@@ -704,8 +704,14 @@ static __poll_t ep_scan_ready_list(struc
* in a lockless way.
*/
write_lock_irq(&ep->lock);
- list_splice_init(&ep->rdllist, &txlist);
WRITE_ONCE(ep->ovflist, NULL);
+ /*
+ * In ep_poll() we use ep_events_available() in a lockless way to decide
+ * if events are available. So we need to preserve that either
+ * ep->oflist != EP_UNACTIVE_PTR or there are events on the ep->rdllist.
+ */
+ smp_wmb();
+ list_splice_init(&ep->rdllist, &txlist);
write_unlock_irq(&ep->lock);
/*
@@ -737,16 +743,21 @@ static __poll_t ep_scan_ready_list(struc
}
}
/*
+ * Quickly re-inject items left on "txlist".
+ */
+ list_splice(&txlist, &ep->rdllist);
+ /*
+ * In ep_poll() we use ep_events_available() in a lockless way to decide
+ * if events are available. So we need to preserve that either
+ * ep->oflist != EP_UNACTIVE_PTR or there are events on the ep->rdllist.
+ */
+ smp_wmb();
+ /*
* We need to set back ep->ovflist to EP_UNACTIVE_PTR, so that after
* releasing the lock, events will be queued in the normal way inside
* ep->rdllist.
*/
WRITE_ONCE(ep->ovflist, EP_UNACTIVE_PTR);
-
- /*
- * Quickly re-inject items left on "txlist".
- */
- list_splice(&txlist, &ep->rdllist);
__pm_relax(ep->ws);
write_unlock_irq(&ep->lock);
The patch titled Subject: epoll: ensure ep_poll() doesn't miss wakeup events has been added to the -mm tree. Its filename is epoll-ensure-ep_poll-doesnt-miss-wakeup-events.patch This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/epoll-ensure-ep_poll-doesnt-miss-wakeup-events.patch and later at http://ozlabs.org/~akpm/mmotm/broken-out/epoll-ensure-ep_poll-doesnt-miss-wakeup-events.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Jason Baron <jbaron@akamai.com> Subject: epoll: ensure ep_poll() doesn't miss wakeup events Now that the ep_events_available() check is done in a lockless way, and we no longer perform wakeups from ep_scan_ready_list(), we need to ensure that either ep->rdllist has items or the overflow list is active. Prior to: commit 339ddb53d373 ("fs/epoll: remove unnecessary wakeups of nested epoll"), we did wake_up(&ep->wq) after manipulating the ep->rdllist and the overflow list. Thus, any waiters would observe the correct state. However, with that wake_up() now removed we need to be more careful to ensure that condition. Here's an example of what could go wrong: We have epoll fds: epfd1, epfd2. And epfd1 is added to epfd2 and epfd2 is added to a socket: epfd1->epfd2->socket. Thread a is doing epoll_wait() on epfd1, and thread b is doing epoll_wait on epfd2. Then: 1) data comes in on socket ep_poll_callback() wakes up threads a and b 2) thread a runs ep_poll() ep_scan_ready_list() ep_send_events_proc() ep_item_poll() ep_scan_ready_list() list_splice_init(&ep->rdllist, &txlist); 3) now thread b is running ep_poll() ep_events_available() returns false schedule_hrtimeout_range() Thus, thread b has now scheduled and missed the wakeup. Link: http://lkml.kernel.org/r/1588360533-11828-1-git-send-email-jbaron@akamai.com Fixes: 339ddb53d373 ("fs/epoll: remove unnecessary wakeups of nested epoll") Signed-off-by: Jason Baron <jbaron@akamai.com> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Heiher <r@hev.cc> Cc: Roman Penyaev <rpenyaev@suse.de> Cc: Khazhismel Kumykov <khazhy@google.com> Cc: Davidlohr Bueso <dbueso@suse.de> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> --- fs/eventpoll.c | 23 +++++++++++++++++------ 1 file changed, 17 insertions(+), 6 deletions(-)