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

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Message-ID: <d14bcceddd9f59a72ef54afced204815e9dd092e.camel@xry111.site>
Date: Thu, 10 Jul 2025 14:54:06 +0800
From: Xi Ruoyao <xry111@...111.site>
To: Nam Cao <namcao@...utronix.de>
Cc: Christian Brauner <brauner@...nel.org>, Frederic Weisbecker	
 <frederic@...nel.org>, Valentin Schneider <vschneid@...hat.com>, Alexander
 Viro	 <viro@...iv.linux.org.uk>, 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>
Subject: Re: [PATCH v3] eventpoll: Fix priority inversion problem

On Thu, 2025-07-10 at 08:21 +0200, Nam Cao wrote:
> On Thu, Jul 10, 2025 at 11:08:18AM +0800, Xi Ruoyao wrote:
> > After upgrading my kernel to the recent mainline I've encountered some
> > stability issue, like:
> > 
> > - When GDM started gnome-shell, the screen often froze and the only
> > thing I could do was to switch into a VT and reboot.
> > - Sometimes gnome-shell started "fine" but then starting an application
> > (like gnome-console) needed to wait for about a minute.
> > - Sometimes the system shutdown process hangs waiting for a service to
> > stop.
> > - Rarely the system boot process hangs for no obvious reason.
> > 
> > Most strangely in all the cases there are nothing alarming in dmesg or
> > system journal.
> > 
> > I'm unsure if this is the culprit but I'm almost sure it's the trigger.
> > Maybe there's some race condition in my userspace that the priority
> > inversion had happened to hide...  but anyway reverting this patch
> > seemed to "fix" the issue.
> > 
> > Any thoughts or pointers to diagnose further?
> 
> I have been running this new epoll on my work machine for weeks by now
> without issue, while you seem to reproduce it reliably. I'm guessing that
> the problem is on some code path which is dead on my system, but executed
> on yours.

I also failed to reproduce it in a VM running the latest Fedora Rawhide
(in 3 attempts).

> I am curious if Gnome is using some epoll options which are unused on my
> system.

> I presume you can still access dmesg despite the freeze. Do you mind
> running the below patch, let me know what's in your dmesg? It may help
> identifying that code path.

Attached the system journal (dmesg was truncated due to too many lines).
I guess the relevant part should be between line 6947 ("New session 2 of
user xry111") and line 8022 ("start operation timed out. Terminating").

-- 
Xi Ruoyao <xry111@...111.site>

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