linux-kernel - Re: [PATCH v4 1/1] eventpoll: Replace rwlock with spinlock

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Message-ID: <20250903084012.A8dd-A5z@linutronix.de>
Date: Wed, 3 Sep 2025 10:40:12 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Nam Cao <namcao@...utronix.de>, Christian Brauner <brauner@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Xi Ruoyao <xry111@...111.site>,
	Frederic Weisbecker <frederic@...nel.org>,
	Valentin Schneider <vschneid@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
	John Ogness <john.ogness@...utronix.de>,
	K Prateek Nayak <kprateek.nayak@....com>,
	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>, stable@...r.kernel.org
Subject: Re: [PATCH v4 1/1] eventpoll: Replace rwlock with spinlock

On 2025-08-26 10:43:20 [+0200], Nam Cao wrote:
> On Tue, Jul 15, 2025 at 02:46:34PM +0200, Nam Cao wrote:
> > The ready event list of an epoll object is protected by read-write
> > semaphore:
> > 
> >   - The consumer (waiter) acquires the write lock and takes items.
> >   - the producer (waker) takes the read lock and adds items.
> > 
> > The point of this design is enabling epoll to scale well with large number
> > of producers, as multiple producers can hold the read lock at the same
> > time.
> > 
> > Unfortunately, this implementation may cause scheduling priority inversion
> > problem. Suppose the consumer has higher scheduling priority than the
> > producer. The consumer needs to acquire the write lock, but may be blocked
> > by the producer holding the read lock. Since read-write semaphore does not
> > support priority-boosting for the readers (even with CONFIG_PREEMPT_RT=y),
> > we have a case of priority inversion: a higher priority consumer is blocked
> > by a lower priority producer. This problem was reported in [1].
> > 
> > Furthermore, this could also cause stall problem, as described in [2].
> > 
> > Fix this problem by replacing rwlock with spinlock.
> 
> Hi Christian,
> 
> May I know your plan with this patch? Are you still waiting for something?
> 
> You may still understandably be paranoid about epoll due to the last
> regression. But it's been weeks, and this patch is quite simple, so I start
> to wonder if it is forgotten.

A friendly reminder.

> Nam

Sebastian