linux-kernel - Re: [PATCH v3] eventpoll: Fix priority inversion problem

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Message-ID: <20250625160227.B9-NAdc2@linutronix.de>
Date: Wed, 25 Jun 2025 18:02:27 +0200
From: Nam Cao <namcao@...utronix.de>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	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 v3] eventpoll: Fix priority inversion problem

On Wed, Jun 25, 2025 at 05:57:18PM +0200, Nam Cao wrote:
> On Wed, Jun 25, 2025 at 05:33:54PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2025-06-25 17:27:02 [+0200], Nam Cao wrote:
> > > To be sure, I tried your suggestion. Systemd sometimes failed to boot, and
> > > my stress test crashed instantly.
> > 
> > I had a trace_printk() there while testing and it never triggered.
> 
> This code path is only executed for broken userspace.

Forgot to mention, my test crashed because the __llist_add(n, &txlist)
below doesn't care if 'n' is already in a list. By changing to from
llist_del_first() to llist_del_first_init(), it is possible for 'n' to be
in a list, therefore __llist_add() would break.

Nam