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Subject[PATCH V2 2/4] 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>
---
include/linux/wait.h | 7 +------
kernel/sched/wait.c | 23 ++++++++++++++++++-----
2 files changed, 19 insertions(+), 11 deletions(-)

diff --git a/include/linux/wait.h b/include/linux/wait.h
index 329f796..5179915 100644
--- a/include/linux/wait.h
+++ b/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; \
diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index 2bbba01..4af0dc8 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -199,15 +199,28 @@ 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 (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
@@ -216,7 +229,7 @@ long prepare_to_wait_event(wait_queue_head_t *q, wait_queue_t *wait, int state)
set_current_state(state);
spin_unlock_irqrestore(&q->lock, flags);

- return 0;
+ return ret;
}
EXPORT_SYMBOL(prepare_to_wait_event);

--
2.5.0
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