kernel/locking/qrwlock.c

   1 /*
   2  * Queued read/write locks
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License as published by
   6  * the Free Software Foundation; either version 2 of the License, or
   7  * (at your option) any later version.
   8  *
   9  * This program is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  * GNU General Public License for more details.
  13  *
  14  * (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
  15  *
  16  * Authors: Waiman Long <waiman.long@hp.com>
  17  */
  18 #include <linux/smp.h>
  19 #include <linux/bug.h>
  20 #include <linux/cpumask.h>
  21 #include <linux/percpu.h>
  22 #include <linux/hardirq.h>
  23 #include <asm/qrwlock.h>
  24
  25 /*
  26  * This internal data structure is used for optimizing access to some of
  27  * the subfields within the atomic_t cnts.
  28  */
  29 struct __qrwlock {
  30         union {
  31                 atomic_t cnts;
  32                 struct {
  33 #ifdef __LITTLE_ENDIAN
  34                         u8 wmode;       /* Writer mode   */
  35                         u8 rcnts[3];    /* Reader counts */
  36 #else
  37                         u8 rcnts[3];    /* Reader counts */
  38                         u8 wmode;       /* Writer mode   */
  39 #endif
  40                 };
  41         };
  42         arch_spinlock_t lock;
  43 };
  44
  45 /**
  46  * rspin_until_writer_unlock - inc reader count & spin until writer is gone
  47  * @lock  : Pointer to queue rwlock structure
  48  * @writer: Current queue rwlock writer status byte
  49  *
  50  * In interrupt context or at the head of the queue, the reader will just
  51  * increment the reader count & wait until the writer releases the lock.
  52  */
  53 static __always_inline void
  54 rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
  55 {
  56         while ((cnts & _QW_WMASK) == _QW_LOCKED) {
  57                 cpu_relax_lowlatency();
  58                 cnts = smp_load_acquire((u32 *)&lock->cnts);
  59         }
  60 }
  61
  62 /**
  63  * queue_read_lock_slowpath - acquire read lock of a queue rwlock
  64  * @lock: Pointer to queue rwlock structure
  65  */
  66 void queue_read_lock_slowpath(struct qrwlock *lock)
  67 {
  68         u32 cnts;
  69
  70         /*
  71          * Readers come here when they cannot get the lock without waiting
  72          */
  73         if (unlikely(in_interrupt())) {
  74                 /*
  75                  * Readers in interrupt context will spin until the lock is
  76                  * available without waiting in the queue.
  77                  */
  78                 cnts = smp_load_acquire((u32 *)&lock->cnts);
  79                 rspin_until_writer_unlock(lock, cnts);
  80                 return;
  81         }
  82         atomic_sub(_QR_BIAS, &lock->cnts);
  83
  84         /*
  85          * Put the reader into the wait queue
  86          */
  87         arch_spin_lock(&lock->lock);
  88
  89         /*
  90          * At the head of the wait queue now, wait until the writer state
  91          * goes to 0 and then try to increment the reader count and get
  92          * the lock. It is possible that an incoming writer may steal the
  93          * lock in the interim, so it is necessary to check the writer byte
  94          * to make sure that the write lock isn't taken.
  95          */
  96         while (atomic_read(&lock->cnts) & _QW_WMASK)
  97                 cpu_relax_lowlatency();
  98
  99         cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS;
 100         rspin_until_writer_unlock(lock, cnts);
 101
 102         /*
 103          * Signal the next one in queue to become queue head
 104          */
 105         arch_spin_unlock(&lock->lock);
 106 }
 107 EXPORT_SYMBOL(queue_read_lock_slowpath);
 108
 109 /**
 110  * queue_write_lock_slowpath - acquire write lock of a queue rwlock
 111  * @lock : Pointer to queue rwlock structure
 112  */
 113 void queue_write_lock_slowpath(struct qrwlock *lock)
 114 {
 115         u32 cnts;
 116
 117         /* Put the writer into the wait queue */
 118         arch_spin_lock(&lock->lock);
 119
 120         /* Try to acquire the lock directly if no reader is present */
 121         if (!atomic_read(&lock->cnts) &&
 122             (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0))
 123                 goto unlock;
 124
 125         /*
 126          * Set the waiting flag to notify readers that a writer is pending,
 127          * or wait for a previous writer to go away.
 128          */
 129         for (;;) {
 130                 struct __qrwlock *l = (struct __qrwlock *)lock;
 131
 132                 if (!READ_ONCE(l->wmode) &&
 133                    (cmpxchg(&l->wmode, 0, _QW_WAITING) == 0))
 134                         break;
 135
 136                 cpu_relax_lowlatency();
 137         }
 138
 139         /* When no more readers, set the locked flag */
 140         for (;;) {
 141                 cnts = atomic_read(&lock->cnts);
 142                 if ((cnts == _QW_WAITING) &&
 143                     (atomic_cmpxchg(&lock->cnts, _QW_WAITING,
 144                                     _QW_LOCKED) == _QW_WAITING))
 145                         break;
 146
 147                 cpu_relax_lowlatency();
 148         }
 149 unlock:
 150         arch_spin_unlock(&lock->lock);
 151 }
 152 EXPORT_SYMBOL(queue_write_lock_slowpath);