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Subject[PATCH] poll: prevent missed events if _qproc is NULL
This patch seems to fix my issue with ppoll() being stuck on my
SMP machine:

The change to sock_poll_wait() in
commit 626cf236608505d376e4799adb4f7eb00a8594af
(poll: add poll_requested_events() and poll_does_not_wait() functions)
seems to have allowed additional cases where the SMP memory barrier
is not issued before checking for readiness.

In my case, this affects the select()-family of functions
which register descriptors once and set _qproc to NULL before
checking events again (after poll_schedule_timeout() returns).
The set_mb() barrier in poll_schedule_timeout() appears to be
insufficient on my SMP x86-64 machine (as it's only an xchg()).

This may also be related to the epoll issue described by
Andreas Voellmy in

Signed-off-by: Eric Wong <>
Cc: Hans Verkuil <>
Cc: Jiri Olsa <>
Cc: Jonathan Corbet <>
Cc: Al Viro <>
Cc: Davide Libenzi <>
Cc: Hans de Goede <>
Cc: Mauro Carvalho Chehab <>
Cc: David Miller <>
Cc: Eric Dumazet <>
Cc: Andrew Morton <>
Cc: Linus Torvalds <>
Cc: Andreas Voellmy <>
Cc: "Junchang(Jason) Wang" <>
If this patch is correct, I think we can just drop the
poll_does_not_wait() function entirely since poll_wait()
does the same check anyways...

include/net/sock.h | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index c945fba..1923e48 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1925,8 +1925,9 @@ static inline bool wq_has_sleeper(struct socket_wq *wq)
static inline void sock_poll_wait(struct file *filp,
wait_queue_head_t *wait_address, poll_table *p)
- if (!poll_does_not_wait(p) && wait_address) {
- poll_wait(filp, wait_address, p);
+ if (wait_address) {
+ if (!poll_does_not_wait(p))
+ poll_wait(filp, wait_address, p);
/* We need to be sure we are in sync with the
* socket flags modification.
Eric Wong

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