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@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>
Closes: https://lore.kernel.org/lkml/6afb936f-17c7-43fa-90e0-b9e780866097@app.fastmail.com/
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: John Stultz <jstultz@google.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);
-- 
2.47.0.338.g60cca15819-goog

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@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>
> Closes: https://lore.kernel.org/lkml/6afb936f-17c7-43fa-90e0-b9e780866097@app.fastmail.com/
> Tested-by: Anders Roxell <anders.roxell@linaro.org>
> Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
> Signed-off-by: John Stultz <jstultz@google.com>
> ---

LGTM.

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

Best,
Juri