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@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Max Kellermann <max.kellermann@xxxxxxxxx>
---
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
