linux-kernel - Re: [PATCH] locking: rtmutex: Fix wake_q logic in task_blocks_on_rt

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Message-ID: <Zzc5N_3jxOZ3ng3m@jlelli-thinkpadt14gen4.remote.csb>
Date: Fri, 15 Nov 2024 12:06:15 +0000
From: Juri Lelli <juri.lelli@...hat.com>
To: John Stultz <jstultz@...gle.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Joel Fernandes <joelaf@...gle.com>,
	Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Valentin Schneider <vschneid@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Benjamin Segall <bsegall@...gle.com>,
	Zimuzo Ezeozue <zezeozue@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Metin Kaya <Metin.Kaya@....com>,
	Xuewen Yan <xuewen.yan94@...il.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>, kernel-team@...roid.com,
	Davidlohr Bueso <dave@...olabs.net>, regressions@...ts.linux.dev,
	Thorsten Leemhuis <linux@...mhuis.info>,
	Anders Roxell <anders.roxell@...aro.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] locking: rtmutex: Fix wake_q logic in
 task_blocks_on_rt_mutex

Hi John,

On 14/11/24 11:00, John Stultz wrote:
> Anders had bisected a crash using PREEMPT_RT with linux-next and
> isolated it down to commit 894d1b3db41c ("locking/mutex: Remove
> wakeups from under mutex::wait_lock"), where it seemed the
> wake_q structure was somehow getting corrupted causing a null
> pointer traversal.
> 
> I was able to easily repoduce this with PREEMPT_RT and managed
> to isolate down that through various call stacks we were
> actually calling wake_up_q() twice on the same wake_q.
> 
> I found that in the problematic commit, I had added the
> wake_up_q() call in task_blocks_on_rt_mutex() around
> __ww_mutex_add_waiter(), following a similar pattern in
> __mutex_lock_common().
> 
> However, its just wrong. We haven't dropped the lock->wait_lock,
> so its contrary to the point of the original patch. And it
> didn't match the __mutex_lock_common() logic of re-initializing
> the wake_q after calling it midway in the stack.
> 
> Looking at it now, the wake_up_q() call is incorrect and should
> just be removed. So drop the erronious logic I had added.
> 
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Joel Fernandes <joelaf@...gle.com>
> Cc: Qais Yousef <qyousef@...alina.io>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Valentin Schneider <vschneid@...hat.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Benjamin Segall <bsegall@...gle.com>
> Cc: Zimuzo Ezeozue <zezeozue@...gle.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Will Deacon <will@...nel.org>
> Cc: Waiman Long <longman@...hat.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: "Paul E. McKenney" <paulmck@...nel.org>
> Cc: Metin Kaya <Metin.Kaya@....com>
> Cc: Xuewen Yan <xuewen.yan94@...il.com>
> Cc: K Prateek Nayak <kprateek.nayak@....com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Cc: kernel-team@...roid.com
> Cc: Davidlohr Bueso <dave@...olabs.net>
> Cc: regressions@...ts.linux.dev
> Cc: Thorsten Leemhuis <linux@...mhuis.info>
> Cc: Anders Roxell <anders.roxell@...aro.org>
> Fixes: 894d1b3db41c ("locking/mutex: Remove wakeups from under mutex::wait_lock")
> Reported-by: Anders Roxell <anders.roxell@...aro.org>
> Reported-by: Arnd Bergmann <arnd@...db.de>
> Closes: https://lore.kernel.org/lkml/6afb936f-17c7-43fa-90e0-b9e780866097@app.fastmail.com/
> Tested-by: Anders Roxell <anders.roxell@...aro.org>
> Tested-by: K Prateek Nayak <kprateek.nayak@....com>
> Signed-off-by: John Stultz <jstultz@...gle.com>
> ---

LGTM.

Reviewed-by: Juri Lelli <juri.lelli@...hat.com>

Best,
Juri