Hi Andrew,

May I ask you to remove "epoll: ensure ep_poll() doesn't miss wakeup events" from your -mm queue? Jason lately found out that the patch does not fully solve the problem and this one patch is a second attempt to do things correctly in a different way (namely to do the final check under the lock). Previous changes are not needed.

Thanks.

-- Roman

On 2020-05-05 10:40, Roman Penyaev wrote:

The original problem was described here: https://lkml.org/lkml/2020/4/27/1121

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 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.

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").

Signed-off-by: Roman Penyaev rpenyaev@suse.de Reported-by: Jason Baron jbaron@akamai.com Cc: Andrew Morton akpm@linux-foundation.org Cc: Khazhismel Kumykov khazhy@google.com Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: linux-fsdevel@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: stable@vger.kernel.org

---

fs/eventpoll.c | 48 ++++++++++++++++++++++++++++-------------------- 1 file changed, 28 insertions(+), 20 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c index aba03ee749f8..8453e5403283 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1879,34 +1879,33 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, * event delivery. */ init_wait(&wait);

• write_lock_irq(&ep->lock);
  

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

• write_unlock_irq(&ep->lock);
  
  

• write_lock_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;

@@ -1927,6 +1926,15 @@ 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

On 5/5/20 4:40 AM, Roman Penyaev wrote:

The original problem was described here: https://lkml.org/lkml/2020/4/27/1121

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 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.

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").

Signed-off-by: Roman Penyaev rpenyaev@suse.de Reported-by: Jason Baron jbaron@akamai.com Cc: Andrew Morton akpm@linux-foundation.org Cc: Khazhismel Kumykov khazhy@google.com Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: linux-fsdevel@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: stable@vger.kernel.org

---

fs/eventpoll.c | 48 ++++++++++++++++++++++++++++-------------------- 1 file changed, 28 insertions(+), 20 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c index aba03ee749f8..8453e5403283 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1879,34 +1879,33 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, * event delivery. */ init_wait(&wait);

• write_lock_irq(&ep->lock);
  

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

• write_unlock_irq(&ep->lock);
  

• write_lock_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; @@ -1927,6 +1926,15 @@ 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

Hi Roman,

Looks good feel free to add: Reviewed-by: Jason Baron jbaron@akamai.com

I think we should also add the fixes tag to assist stable backports: Fixes: 339ddb53d373 ("fs/epoll: remove unnecessary wakeups of nested epoll")

Thanks,

-Jason