rwsem: simplify __rwsem_do_wake
This is mostly for cleanup value: - We don't need several gotos to handle the case where the first waiter is a writer. Two simple tests will do (and generate very similar code). - In the remainder of the function, we know the first waiter is a reader, so we don't have to double check that. We can use do..while loops to iterate over the readers to wake (generates slightly better code). Signed-off-by: Michel Lespinasse <walken@google.com> Reviewed-by: Peter Hurley <peter@hurleysoftware.com> Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -70,26 +70,17 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
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waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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if (!wakewrite) {
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if (waiter->type == RWSEM_WAITING_FOR_WRITE)
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goto out;
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goto dont_wake_writers;
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}
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/*
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* as we support write lock stealing, we can't set sem->activity
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* to -1 here to indicate we get the lock. Instead, we wake it up
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* to let it go get it again.
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*/
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if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
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wake_up_process(waiter->task);
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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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dont_wake_writers:
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woken = 0;
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while (waiter->type == RWSEM_WAITING_FOR_READ) {
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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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@ -99,10 +90,10 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
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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 (list_empty(&sem->wait_list))
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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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}
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} while (waiter->type != RWSEM_WAITING_FOR_WRITE);
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sem->activity += woken;
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26
lib/rwsem.c
26
lib/rwsem.c
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@ -68,20 +68,17 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
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signed long woken, loop, adjustment;
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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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goto readers_only;
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if (wake_type == RWSEM_WAKE_READ_OWNED)
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/* Another active reader was observed, so wakeup is not
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* likely to succeed. Save the atomic op.
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*/
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if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
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if (wake_type != RWSEM_WAKE_READ_OWNED)
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/* Wake writer at the front of the queue, but do not
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* grant it the lock yet as we want other writers
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* to be able to steal it. Readers, on the other hand,
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* will block as they will notice the queued writer.
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*/
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wake_up_process(waiter->task);
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goto out;
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}
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/* Wake up the writing waiter and let the task grab the sem: */
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wake_up_process(waiter->task);
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goto out;
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readers_only:
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/* If we come here from up_xxxx(), another thread might have reached
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* rwsem_down_failed_common() before we acquired the spinlock and
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* woken up a waiter, making it now active. We prefer to check for
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@ -125,7 +122,8 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
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rwsem_atomic_add(adjustment, sem);
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next = sem->wait_list.next;
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for (loop = woken; loop > 0; loop--) {
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loop = woken;
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do {
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waiter = list_entry(next, struct rwsem_waiter, list);
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next = waiter->list.next;
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tsk = waiter->task;
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@ -133,7 +131,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
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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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}
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} while (--loop);
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sem->wait_list.next = next;
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next->prev = &sem->wait_list;
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