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From:   John Stultz <jstultz@google.com>
Date:   Wed, 8 Mar 2023 12:04:23 -0800
Message-ID: <CANDhNCpw+M_bwLpxb-vdL__xZrQsAVCMBozVt3+v4ncUVKbfAA@mail.gmail.com>
Subject: Re: [PATCH] pstore: Revert pmsg_lock back to a normal mutex
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     LKML <linux-kernel@vger.kernel.org>, Wei Wang <wvw@google.com>,
        Midas Chien <midaschieh@google.com>,
        Kees Cook <keescook@chromium.org>,
        Anton Vorontsov <anton@enomsg.org>,
        "Guilherme G. Piccoli" <gpiccoli@igalia.com>,
        Tony Luck <tony.luck@intel.com>, kernel-team@android.com,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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On Tue, Mar 7, 2023 at 5:31=E2=80=AFPM Steven Rostedt <rostedt@goodmis.org>=
 wrote:
>
> On Thu, 2 Mar 2023 20:01:36 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > @@ -1421,11 +1425,23 @@ static bool rtmutex_spin_on_owner(struct rt_mut=
ex_base *lock,
> >                *    for CONFIG_PREEMPT_RCU=3Dy)
> >                *  - the VCPU on which owner runs is preempted
> >                */
> > -             if (!owner_on_cpu(owner) || need_resched() ||
> > -                 !rt_mutex_waiter_is_top_waiter(lock, waiter)) {
> > +             if (!owner_on_cpu(owner) || need_resched()) {
> >                       res =3D false;
> >                       break;
> >               }
> > +             top_waiter =3D rt_mutex_top_waiter(lock);
>
> rt_mutex_top_waiter() can not be called outside the wait_lock, as it may
> trigger that BUG_ON() you saw.
>
> New patch below.

Hey Steven!
  Thanks for the new version! It avoids the crash issue. However, with
my sef-created reproducer, I was still seeing similar regression going
between mutex to rtmutex.

I'm interested in continuing to see if we can further tweak it, but
I've got some other work I need to focus on, so I think I'm going to
advocate for the revert in the short-term and look at finer grained
locking (along with rtmutex to address the priority inversion issue)
in the longer term.

thanks
-john