--- /dev/null
+/*
+ * Ldisc rw semaphore
+ *
+ * The ldisc semaphore is semantically a rw_semaphore but which enforces
+ * an alternate policy, namely:
+ * 1) Supports lock wait timeouts
+ * 2) Write waiter has priority
+ * 3) Downgrading is not supported
+ *
+ * Implementation notes:
+ * 1) Upper half of semaphore count is a wait count (differs from rwsem
+ * in that rwsem normalizes the upper half to the wait bias)
+ * 2) Lacks overflow checking
+ *
+ * The generic counting was copied and modified from include/asm-generic/rwsem.h
+ * by Paul Mackerras <paulus@samba.org>.
+ *
+ * The scheduling policy was copied and modified from lib/rwsem.c
+ * Written by David Howells (dhowells@redhat.com).
+ *
+ * This implementation incorporates the write lock stealing work of
+ * Michel Lespinasse <walken@google.com>.
+ *
+ * Copyright (C) 2013 Peter Hurley <peter@hurleysoftware.com>
+ *
+ * This file may be redistributed under the terms of the GNU General Public
+ * License v2.
+ */
+
+#include <linux/list.h>
+#include <linux/spinlock.h>
+#include <linux/atomic.h>
+#include <linux/tty.h>
+#include <linux/sched.h>
+
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+# define __acq(l, s, t, r, c, n, i) \
+ lock_acquire(&(l)->dep_map, s, t, r, c, n, i)
+# define __rel(l, n, i) \
+ lock_release(&(l)->dep_map, n, i)
+# ifdef CONFIG_PROVE_LOCKING
+# define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 2, NULL, i)
+# define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 2, n, i)
+# define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 2, NULL, i)
+# define lockdep_release(l, n, i) __rel(l, n, i)
+# else
+# define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 1, NULL, i)
+# define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 1, n, i)
+# define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 1, NULL, i)
+# define lockdep_release(l, n, i) __rel(l, n, i)
+# endif
+#else
+# define lockdep_acquire(l, s, t, i) do { } while (0)
+# define lockdep_acquire_nest(l, s, t, n, i) do { } while (0)
+# define lockdep_acquire_read(l, s, t, i) do { } while (0)
+# define lockdep_release(l, n, i) do { } while (0)
+#endif
+
+#ifdef CONFIG_LOCK_STAT
+# define lock_stat(_lock, stat) lock_##stat(&(_lock)->dep_map, _RET_IP_)
+#else
+# define lock_stat(_lock, stat) do { } while (0)
+#endif
+
+
+#if BITS_PER_LONG == 64
+# define LDSEM_ACTIVE_MASK 0xffffffffL
+#else
+# define LDSEM_ACTIVE_MASK 0x0000ffffL
+#endif
+
+#define LDSEM_UNLOCKED 0L
+#define LDSEM_ACTIVE_BIAS 1L
+#define LDSEM_WAIT_BIAS (-LDSEM_ACTIVE_MASK-1)
+#define LDSEM_READ_BIAS LDSEM_ACTIVE_BIAS
+#define LDSEM_WRITE_BIAS (LDSEM_WAIT_BIAS + LDSEM_ACTIVE_BIAS)
+
+struct ldsem_waiter {
+ struct list_head list;
+ struct task_struct *task;
+};
+
+static inline long ldsem_atomic_update(long delta, struct ld_semaphore *sem)
+{
+ return atomic_long_add_return(delta, (atomic_long_t *)&sem->count);
+}
+
+static inline int ldsem_cmpxchg(long *old, long new, struct ld_semaphore *sem)
+{
+ long tmp = *old;
+ *old = atomic_long_cmpxchg(&sem->count, *old, new);
+ return *old == tmp;
+}
+
+/*
+ * Initialize an ldsem:
+ */
+void __init_ldsem(struct ld_semaphore *sem, const char *name,
+ struct lock_class_key *key)
+{
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ /*
+ * Make sure we are not reinitializing a held semaphore:
+ */
+ debug_check_no_locks_freed((void *)sem, sizeof(*sem));
+ lockdep_init_map(&sem->dep_map, name, key, 0);
+#endif
+ sem->count = LDSEM_UNLOCKED;
+ sem->wait_readers = 0;
+ raw_spin_lock_init(&sem->wait_lock);
+ INIT_LIST_HEAD(&sem->read_wait);
+ INIT_LIST_HEAD(&sem->write_wait);
+}
+
+static void __ldsem_wake_readers(struct ld_semaphore *sem)
+{
+ struct ldsem_waiter *waiter, *next;
+ struct task_struct *tsk;
+ long adjust, count;
+
+ /* Try to grant read locks to all readers on the read wait list.
+ * Note the 'active part' of the count is incremented by
+ * the number of readers before waking any processes up.
