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49e4b2bcf7
The need for the smp_mb() in __rwsem_do_wake() should be properly documented. Applies to both xadd and spinlock variants. Signed-off-by: Davidlohr Bueso <dbueso@suse.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Jason Low <jason.low2@hp.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Michel Lespinasse <walken@google.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Tim Chen <tim.c.chen@linux.intel.com> Link: http://lkml.kernel.org/r/1422609267-15102-3-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
304 lines
6.7 KiB
C
304 lines
6.7 KiB
C
/* rwsem-spinlock.c: R/W semaphores: contention handling functions for
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* generic spinlock implementation
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*
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* Copyright (c) 2001 David Howells (dhowells@redhat.com).
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* - Derived partially from idea by Andrea Arcangeli <andrea@suse.de>
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* - Derived also from comments by Linus
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*/
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#include <linux/rwsem.h>
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#include <linux/sched.h>
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#include <linux/export.h>
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enum rwsem_waiter_type {
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RWSEM_WAITING_FOR_WRITE,
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RWSEM_WAITING_FOR_READ
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};
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struct rwsem_waiter {
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struct list_head list;
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struct task_struct *task;
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enum rwsem_waiter_type type;
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};
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int rwsem_is_locked(struct rw_semaphore *sem)
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{
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int ret = 1;
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unsigned long flags;
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if (raw_spin_trylock_irqsave(&sem->wait_lock, flags)) {
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ret = (sem->count != 0);
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raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
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}
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return ret;
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}
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EXPORT_SYMBOL(rwsem_is_locked);
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/*
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* initialise the semaphore
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*/
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void __init_rwsem(struct rw_semaphore *sem, const char *name,
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struct lock_class_key *key)
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{
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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/*
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* Make sure we are not reinitializing a held semaphore:
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*/
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debug_check_no_locks_freed((void *)sem, sizeof(*sem));
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lockdep_init_map(&sem->dep_map, name, key, 0);
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#endif
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sem->count = 0;
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raw_spin_lock_init(&sem->wait_lock);
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INIT_LIST_HEAD(&sem->wait_list);
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}
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EXPORT_SYMBOL(__init_rwsem);
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/*
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* handle the lock release when processes blocked on it that can now run
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* - if we come here, then:
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* - the 'active count' _reached_ zero
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* - the 'waiting count' is non-zero
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* - the spinlock must be held by the caller
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* - woken process blocks are discarded from the list after having task zeroed
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* - writers are only woken if wakewrite is non-zero
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*/
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static inline struct rw_semaphore *
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__rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
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{
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struct rwsem_waiter *waiter;
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struct task_struct *tsk;
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int woken;
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waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
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if (wakewrite)
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/* Wake up a writer. Note that we do not grant it the
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* lock - it will have to acquire it when it runs. */
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wake_up_process(waiter->task);
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goto out;
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}
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/* grant an infinite number of read locks to the front of the queue */
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woken = 0;
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do {
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struct list_head *next = waiter->list.next;
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list_del(&waiter->list);
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tsk = waiter->task;
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/*
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* Make sure we do not wakeup the next reader before
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* setting the nil condition to grant the next reader;
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* otherwise we could miss the wakeup on the other
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* side and end up sleeping again. See the pairing
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* in rwsem_down_read_failed().
