On Thu, Nov 24, 2022 at 12:11:22AM +0000, Rishabh Bhatnagar wrote:

From: Roman Penyaev rpenyaev@suse.de

Commit 65759097d804d2a9ad2b687db436319704ba7019 upstream.

There is a possible race when ep_scan_ready_list() leaves ->rdllist and ->obflist empty for a short period of time although some events are pending. It is quite likely that ep_events_available() observes empty lists and goes to sleep.

Since commit 339ddb53d373 ("fs/epoll: remove unnecessary wakeups of nested epoll") we are conservative in wakeups (there is only one place for wakeup and this is ep_poll_callback()), thus ep_events_available() must always observe correct state of two lists.

The easiest and correct way is to do the final check under the lock. This does not impact the performance, since lock is taken anyway for adding a wait entry to the wait queue.

The discussion of the problem can be found here:

https://lore.kernel.org/linux-fsdevel/a2f22c3c-c25a-4bda-8339-a7bdaf17849e@a...

In this patch barrierless __set_current_state() is used. This is safe since waitqueue_active() is called under the same lock on wakeup side.

Short-circuit for fatal signals (i.e. fatal_signal_pending() check) is moved to the line just before actual events harvesting routine. This is fully compliant to what is said in the comment of the patch where the actual fatal_signal_pending() check was added: c257a340ede0 ("fs, epoll: short circuit fetching events if thread has been killed").

Fixes: 339ddb53d373 ("fs/epoll: remove unnecessary wakeups of nested epoll")

We may want to consider pulling in this commit as well to 5.4. 339ddb53d373 was merged in v5.5, but there are at least two other commits already in 5.4 stable that reference it, e.g.

$ git log --oneline v5.4..stable/linux-5.4.y --grep '339ddb53d373' ee922a2f6be9 eventpoll: fix missing wakeup for ovflist in ep_poll_callback 5d77631de15a epoll: atomically remove wait entry on wake up

Technically it broke things originally, but it smells bad to have several fixes in the tree for removed code that is still there - any thoughts?

Cheers, Sam

Reported-by: Jason Baron jbaron@akamai.com Reported-by: Randy Dunlap rdunlap@infradead.org Signed-off-by: Roman Penyaev rpenyaev@suse.de Signed-off-by: Andrew Morton akpm@linux-foundation.org Reviewed-by: Jason Baron jbaron@akamai.com Cc: Khazhismel Kumykov khazhy@google.com Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: stable@vger.kernel.org Link: http://lkml.kernel.org/r/20200505145609.1865152-1-rpenyaev@suse.de Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Rishabh Bhatnagar risbhat@amazon.com

---

fs/eventpoll.c | 47 +++++++++++++++++++++++++++-------------------- 1 file changed, 27 insertions(+), 20 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 7e11135bc915..e5496483a882 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1905,33 +1905,31 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, init_wait(&wait); wait.func = ep_autoremove_wake_function; write_lock_irq(&ep->lock);

• __add_wait_queue_exclusive(&ep->wq, &wait);
  

• write_unlock_irq(&ep->lock);
  

• /*

•  * We don't want to sleep if the ep_poll_callback() sends us
  

•  * a wakeup in between. That's why we set the task state
  

•  * to TASK_INTERRUPTIBLE before doing the checks.
  

•  * Barrierless variant, waitqueue_active() is called under
  

•  * the same lock on wakeup ep_poll_callback() side, so it
  

•  * is safe to avoid an explicit barrier.
  

  */

• set_current_state(TASK_INTERRUPTIBLE);
  

• __set_current_state(TASK_INTERRUPTIBLE);
  

• /*

•  * Always short-circuit for fatal signals to allow
  

•  * threads to make a timely exit without the chance of
  

•  * finding more events available and fetching
  

•  * repeatedly.
  

•  * Do the final check under the lock. ep_scan_ready_list()
  

•  * plays with two lists (->rdllist and ->ovflist) and there
  

•  * is always a race when both lists are empty for short
  

•  * period of time although events are pending, so lock is
  

•  * important.
  

  */

• if (fatal_signal_pending(current)) {
  

• 	res = -EINTR;
  

• 	break;
  

• eavail = ep_events_available(ep);
  

• if (!eavail) {
  

• 	if (signal_pending(current))
  

• 		res = -EINTR;
  

• 	else
  

• 		__add_wait_queue_exclusive(&ep->wq, &wait);
  

  }

• write_unlock_irq(&ep->lock);
  

• eavail = ep_events_available(ep);
  

• if (eavail)
  

• 	break;
  

• if (signal_pending(current)) {
  

• 	res = -EINTR;
  

• if (eavail || res)
  break;
  

• }
  

if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS)) { timed_out = 1; @@ -1952,6 +1950,15 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, } send_events:

• if (fatal_signal_pending(current)) {

• /*
  

•  * Always short-circuit for fatal signals to allow
  

•  * threads to make a timely exit without the chance of
  

•  * finding more events available and fetching
  

•  * repeatedly.
  