+ */
+ adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS);
+ count = ldsem_atomic_update(adjust, sem);
+ do {
+ if (count > 0)
+ break;
+ if (ldsem_cmpxchg(&count, count - adjust, sem))
+ return;
+ } while (1);
+
+ list_for_each_entry_safe(waiter, next, &sem->read_wait, list) {
+ tsk = waiter->task;
+ smp_mb();
+ waiter->task = NULL;
+ wake_up_process(tsk);
+ put_task_struct(tsk);
+ }
+ INIT_LIST_HEAD(&sem->read_wait);
+ sem->wait_readers = 0;
+}
+
+static inline int writer_trylock(struct ld_semaphore *sem)
+{
+ /* only wake this writer if the active part of the count can be
+ * transitioned from 0 -> 1
+ */
+ long count = ldsem_atomic_update(LDSEM_ACTIVE_BIAS, sem);
+ do {
+ if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS)
+ return 1;
+ if (ldsem_cmpxchg(&count, count - LDSEM_ACTIVE_BIAS, sem))
+ return 0;
+ } while (1);
+}
+
+static void __ldsem_wake_writer(struct ld_semaphore *sem)
+{
+ struct ldsem_waiter *waiter;
+
+ waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list);
+ wake_up_process(waiter->task);
+}
+
+/*
+ * handle the lock release when processes blocked on it that can now run
+ * - if we come here from up_xxxx(), then:
+ * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
+ * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
+ * - the spinlock must be held by the caller
+ * - woken process blocks are discarded from the list after having task zeroed
+ */
+static void __ldsem_wake(struct ld_semaphore *sem)
+{
+ if (!list_empty(&sem->write_wait))
+ __ldsem_wake_writer(sem);
+ else if (!list_empty(&sem->read_wait))
+ __ldsem_wake_readers(sem);
+}
+
+static void ldsem_wake(struct ld_semaphore *sem)
+{
+ unsigned long flags;
+
+ raw_spin_lock_irqsave(&sem->wait_lock, flags);
+ __ldsem_wake(sem);
+ raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+}
+
+/*
+ * wait for the read lock to be granted
+ */
+static struct ld_semaphore __sched *
+down_read_failed(struct ld_semaphore *sem, long count, long timeout)
+{
+ struct ldsem_waiter waiter;
+ struct task_struct *tsk = current;
+ long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS;
+
+ /* set up my own style of waitqueue */
+ raw_spin_lock_irq(&sem->wait_lock);
+
+ /* Try to reverse the lock attempt but if the count has changed
+ * so that reversing fails, check if there are are no waiters,
+ * and early-out if not */
+ do {
+ if (ldsem_cmpxchg(&count, count + adjust, sem))
+ break;
+ if (count > 0) {
+ raw_spin_unlock_irq(&sem->wait_lock);
+ return sem;
+ }
+ } while (1);
+
+ list_add_tail(&waiter.list, &sem->read_wait);
+ sem->wait_readers++;
+
+ waiter.task = tsk;
+ get_task_struct(tsk);
+
+ /* if there are no active locks, wake the new lock owner(s) */
+ if ((count & LDSEM_ACTIVE_MASK) == 0)
+ __ldsem_wake(sem);
+
+ raw_spin_unlock_irq(&sem->wait_lock);
+
+ /* wait to be given the lock */
+ for (;;) {
+ set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+
+ if (!waiter.task)
+ break;
+ if (!timeout)
+ break;
+ timeout = schedule_timeout(timeout);
+ }
+
+ __set_task_state(tsk, TASK_RUNNING);
+
+ if (!timeout) {
+ /* lock timed out but check if this task was just
+ * granted lock ownership - if so, pretend there
+ * was no timeout; otherwise, cleanup lock wait */
+ raw_spin_lock_irq(&sem->wait_lock);
+ if (waiter.task) {
+ ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
+ list_del(&waiter.list);
+ raw_spin_unlock_irq(&sem->wait_lock);
+ put_task_struct(waiter.task);
+ return NULL;
+ }
+ raw_spin_unlock_irq(&sem->wait_lock);
+ }
+
+ return sem;
+}
+
+/*
+ * wait for the write lock to be granted
+ */
+static struct ld_semaphore __sched *
+down_write_failed(struct ld_semaphore *sem, long count, long timeout)
+{
+ struct ldsem_waiter waiter;
+ struct task_struct *tsk = current;
+ long adjust = -LDSEM_ACTIVE_BIAS;
+ int locked = 0;
+
+ /* set up my own style of waitqueue */
+ raw_spin_lock_irq(&sem->wait_lock);
+
+ /* Try to reverse the lock attempt but if the count has changed
