linux-kernel - Re: [PATCH] pstore: Revert pmsg

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Message-ID: <20230302163603.223313ba@gandalf.local.home>
Date:   Thu, 2 Mar 2023 16:36:03 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     John Stultz <jstultz@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Wei Wang <wvw@...gle.com>,
        Midas Chien <midaschieh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Anton Vorontsov <anton@...msg.org>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Tony Luck <tony.luck@...el.com>, kernel-team@...roid.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH] pstore: Revert pmsg_lock back to a normal mutex

On Thu, 2 Mar 2023 16:32:53 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

>    CPU1					CPU2
>    ----					----
> rt_mutex_slowlock_block() {
>   raw_spin_lock_irq(wait_lock);
>   owner = rt_mutex_owner();
>   raw_spin_unlock_irq(wait_lock);
>
>   rtmutex_spin_on_owner(owner) {

I just noticed there's an rcu_read_lock() here around the loop.
I'm guessing that's to keep this race from happening.
Would have been nice to have a comment there stating such.

-- Steve

>     owner = rt_mutex_owner();
> 
>     [ task preempted! (could also be a long interrupt) ]
> 
> 				   owner releases lock and exits
> 				   owner is freed
> 
>     [ task resumes ]
> 
>     if (!owner_on_cpu(owner)
> 
>       READ_ONCE(owner->on_cpu)
>      *** BOOM invalid pointer dereference ***