•  */
  

• res = -EINTR;
  

• } /*
  • Try to transfer events to user space. In case we get 0 events and
  • there's still timeout left over, we go trying again in search of

-- 2.37.1

On Thu, Nov 24, 2022 at 12:11:23AM +0000, Rishabh Bhatnagar wrote:

From: Soheil Hassas Yeganeh soheil@google.com

Commit 289caf5d8f6c61c6d2b7fd752a7f483cd153f182 upstream.

Patch series "simplify ep_poll".

This patch series is a followup based on the suggestions and feedback by Linus: https://lkml.kernel.org/r/CAHk-=wizk=OxUyQPbO8MS41w2Pag1kniUV5WdD5qWL-gq1kjD...

The first patch in the series is a fix for the epoll race in presence of timeouts, so that it can be cleanly backported to all affected stable kernels.

The rest of the patch series simplify the ep_poll() implementation. Some of these simplifications result in minor performance enhancements as well. We have kept these changes under self tests and internal benchmarks for a few days, and there are minor (1-2%) performance enhancements as a result.

This patch (of 8):

After abc610e01c66 ("fs/epoll: avoid barrier after an epoll_wait(2) timeout"), we break out of the ep_poll loop upon timeout, without checking whether there is any new events available. Prior to that patch-series we always called ep_events_available() after exiting the loop.

This can cause races and missed wakeups. For example, consider the following scenario reported by Guantao Liu:

Suppose we have an eventfd added using EPOLLET to an epollfd.

Thread 1: Sleeps for just below 5ms and then writes to an eventfd. Thread 2: Calls epoll_wait with a timeout of 5 ms. If it sees an event of the eventfd, it will write back on that fd. Thread 3: Calls epoll_wait with a negative timeout.

Prior to abc610e01c66, it is guaranteed that Thread 3 will wake up either by Thread 1 or Thread 2. After abc610e01c66, Thread 3 can be blocked indefinitely if Thread 2 sees a timeout right before the write to the eventfd by Thread 1. Thread 2 will be woken up from schedule_hrtimeout_range and, with evail 0, it will not call ep_send_events().

To fix this issue:

1. Simplify the timed_out case as suggested by Linus.
2. while holding the lock, recheck whether the thread was woken up after its time out has reached.

Note that (2) is different from Linus' original suggestion: It do not set "eavail = ep_events_available(ep)" to avoid unnecessary contention (when there are too many timed-out threads and a small number of events), as well as races mentioned in the discussion thread.

This is the first patch in the series so that the backport to stable releases is straightforward.

Link: https://lkml.kernel.org/r/20201106231635.3528496-1-soheil.kdev@gmail.com Link: https://lkml.kernel.org/r/CAHk-=wizk=OxUyQPbO8MS41w2Pag1kniUV5WdD5qWL-gq1kjD... Link: https://lkml.kernel.org/r/20201106231635.3528496-2-soheil.kdev@gmail.com Fixes: abc610e01c66 ("fs/epoll: avoid barrier after an epoll_wait(2) timeout") Signed-off-by: Soheil Hassas Yeganeh soheil@google.com Tested-by: Guantao Liu guantaol@google.com Suggested-by: Linus Torvalds torvalds@linux-foundation.org Reported-by: Guantao Liu guantaol@google.com Reviewed-by: Eric Dumazet edumazet@google.com Reviewed-by: Willem de Bruijn willemb@google.com Reviewed-by: Khazhismel Kumykov khazhy@google.com Reviewed-by: Davidlohr Bueso dbueso@suse.de Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Rishabh Bhatnagar risbhat@amazon.com

Acked-by: Thadeu Lima de Souza Cascardo cascardo@canonical.com

---

fs/eventpoll.c | 25 ++++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c index e5496483a882..877f9f61a4e8 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1928,23 +1928,30 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, } write_unlock_irq(&ep->lock);

• if (eavail || res)
  

• 	break;
  

• if (!eavail && !res)
  

• 	timed_out = !schedule_hrtimeout_range(to, slack,
  

• 					      HRTIMER_MODE_ABS);
  

• if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS)) {
  

• 	timed_out = 1;
  

• 	break;
  

• }
  

• /* We were woken up, thus go and try to harvest some events */
  

• /*
  

•  * We were woken up, thus go and try to harvest some events.
  

•  * If timed out and still on the wait queue, recheck eavail
  

•  * carefully under lock, below.
  

•  */
  

  eavail = 1;

• } while (0);

__set_current_state(TASK_RUNNING); if (!list_empty_careful(&wait.entry)) { write_lock_irq(&ep->lock);

• /*
  

•  * If the thread timed out and is not on the wait queue, it
  

•  * means that the thread was woken up after its timeout expired
  

•  * before it could reacquire the lock. Thus, when wait.entry is
  

•  * empty, it needs to harvest events.
  

•  */
  

• if (timed_out)
  

• 	eavail = list_empty(&wait.entry);
  

  __remove_wait_queue(&ep->wq, &wait); write_unlock_irq(&ep->lock); }

-- 2.37.1