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

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 20, 2024 at 10:46:15AM -0800, 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>
> Reviewed-by: Juri Lelli <juri.lelli@redhat.com>
> Signed-off-by: John Stultz <jstultz@google.com>
> ---
> Apologies for being noisy, I recognize its only been a week, but
> I wanted to resend this now as the problematic commit just
> landed in Linus' tree and I've not seen this get queued yet.

Right, I picked up the old one a few days ago, but will not be sticking
it in any git tree until -rc1.