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

[John Stultz]

On Wed, Nov 13, 2024 at 10:37 PM K Prateek Nayak <kprateek.nayak@xxxxxxx> wrote:
> 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.
> >
...
>
> 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@xxxxxxx>
>

Thank you so much for testing! I really appreciate it!
I'll resend with the provided tags and without the RFC here soon.
-john