On Tue 29-04-25 20:58:27, Max Kellermann wrote:

After commit 0a65bc27bd64 ("eventpoll: Set epoll timeout if it's in the future"), the following program would immediately enter a busy loop in the kernel:

int main() {
  int e = epoll_create1(0);
  struct epoll_event event = {.events = EPOLLIN};
  epoll_ctl(e, EPOLL_CTL_ADD, 0, &event);
  const struct timespec timeout = {.tv_nsec = 1};
  epoll_pwait2(e, &event, 1, &timeout, 0);
}

This happens because the given (non-zero) timeout of 1 nanosecond usually expires before ep_poll() is entered and then ep_schedule_timeout() returns false, but `timed_out` is never set because the code line that sets it is skipped. This quickly turns into a soft lockup, RCU stalls and deadlocks, inflicting severe headaches to the whole system.

When the timeout has expired, we don't need to schedule a hrtimer, but we should set the `timed_out` variable. Therefore, I suggest moving the ep_schedule_timeout() check into the `timed_out` expression instead of skipping it.

Fixes: 0a65bc27bd64 ("eventpoll: Set epoll timeout if it's in the future") Cc: Joe Damato jdamato@fastly.com Cc: stable@vger.kernel.org Signed-off-by: Max Kellermann max.kellermann@ionos.com

I agree this makes the logic somewhat more obvious than Joe's fix so feel free to add:

Reviewed-by: Jan Kara jack@suse.cz

Thanks!

Honza

---

fs/eventpoll.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 4bc264b854c4..d4dbffdedd08 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -2111,9 +2111,10 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, write_unlock_irq(&ep->lock);

• if (!eavail && ep_schedule_timeout(to))
  

• 	timed_out = !schedule_hrtimeout_range(to, slack,
  

• 					      HRTIMER_MODE_ABS);
  

• if (!eavail)
  

• 	timed_out = !ep_schedule_timeout(to) ||
  

• 		!schedule_hrtimeout_range(to, slack,
  

• 					  HRTIMER_MODE_ABS);
  

  __set_current_state(TASK_RUNNING);

/* -- 2.47.2