Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote: > On Mon, 2012-12-31 at 13:21 +0000, Eric Wong wrote: > > 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/ > > Hmm, the change seems not very logical to me. My original description was not complete and I'm still bisecting my problem (ppoll + send stuck). However, my patch does solve the issue Andreas encountered and I now understand why. > If it helps, I would like to understand the real issue. > > commit 626cf236608505d376e4799adb4f7eb00a8594af should not have this > side effect, at least for poll()/select() functions. The epoll() changes > I am not yet very confident. I have a better explanation of the epoll problem below. An alternate version (limited to epoll) would be: diff --git a/fs/eventpoll.c b/fs/eventpoll.c index cd96649..ca5f3d0 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1299,6 +1299,7 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even * Get current event bits. We can safely use the file* here because * its usage count has been increased by the caller of this function. */ + smp_mb(); revents = epi->ffd.file->f_op->poll(epi->ffd.file, &pt); /* > I suspect a race already existed before this commit, it would be nice to > track it properly. I don't believe this race existed before that change. Updated commit message below: >From 87bca82bc39a941d9b8d5b8bc08b39a071a9884f Mon Sep 17 00:00:00 2001 From: Eric Wong <normalperson@xxxxxxxx> Date: Mon, 31 Dec 2012 13:20:23 +0000 Subject: [PATCH] epoll: prevent missed events on EPOLL_CTL_MOD ep_modify() works on files that are already registered with a wait queue (and thus should not reregister). For sockets, this means sk_sleep() will return a non-NULL wait address. ep_modify() must check for events that were received and ignored _before_ ep_modify() was called. So it must call f_op->poll() to fish for events _after_ changing epi->event.events. When f_op->poll() calls tcp_poll() (and thus sock_poll_wait()), wait_address is non-NULL because the socket was already registered by epoll. Thus, ep_modify() passes a NULL pt to prevent re-registration. When ep_modify() is called, sock_poll_wait() will see a wait_address, but a NULL pt, and this caused the memory barrier to get skipped and events to be missed (this memory barrier is described in the documentation for wq_has_sleeper). This regression appeared with the change to sock_poll_wait() in commit 626cf236608505d376e4799adb4f7eb00a8594af (poll: add poll_requested_events() and poll_does_not_wait() functions) This issue was encountered by Andreas Voellmy and Junchang(Jason) Wang: http://thread.gmane.org/gmane.linux.kernel/1408782/ Signed-off-by: Eric Wong <normalperson@xxxxxxxx> Cc: Hans Verkuil <hans.verkuil@xxxxxxxxx> Cc: Jiri Olsa <jolsa@xxxxxxxxxx> Cc: Jonathan Corbet <corbet@xxxxxxx> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Davide Libenzi <davidel@xxxxxxxxxxxxxxx> Cc: Hans de Goede <hdegoede@xxxxxxxxxx> Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxx> Cc: David Miller <davem@xxxxxxxxxxxxx> Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Tested-by: Andreas Voellmy <andreas.voellmy@xxxxxxxx> Tested-by: "Junchang(Jason) Wang" <junchang.wang@xxxxxxxx> Cc: netdev@xxxxxxxxxxxxxxx Cc: linux-fsdevel@xxxxxxxxxxxxxxx --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
