                                            struct list_head *pending)
{
        struct drbd_request *req, *tmp;
        int wake = 0;
        int err;

        spin_lock_irq(&device->al_lock);
        list_for_each_entry_safe(req, tmp, incoming, tl_requests) {
                err = drbd_al_begin_io_nonblock(device, &req->i);
                if (err == -EBUSY)
                        wake = 1;
                if (err)
                        continue;
                req->rq_state |= RQ_IN_ACT_LOG;
                list_move_tail(&req->tl_requests, pending);
        }
        spin_unlock_irq(&device->al_lock);
        if (wake)
                wake_up(&device->al_wait);

        return !list_empty(pending);
}

void do_submit(struct work_struct *ws)
{
        struct drbd_device *device = container_of(ws, struct drbd_device, submit.worker);
        LIST_HEAD(incoming);
        LIST_HEAD(pending);
        struct drbd_request *req, *tmp;

        for (;;) {
                spin_lock(&device->submit.lock);
                list_splice_tail_init(&device->submit.writes, &incoming);
                spin_unlock(&device->submit.lock);

                submit_fast_path(device, &incoming);
                if (list_empty(&incoming))
                        break;

skip_fast_path:
                wait_event(device->al_wait, prepare_al_transaction_nonblock(device, &incoming, &pending));
                /* Maybe more was queued, while we prepared the transaction?
                 * Try to stuff them into this transaction as well.
                 * Be strictly non-blocking here, no wait_event, we already
                 * have something to commit.
                 * Stop if we don't make any more progres.
                 */
                for (;;) {
                        LIST_HEAD(more_pending);
                        LIST_HEAD(more_incoming);
                        bool made_progress;

                        /* It is ok to look outside the lock,
                         * it's only an optimization anyways */
                        if (list_empty(&device->submit.writes))
                                break;

                        spin_lock(&device->submit.lock);
                        list_splice_tail_init(&device->submit.writes, &more_incoming);
                        spin_unlock(&device->submit.lock);

                        if (list_empty(&more_incoming))
                                break;

                        made_progress = prepare_al_transaction_nonblock(device, &more_incoming, &more_pending);

                        list_splice_tail_init(&more_pending, &pending);
                        list_splice_tail_init(&more_incoming, &incoming);

                        if (!made_progress)
                                break;
                }
                drbd_al_begin_io_commit(device);

                list_for_each_entry_safe(req, tmp, &pending, tl_requests) {
                        list_del_init(&req->tl_requests);
                        drbd_send_and_submit(device, req);
                }

                /* If all currently hot activity log extents are kept busy by
                 * incoming requests, we still must not totally starve new
                 * requests to cold extents. In that case, prepare one request
                 * in blocking mode. */
                list_for_each_entry_safe(req, tmp, &incoming, tl_requests) {
                        list_del_init(&req->tl_requests);
                        req->rq_state |= RQ_IN_ACT_LOG;
                        if (!drbd_al_begin_io_prepare(device, &req->i)) {
                                /* Corresponding extent was hot after all? */
                                drbd_send_and_submit(device, req);
                        } else {
                                /* Found a request to a cold extent.
                                 * Put on "pending" list,
                                 * and try to cumulate with more. */
                                list_add(&req->tl_requests, &pending);
                                goto skip_fast_path;
                        }
                }
        }
}

void drbd_make_request(struct request_queue *q, struct bio *bio)
{
        struct drbd_device *device = (struct drbd_device *) q->queuedata;
        unsigned long start_time;

        start_time = jiffies;

        /*
         * what we "blindly" assume:
         */
        D_ASSERT(device, IS_ALIGNED(bio->bi_iter.bi_size, 512));

        inc_ap_bio(device);
        __drbd_make_request(device, bio, start_time);
}

/* This is called by bio_add_page().
 *
 * q->max_hw_sectors and other global limits are already enforced there.
 *
 * We need to call down to our lower level device,
 * in case it has special restrictions.
 *
 * We also may need to enforce configured max-bio-bvecs limits.
 *
 * As long as the BIO is empty we have to allow at least one bvec,
 * regardless of size and offset, so no need to ask lower levels.
 */
int drbd_merge_bvec(struct request_queue *q, struct bvec_merge_data *bvm, struct bio_vec *bvec)
{
        struct drbd_device *device = (struct drbd_device *) q->queuedata;
        unsigned int bio_size = bvm->bi_size;
        int limit = DRBD_MAX_BIO_SIZE;
        int backing_limit;

