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Date: Mon, 03 Feb 2020 18:33:27 +0100
From: Roman Penyaev <rpenyaev@...e.de>
To: Max Neunhoeffer <max@...ngodb.com>
Cc: Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
Christopher Kohlhoff <chris.kohlhoff@...arpool.io>
Subject: Re: epoll_wait misses edge-triggered eventfd events: bug in Linux 5.3
and 5.4
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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