linux-kernel - Re: [PATCH v2] eventpoll: Fix priority inversion problem

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Message-ID: <20250528061252.AeDA23yH@linutronix.de>
Date: Wed, 28 May 2025 08:12:52 +0200
From: Nam Cao <namcao@...utronix.de>
To: Holger Hoffstätte <holger@...lied-asynchrony.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	John Ogness <john.ogness@...utronix.de>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
	linux-rt-users@...r.kernel.org, Joe Damato <jdamato@...tly.com>,
	Martin Karsten <mkarsten@...terloo.ca>,
	Jens Axboe <axboe@...nel.dk>,
	Frederic Weisbecker <frederic@...nel.org>,
	Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH v2] eventpoll: Fix priority inversion problem

On Wed, May 28, 2025 at 07:57:26AM +0200, Holger Hoffstätte wrote:
> I have been running with v2 on 6.15.0 without any issues so far, but just
> found this in my server's kern.log:

Thanks for testing!

> It seems the condition (!n) in __ep_remove is not always true and the WARN_ON triggers.
> This is the first and only time I've seen this. Currently rebuilding with v3.

Yeah this means __ep_remove() thinks the item is in epoll's rdllist and
attempt to remove it, but then couldn't actually find the item in the list.

__ep_remove() relies on the 'ready' flag, and this flags is quite
complicated. And as my colleague pointed out off-list, I got memory
ordering wrong for this flag. Therefore it is likely that you stepped on a
bug with this flag.

I got rid of this flag in v3, so hopefully the problem goes away.

Best regards,
Nam