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

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Message-ID: <20250711122123.qXVK-EkF@linutronix.de>
Date: Fri, 11 Jul 2025 14:21:23 +0200
From: Nam Cao <namcao@...utronix.de>
To: Xi Ruoyao <xry111@...111.site>
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 Fri, Jul 11, 2025 at 08:09:12PM +0800, Xi Ruoyao wrote:
> On Fri, 2025-07-11 at 11:58 +0200, Nam Cao wrote:
> > On Fri, Jul 11, 2025 at 05:48:56PM +0800, Xi Ruoyao wrote:
> > > Sadly, still no luck.
> > 
> > That's unfortunate.
> > 
> > I'm still unable to reproduce the issue, so all I can do is staring at the
> > code and guessing. But I'm out of idea for now.
> 
> Same as I.  I tried to reproduce in a VM running Fedora Rawhide but
> failed.
> 
> > This one is going to be hard to figure out..
> 
> And I'm afraid this may be a bug in my userspace...

Yeah I expect the same thing. In the log you sent me, there are some
strange-looking event masks that look like a userspace bug.

I tried those strange looking event masks, but epoll_ctl() correctly
rejects them with EINVAL. So those masks probably aren't the reason, but
they still suggest that userspace may also be broken somewhere else and
triggers hard-to-reproduce problems in kernel's epoll.

> Then I'd feel guilty if this is reverted because of an invalid bug report
> from I :(.

But even when userspace is broken, if the kernel patch makes userspace
non-functional, I would still call it a kernel bug. So it's a valid bug
report alright, don't worry about it.

Until we know exactly the root cause, I think revert is the right thing to
do.

Best regards,
Nam