Re: [RFC][PATCH] locking: rtmutex: Fix wake_q logic in task_blocks_on_rt_mutex

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello John,

On 11/14/2024 3:22 AM, 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.
> 
> Anders: Can you double check this resolves the issue for you?
> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Joel Fernandes <joelaf@google.com>
> Cc: Qais Yousef <qyousef@layalina.io>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Benjamin Segall <bsegall@google.com>
> Cc: Zimuzo Ezeozue <zezeozue@google.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Will Deacon <will@kernel.org>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Metin Kaya <Metin.Kaya@arm.com>
> Cc: Xuewen Yan <xuewen.yan94@gmail.com>
> Cc: K Prateek Nayak <kprateek.nayak@amd.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
> Cc: kernel-team@android.com
> Cc: Davidlohr Bueso <dave@stgolabs.net>
> Cc: regressions@lists.linux.dev
> Cc: Thorsten Leemhuis <linux@leemhuis.info>
> Cc: Anders Roxell <anders.roxell@linaro.org>
> Fixes: 894d1b3db41c ("locking/mutex: Remove wakeups from under mutex::wait_lock")
> Reported-by: Anders Roxell <anders.roxell@linaro.org>
> Reported-by: Arnd Bergmann <arnd@arndb.de>
> Link: https://lore.kernel.org/lkml/6afb936f-17c7-43fa-90e0-b9e780866097@app.fastmail.com/
> Signed-off-by: John Stultz <jstultz@google.com>

I've been running rtmutex_lock torture test in addition to a few
standard micro-benchmarks with the fix on my system on top of
tip:sched/core and I haven't encountered any splats there. Feel free to
add:

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>

-- 
Thanks and Regards,
Prateek

> ---
>   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);