fs/eventpoll: fix endless busy loop after timeout has expired

After commit 0a65bc2 ("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.

brauner: Note that there was an earlier fix by Joe Damato in response to
my bug report in [1].

Fixes: 0a65bc2 ("eventpoll: Set epoll timeout if it's in the future")
Cc: Joe Damato <[email protected]>
Cc: [email protected]
Signed-off-by: Max Kellermann <[email protected]>
Link: https://lore.kernel.org/[email protected] [1]
Link: https://lore.kernel.org/[email protected]
Reviewed-by: Jan Kara <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: MaxKellermann

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);
 
 		/*