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.
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: Andrew Morton akpm@linux-foundation.org Cc: Davidlohr Bueso dbueso@suse.de Cc: stable@vger.kernel.org --- fs/eventpoll.c | 23 +++++++++++++++++------ 1 file changed, 17 insertions(+), 6 deletions(-)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c index aba03ee749f8..4af2d020f548 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -704,8 +704,14 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep, * 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(struct eventpoll *ep, } } /* + * 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);