        if (bio_size && get_ldev(device)) {
                unsigned int max_hw_sectors = queue_max_hw_sectors(q);
                struct request_queue * const b =
                        device->ldev->backing_bdev->bd_disk->queue;
                if (b->merge_bvec_fn) {
                        backing_limit = b->merge_bvec_fn(b, bvm, bvec);
                        limit = min(limit, backing_limit);
                }
                put_ldev(device);
                if ((limit >> 9) > max_hw_sectors)
                        limit = max_hw_sectors << 9;
        }
        return limit;
}

static void find_oldest_requests(
                struct drbd_connection *connection,
                struct drbd_device *device,
                struct drbd_request **oldest_req_waiting_for_peer,
                struct drbd_request **oldest_req_waiting_for_disk)
{
        struct drbd_request *r;
        *oldest_req_waiting_for_peer = NULL;
        *oldest_req_waiting_for_disk = NULL;
        list_for_each_entry(r, &connection->transfer_log, tl_requests) {
                const unsigned s = r->rq_state;
                if (!*oldest_req_waiting_for_peer
                && ((s & RQ_NET_MASK) && !(s & RQ_NET_DONE)))
                        *oldest_req_waiting_for_peer = r;

                if (!*oldest_req_waiting_for_disk
                && (s & RQ_LOCAL_PENDING) && r->device == device)
                        *oldest_req_waiting_for_disk = r;

                if (*oldest_req_waiting_for_peer && *oldest_req_waiting_for_disk)
                        break;
        }
}

void request_timer_fn(unsigned long data)
{
        struct drbd_device *device = (struct drbd_device *) data;
        struct drbd_connection *connection = first_peer_device(device)->connection;
        struct drbd_request *req_disk, *req_peer; /* oldest request */
        struct net_conf *nc;
        unsigned long ent = 0, dt = 0, et, nt; /* effective timeout = ko_count * timeout */
        unsigned long now;

        rcu_read_lock();
        nc = rcu_dereference(connection->net_conf);
        if (nc && device->state.conn >= C_WF_REPORT_PARAMS)
                ent = nc->timeout * HZ/10 * nc->ko_count;

        if (get_ldev(device)) { /* implicit state.disk >= D_INCONSISTENT */
                dt = rcu_dereference(device->ldev->disk_conf)->disk_timeout * HZ / 10;
                put_ldev(device);
        }
        rcu_read_unlock();

        et = min_not_zero(dt, ent);

        if (!et)
                return; /* Recurring timer stopped */

        now = jiffies;

        spin_lock_irq(&device->resource->req_lock);
        find_oldest_requests(connection, device, &req_peer, &req_disk);
        if (req_peer == NULL && req_disk == NULL) {
                spin_unlock_irq(&device->resource->req_lock);
                mod_timer(&device->request_timer, now + et);
                return;
        }

        /* The request is considered timed out, if
         * - we have some effective timeout from the configuration,
         *   with above state restrictions applied,
         * - the oldest request is waiting for a response from the network
         *   resp. the local disk,
         * - the oldest request is in fact older than the effective timeout,
         * - the connection was established (resp. disk was attached)
         *   for longer than the timeout already.
         * Note that for 32bit jiffies and very stable connections/disks,
         * we may have a wrap around, which is catched by
         *   !time_in_range(now, last_..._jif, last_..._jif + timeout).
         *
         * Side effect: once per 32bit wrap-around interval, which means every
         * ~198 days with 250 HZ, we have a window where the timeout would need
         * to expire twice (worst case) to become effective. Good enough.
         */
        if (ent && req_peer &&
                 time_after(now, req_peer->start_time + ent) &&
                !time_in_range(now, connection->last_reconnect_jif, connection->last_reconnect_jif + ent)) {
                drbd_warn(device, "Remote failed to finish a request within ko-count * timeout\n");
                _drbd_set_state(_NS(device, conn, C_TIMEOUT), CS_VERBOSE | CS_HARD, NULL);
        }
        if (dt && req_disk &&
                 time_after(now, req_disk->start_time + dt) &&
                !time_in_range(now, device->last_reattach_jif, device->last_reattach_jif + dt)) {
                drbd_warn(device, "Local backing device failed to meet the disk-timeout\n");
                __drbd_chk_io_error(device, DRBD_FORCE_DETACH);
        }

        /* Reschedule timer for the nearest not already expired timeout.
         * Fallback to now + min(effective network timeout, disk timeout). */
        ent = (ent && req_peer && time_before(now, req_peer->start_time + ent))
                ? req_peer->start_time + ent : now + et;
        dt = (dt && req_disk && time_before(now, req_disk->start_time + dt))
                ? req_disk->start_time + dt : now + et;
        nt = time_before(ent, dt) ? ent : dt;
        spin_unlock_irq(&connection->resource->req_lock);
        mod_timer(&device->request_timer, nt);
}