linux-kernel - Re: [PATCH] fs/eventpoll: fix endless busy loop after timeout has expired

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Message-ID: <diilyq37i35qlll7hu3si6dqrjntiif5gzajazkgtqvfsi4kgg@yr3v562urmjp>
Date: Wed, 30 Apr 2025 10:27:45 +0200
From: Jan Kara <jack@...e.cz>
To: Max Kellermann <max.kellermann@...os.com>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, Joe Damato <jdamato@...tly.com>, 
	stable@...r.kernel.org
Subject: Re: [PATCH] fs/eventpoll: fix endless busy loop after timeout has
 expired

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@...tly.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Max Kellermann <max.kellermann@...os.com>

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

Reviewed-by: Jan Kara <jack@...e.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
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR