Messages in this thread Patch in this message | | | From | Eric Wong <> | Subject | [PATCH] poll: prevent missed events if _qproc is NULL | Date | Mon, 31 Dec 2012 13:21:00 +0000 |
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This patch seems to fix my issue with ppoll() being stuck on my SMP machine: http://article.gmane.org/gmane.linux.file-systems/70414
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 http://thread.gmane.org/gmane.linux.kernel/1408782/
Signed-off-by: Eric Wong <normalperson@yhbt.net> Cc: Hans Verkuil <hans.verkuil@cisco.com> Cc: Jiri Olsa <jolsa@redhat.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Davide Libenzi <davidel@xmailserver.org> Cc: Hans de Goede <hdegoede@redhat.com> Cc: Mauro Carvalho Chehab <mchehab@infradead.org> Cc: David Miller <davem@davemloft.net> Cc: Eric Dumazet <eric.dumazet@gmail.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andreas Voellmy <andreas.voellmy@yale.edu> Cc: "Junchang(Jason) Wang" <junchang.wang@yale.edu> Cc: netdev@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org --- 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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