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*/
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smp_mb();
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waiter->task = NULL;
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wake_up_process(tsk);
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put_task_struct(tsk);
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woken++;
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if (next == &sem->wait_list)
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break;
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waiter = list_entry(next, struct rwsem_waiter, list);
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} while (waiter->type != RWSEM_WAITING_FOR_WRITE);
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sem->count += woken;
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out:
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return sem;
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}
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/*
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* wake a single writer
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*/
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static inline struct rw_semaphore *
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__rwsem_wake_one_writer(struct rw_semaphore *sem)
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{
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struct rwsem_waiter *waiter;
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waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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wake_up_process(waiter->task);
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return sem;
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}
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/*
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* get a read lock on the semaphore
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*/
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void __sched __down_read(struct rw_semaphore *sem)
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{
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struct rwsem_waiter waiter;
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struct task_struct *tsk;
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unsigned long flags;
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raw_spin_lock_irqsave(&sem->wait_lock, flags);
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if (sem->count >= 0 && list_empty(&sem->wait_list)) {
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/* granted */
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sem->count++;
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raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
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goto out;
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}
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tsk = current;
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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/* set up my own style of waitqueue */
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waiter.task = tsk;
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waiter.type = RWSEM_WAITING_FOR_READ;
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get_task_struct(tsk);
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list_add_tail(&waiter.list, &sem->wait_list);
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/* we don't need to touch the semaphore struct anymore */
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raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
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/* wait to be given the lock */
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for (;;) {
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if (!waiter.task)
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break;
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schedule();
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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}
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__set_task_state(tsk, TASK_RUNNING);
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out:
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;
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}
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/*
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* trylock for reading -- returns 1 if successful, 0 if contention
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*/
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int __down_read_trylock(struct rw_semaphore *sem)
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{
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unsigned long flags;
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int ret = 0;
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raw_spin_lock_irqsave(&sem->wait_lock, flags);
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if (sem->count >= 0 && list_empty(&sem->wait_list)) {
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/* granted */
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sem->count++;
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ret = 1;
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}
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raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
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return ret;
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}
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/*
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* get a write lock on the semaphore
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*/
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void __sched __down_write_nested(struct rw_semaphore *sem, int subclass)
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{
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struct rwsem_waiter waiter;
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struct task_struct *tsk;
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unsigned long flags;
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raw_spin_lock_irqsave(&sem->wait_lock, flags);
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/* set up my own style of waitqueue */
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tsk = current;
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waiter.task = tsk;
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waiter.type = RWSEM_WAITING_FOR_WRITE;
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list_add_tail(&waiter.list, &sem->wait_list);
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/* wait for someone to release the lock */
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for (;;) {
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/*
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* That is the key to support write lock stealing: allows the
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* task already on CPU to get the lock soon rather than put
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* itself into sleep and waiting for system woke it or someone
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* else in the head of the wait list up.
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*/
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if (sem->count == 0)
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break;
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
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schedule();
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raw_spin_lock_irqsave(&sem->wait_lock, flags);
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}
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/* got the lock */
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sem->count = -1;
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list_del(&waiter.list);
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raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
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}
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void __sched __down_write(struct rw_semaphore *sem)
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{
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__down_write_nested(sem, 0);
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}
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/*
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* trylock for writing -- returns 1 if successful, 0 if contention
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*/
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int __down_write_trylock(struct rw_semaphore *sem)
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{
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unsigned long flags;
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int ret = 0;
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raw_spin_lock_irqsave(&sem->wait_lock, flags);
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if (sem->count == 0) {
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/* got the lock */
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sem->count = -1;
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ret = 1;
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}
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raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
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return ret;
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}
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/*
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* release a read lock on the semaphore
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*/
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void __up_read(struct rw_semaphore *sem)
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{
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unsigned long flags;
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raw_spin_lock_irqsave(&sem->wait_lock, flags);
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if (--sem->count == 0 && !list_empty(&sem->wait_list))
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sem = __rwsem_wake_one_writer(sem);
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raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
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}
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/*
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* release a write lock on the semaphore
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*/
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void __up_write(struct rw_semaphore *sem)
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{
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unsigned long flags;
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raw_spin_lock_irqsave(&sem->wait_lock, flags);
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sem->count = 0;
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if (!list_empty(&sem->wait_list))
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sem = __rwsem_do_wake(sem, 1);
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raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
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}
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/*
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* downgrade a write lock into a read lock
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* - just wake up any readers at the front of the queue
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*/
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void __downgrade_write(struct rw_semaphore *sem)
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{
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unsigned long flags;
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raw_spin_lock_irqsave(&sem->wait_lock, flags);
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sem->count = 1;
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if (!list_empty(&sem->wait_list))
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sem = __rwsem_do_wake(sem, 0);
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raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
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}
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