+ * so that reversing fails, check if the lock is now owned,
+ * and early-out if so */
+ do {
+ if (ldsem_cmpxchg(&count, count + adjust, sem))
+ break;
+ if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) {
+ raw_spin_unlock_irq(&sem->wait_lock);
+ return sem;
+ }
+ } while (1);
+
+ list_add_tail(&waiter.list, &sem->write_wait);
+
+ waiter.task = tsk;
+
+ set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+ for (;;) {
+ if (!timeout)
+ break;
+ raw_spin_unlock_irq(&sem->wait_lock);
+ timeout = schedule_timeout(timeout);
+ raw_spin_lock_irq(&sem->wait_lock);
+ set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+ if ((locked = writer_trylock(sem)))
+ break;
+ }
+
+ if (!locked)
+ ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
+ list_del(&waiter.list);
+ raw_spin_unlock_irq(&sem->wait_lock);
+
+ __set_task_state(tsk, TASK_RUNNING);
+
+ /* lock wait may have timed out */
+ if (!locked)
+ return NULL;
+ return sem;
+}
+
+
+
+static inline int __ldsem_down_read_nested(struct ld_semaphore *sem,
+ int subclass, long timeout)
+{
+ long count;
+
+ lockdep_acquire_read(sem, subclass, 0, _RET_IP_);
+
+ count = ldsem_atomic_update(LDSEM_READ_BIAS, sem);
+ if (count <= 0) {
+ lock_stat(sem, contended);
+ if (!down_read_failed(sem, count, timeout)) {
+ lockdep_release(sem, 1, _RET_IP_);
+ return 0;
+ }
+ }
+ lock_stat(sem, acquired);
+ return 1;
+}
+
+static inline int __ldsem_down_write_nested(struct ld_semaphore *sem,
+ int subclass, long timeout)
+{
+ long count;
+
+ lockdep_acquire(sem, subclass, 0, _RET_IP_);
+
+ count = ldsem_atomic_update(LDSEM_WRITE_BIAS, sem);
+ if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) {
+ lock_stat(sem, contended);
+ if (!down_write_failed(sem, count, timeout)) {
+ lockdep_release(sem, 1, _RET_IP_);
+ return 0;
+ }
+ }
+ lock_stat(sem, acquired);
+ return 1;
+}
+
+
+/*
+ * lock for reading -- returns 1 if successful, 0 if timed out
+ */
+int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout)
+{
+ might_sleep();
+ return __ldsem_down_read_nested(sem, 0, timeout);
+}
+
+/*
+ * trylock for reading -- returns 1 if successful, 0 if contention
+ */
+int ldsem_down_read_trylock(struct ld_semaphore *sem)
+{
+ long count = sem->count;
+
+ while (count >= 0) {
+ if (ldsem_cmpxchg(&count, count + LDSEM_READ_BIAS, sem)) {
+ lockdep_acquire_read(sem, 0, 1, _RET_IP_);
+ lock_stat(sem, acquired);
+ return 1;
+ }
+ }
+ return 0;
+}
+
+/*
+ * lock for writing -- returns 1 if successful, 0 if timed out
+ */
+int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout)
+{
+ might_sleep();
+ return __ldsem_down_write_nested(sem, 0, timeout);
+}
+
+/*
+ * trylock for writing -- returns 1 if successful, 0 if contention
+ */
+int ldsem_down_write_trylock(struct ld_semaphore *sem)
+{
+ long count = sem->count;
+
+ while ((count & LDSEM_ACTIVE_MASK) == 0) {
+ if (ldsem_cmpxchg(&count, count + LDSEM_WRITE_BIAS, sem)) {
+ lockdep_acquire(sem, 0, 1, _RET_IP_);
+ lock_stat(sem, acquired);
+ return 1;
+ }
+ }
+ return 0;
+}
+
+/*
+ * release a read lock
+ */
+void ldsem_up_read(struct ld_semaphore *sem)
+{
+ long count;
+
+ lockdep_release(sem, 1, _RET_IP_);
+
+ count = ldsem_atomic_update(-LDSEM_READ_BIAS, sem);
+ if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0)
+ ldsem_wake(sem);
+}
+
+/*
+ * release a write lock
+ */
+void ldsem_up_write(struct ld_semaphore *sem)
+{
+ long count;
+
+ lockdep_release(sem, 1, _RET_IP_);
+
+ count = ldsem_atomic_update(-LDSEM_WRITE_BIAS, sem);
+ if (count < 0)
+ ldsem_wake(sem);
+}
+
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+
+int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout)
+{
+ might_sleep();
+ return __ldsem_down_read_nested(sem, subclass, timeout);
+}
+
+int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
+ long timeout)
+{
+ might_sleep();
+ return __ldsem_down_write_nested(sem, subclass, timeout);
+}
+
+#endif
projects / deliverable / linux.git / commitdiff
