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

[Anders Roxell]

On Wed, 13 Nov 2024 at 22:53, John Stultz <jstultz@xxxxxxxxxx> 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@xxxxxxxxxx>

Cheers,
Anders

>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
> Cc: Qais Yousef <qyousef@xxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Benjamin Segall <bsegall@xxxxxxxxxx>
> Cc: Zimuzo Ezeozue <zezeozue@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Waiman Long <longman@xxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Metin Kaya <Metin.Kaya@xxxxxxx>
> Cc: Xuewen Yan <xuewen.yan94@xxxxxxxxx>
> Cc: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> Cc: kernel-team@xxxxxxxxxxx
> Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> Cc: regressions@xxxxxxxxxxxxxxx
> Cc: Thorsten Leemhuis <linux@xxxxxxxxxxxxx>
> Cc: Anders Roxell <anders.roxell@xxxxxxxxxx>
> Fixes: 894d1b3db41c ("locking/mutex: Remove wakeups from under mutex::wait_lock")
> Reported-by: Anders Roxell <anders.roxell@xxxxxxxxxx>
> Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
> Link: https://lore.kernel.org/lkml/6afb936f-17c7-43fa-90e0-b9e780866097@xxxxxxxxxxxxxxxx/
> Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
> ---
>  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
>