Messages in this thread Patch in this message | | | Date | Mon, 03 Feb 2020 18:33:27 +0100 | From | Roman Penyaev <> | Subject | Re: epoll_wait misses edge-triggered eventfd events: bug in Linux 5.3 and 5.4 |
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Hi Max and all,
I can reproduce the issue. My epoll optimization which you referenced did not consider the case of wakeups on epoll_ctl() path, only the fd update path.
I will send the fix upstream today/tomorrow (already tested on the epollbug.c), the exemplary patch at the bottom of the current email.
Also I would like to submit the epollbug.c as a test case for the epoll test suite. Does the author of epollbug have any objections?
Thanks.
-- Roman
diff --git a/fs/eventpoll.c b/fs/eventpoll.c index c4159bcc05d9..a90f8b8a5def 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -745,7 +745,7 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep, * the ->poll() wait list (delayed after we release the lock). */ if (waitqueue_active(&ep->wq)) - wake_up(&ep->wq); + wake_up_locked(&ep->wq); if (waitqueue_active(&ep->poll_wait)) pwake++; } @@ -1200,7 +1200,7 @@ static inline bool chain_epi_lockless(struct epitem *epi) * Another thing worth to mention is that ep_poll_callback() can be called * concurrently for the same @epi from different CPUs if poll table was inited * with several wait queues entries. Plural wakeup from different CPUs of a - * single wait queue is serialized by wq.lock, but the case when multiple wait + * single wait queue is serialized by ep->lock, but the case when multiple wait * queues are used should be detected accordingly. This is detected using * cmpxchg() operation. */ @@ -1275,6 +1275,13 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v break; } } + /* + * Since here we have the read lock (ep->lock) taken, plural + * wakeup from different CPUs can occur, thus we call wake_up() + * variant which implies its own lock on wqueue. All other paths + * take write lock, thus modifications on ep->wq are serialized + * by rw lock. + */ wake_up(&ep->wq); } if (waitqueue_active(&ep->poll_wait)) @@ -1578,7 +1585,7 @@ static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
/* Notify waiting tasks that events are available */ if (waitqueue_active(&ep->wq)) - wake_up(&ep->wq); + wake_up_locked(&ep->wq); if (waitqueue_active(&ep->poll_wait)) pwake++; } @@ -1684,7 +1691,7 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi,
/* Notify waiting tasks that events are available */ if (waitqueue_active(&ep->wq)) - wake_up(&ep->wq); + wake_up_locked(&ep->wq); if (waitqueue_active(&ep->poll_wait)) pwake++; } @@ -1881,9 +1888,9 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, waiter = true; init_waitqueue_entry(&wait, current);
- spin_lock_irq(&ep->wq.lock); + write_lock_irq(&ep->lock); __add_wait_queue_exclusive(&ep->wq, &wait); - spin_unlock_irq(&ep->wq.lock); + write_unlock_irq(&ep->lock); }
for (;;) { @@ -1931,9 +1938,9 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, goto fetch_events;
if (waiter) { - spin_lock_irq(&ep->wq.lock); + write_lock_irq(&ep->lock); __remove_wait_queue(&ep->wq, &wait); - spin_unlock_irq(&ep->wq.lock); + write_unlock_irq(&ep->lock); }
return res;
On 2020-02-03 16:15, Max Neunhoeffer wrote: > Dear Jakub and all, > > I have done a git bisect and found that this commit introduced the > epoll > bug: > > https://github.com/torvalds/linux/commit/a218cc4914209ac14476cb32769b31a556355b22 > > I Cc the author of the commit. > > This makes sense, since the commit introduces a new rwlock to reduce > contention in ep_poll_callback. I do not fully understand the details > but this sounds all very close to this bug. > > I have also verified that the bug is still present in the latest master > branch in Linus' repository. > > Furthermore, Chris Kohlhoff has provided yet another reproducing > program > which is no longer using edge-triggered but standard level-triggered > events and epoll_wait. This makes the bug all the more urgent, since > potentially more programs could run into this problem and could end up > with sleeping barbers. > > I have added all the details to the bugzilla bugreport: > > https://bugzilla.kernel.org/show_bug.cgi?id=205933 > > Hopefully, we can resolve this now equipped with this amount of > information. > > Best regards, > Max. > > On 20/02/01 12:16, Jakub Kicinski wrote: >> On Fri, 31 Jan 2020 14:57:30 +0100, Max Neunhoeffer wrote: >> > Dear All, >> > >> > I believe I have found a bug in Linux 5.3 and 5.4 in epoll_wait/epoll_ctl >> > when an eventfd together with edge-triggered or the EPOLLONESHOT policy >> > is used. If an epoll_ctl call to rearm the eventfd happens approximately >> > at the same time as the epoll_wait goes to sleep, the event can be lost, >> > even though proper protection through a mutex is employed. >> > >> > The details together with two programs showing the problem can be found >> > here: >> > >> > https://bugzilla.kernel.org/show_bug.cgi?id=205933 >> > >> > Older kernels seem not to have this problem, although I did not test all >> > versions. I know that 4.15 and 5.0 do not show the problem. >> > >> > Note that this method of using epoll_wait/eventfd is used by >> > boost::asio to wake up event loops in case a new completion handler >> > is posted to an io_service, so this is probably relevant for many >> > applications. >> > >> > Any help with this would be appreciated. >> >> Could be networking related but let's CC FS folks just in case. >> >> Would you be able to perform bisection to narrow down the search >> for a buggy change?
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