linux-kernel - Re: [PATCH vfs.fixes] eventpoll: Prevent hang in epoll

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Message-ID: <CAKPOu+980gvzd-uXUARnYQ4V++08spfBVj26nZapExVF80ryYg@mail.gmail.com>
Date: Tue, 29 Apr 2025 21:28:50 +0200
From: Max Kellermann <max.kellermann@...os.com>
To: Joe Damato <jdamato@...tly.com>
Cc: linux-fsdevel@...r.kernel.org, Christian Brauner <brauner@...nel.org>, 
	Mike Pagano <mpagano@...too.org>, Carlos Llamas <cmllamas@...gle.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH vfs.fixes] eventpoll: Prevent hang in epoll_wait

On Tue, Apr 29, 2025 at 9:22 PM Joe Damato <jdamato@...tly.com> wrote:
> In commit 0a65bc27bd64 ("eventpoll: Set epoll timeout if it's in the
> future"), a bug was introduced causing the loop in ep_poll to hang under
> certain circumstances.
>
> When the timeout is non-NULL and ep_schedule_timeout returns false, the
> flag timed_out was not set to true. This causes a hang.
>
> Adjust the logic and set timed_out, if needed, fixing the original code.

Hi Joe,

we have been working on the fix at the same time, this is my fix:

 https://lore.kernel.org/linux-fsdevel/20250429185827.3564438-1-max.kellermann@ionos.com/T/#u

I think mine is better because it checks "eavail" before setting
"timed_out", preserving the old behavior (before commit 0a65bc27bd64).
Your version may set "timed_out" and thus does an unnecessary
list_empty() call in the following block. (And maybe it can reset
"evail" to false?)

Max