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Subject[PATCH] perf: do poll_wait() before checking condition in perf_poll()
Date
One should first enqueue to the waitqueue and then check for the
condition. If the condition gets true after mutex_unlock() but before
poll_wait() then we lose it and would have wait for another wakeup.

This has been like this since v2.6.31-rc1 commit c7138f37f9 ("perf_counter:
fix perf_poll()"). Before that it was slightly worse. I guess we get enough
wakeups so if we miss here one it doesn't really matter. It is still a
bad example.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
I haven't Cc stable@vger.kernel.org since nobody cared about this since
day one.

kernel/events/core.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 6b17ac1..4aa3375 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3528,6 +3528,7 @@ static unsigned int perf_poll(struct file *file, poll_table *wait)
struct ring_buffer *rb;
unsigned int events = POLL_HUP;

+ poll_wait(file, &event->waitq, wait);
/*
* Pin the event->rb by taking event->mmap_mutex; otherwise
* perf_event_set_output() can swizzle our rb and make us miss wakeups.
@@ -3537,9 +3538,6 @@ static unsigned int perf_poll(struct file *file, poll_table *wait)
if (rb)
events = atomic_xchg(&rb->poll, 0);
mutex_unlock(&event->mmap_mutex);
-
- poll_wait(file, &event->waitq, wait);
-
return events;
}

--
2.0.1


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