sched/wait: Avoid abort_exclusive_wait() in ___wait_event()

___wait_event() doesn't really need abort_exclusive_wait(), we can simply
change prepare_to_wait_event() to remove the waiter from q->task_list if
it was interrupted.

This simplifies the code/logic, and this way prepare_to_wait_event() can
have more users, see the next change.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Neil Brown <neilb@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20160908164815.GA18801@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
--
 include/linux/wait.h |    7 +------
 kernel/sched/wait.c  |   35 +++++++++++++++++++++++++----------
 2 files changed, 26 insertions(+), 16 deletions(-)

include/linux/wait.h

@@ -280,12 +280,7 @@ wait_queue_head_t *bit_waitqueue(void *, int);
 									\
 		if (___wait_is_interruptible(state) && __int) {		\
 			__ret = __int;					\
-			if (exclusive) {				\
-				abort_exclusive_wait(&wq, &__wait, 	\
-						     NULL);		\
-				goto __out;				\
-			}						\
-			break;						\
+			goto __out;					\
 		}							\
 									\
 		cmd;							\

kernel/sched/wait.c

@@ -199,24 +199,39 @@ EXPORT_SYMBOL(prepare_to_wait_exclusive);
 long prepare_to_wait_event(wait_queue_head_t *q, wait_queue_t *wait, int state)
 {
 	unsigned long flags;
-
-	if (signal_pending_state(state, current))
-		return -ERESTARTSYS;
+	long ret = 0;
 
 	wait->private = current;
 	wait->func = autoremove_wake_function;
 
 	spin_lock_irqsave(&q->lock, flags);
-	if (list_empty(&wait->task_list)) {
-		if (wait->flags & WQ_FLAG_EXCLUSIVE)
-			__add_wait_queue_tail(q, wait);
-		else
-			__add_wait_queue(q, wait);
+	if (unlikely(signal_pending_state(state, current))) {
+		/*
+		 * Exclusive waiter must not fail if it was selected by wakeup,
+		 * it should "consume" the condition we were waiting for.
+		 *
+		 * The caller will recheck the condition and return success if
+		 * we were already woken up, we can not miss the event because
+		 * wakeup locks/unlocks the same q->lock.
+		 *
+		 * But we need to ensure that set-condition + wakeup after that
+		 * can't see us, it should wake up another exclusive waiter if
+		 * we fail.
+		 */
+		list_del_init(&wait->task_list);
+		ret = -ERESTARTSYS;
+	} else {
+		if (list_empty(&wait->task_list)) {
+			if (wait->flags & WQ_FLAG_EXCLUSIVE)
+				__add_wait_queue_tail(q, wait);
+			else
+				__add_wait_queue(q, wait);
+		}
+		set_current_state(state);
 	}
-	set_current_state(state);
 	spin_unlock_irqrestore(&q->lock, flags);
 
-	return 0;
+	return ret;
 }
 EXPORT_SYMBOL(prepare_to_wait_event);