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

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Message-ID: <CADYN=9+mRq1Z3nR42-CR8CkmS1Oq456_vfKey0QnOg9nrfZuHA@mail.gmail.com>
Date: Wed, 13 Nov 2024 23:49:33 +0100
From: Anders Roxell <anders.roxell@...aro.org>
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>, 
	Juri Lelli <juri.lelli@...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>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC][PATCH] locking: rtmutex: Fix wake_q logic in task_blocks_on_rt_mutex

On Wed, 13 Nov 2024 at 22:53, John Stultz <jstultz@...gle.com> 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.
>
> Anders: Can you double check this resolves the issue for you?

Thank you John for looking into the issue
It booted just fine on the rockpi4 now, ontop of tag next-2024111.

Tested-by: Anders Roxell <anders.roxell@...aro.org>

Cheers,
Anders

>
> 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>
> Link: https://lore.kernel.org/lkml/6afb936f-17c7-43fa-90e0-b9e780866097@app.fastmail.com/
> Signed-off-by: John Stultz <jstultz@...gle.com>
> ---
>  kernel/locking/rtmutex.c | 3 ---
>  1 file changed, 3 deletions(-)
>
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index c7de80ee1f9d..a01e81179df0 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -1248,10 +1248,7 @@ static int __sched task_blocks_on_rt_mutex(struct rt_mutex_base *lock,
>
>                 /* Check whether the waiter should back out immediately */
>                 rtm = container_of(lock, struct rt_mutex, rtmutex);
> -               preempt_disable();
>                 res = __ww_mutex_add_waiter(waiter, rtm, ww_ctx, wake_q);
> -               wake_up_q(wake_q);
> -               preempt_enable();
>                 if (res) {
>                         raw_spin_lock(&task->pi_lock);
>                         rt_mutex_dequeue(lock, waiter);
> --
> 2.47.0.277.g8800431eea-goog
>