linux-kernel - Re: [PATCH] panic: Reenable preemption in WARN slowpath

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Message-ID: <20230915093855.GB6743@noisy.programming.kicks-ass.net>
Date:   Fri, 15 Sep 2023 11:38:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] panic: Reenable preemption in WARN slowpath

On Fri, Sep 15, 2023 at 09:55:39AM +0200, Lukas Wunner wrote:
> Commit 5a5d7e9badd2 ("cpuidle: lib/bug: Disable rcu_is_watching() during
> WARN/BUG") amended warn_slowpath_fmt() to disable preemption until the
> WARN splat has been emitted.
> 
> However the commit neglected to reenable preemption in the !fmt codepath,
> i.e. when a WARN splat is emitted without additional format string.
> 
> One consequence is that users may see more splats than intended.  E.g. a
> WARN splat emitted in a work item results in at least two extra splats:
> 
>   BUG: workqueue leaked lock or atomic
>   (emitted by process_one_work())
> 
>   BUG: scheduling while atomic
>   (emitted by worker_thread() -> schedule())
> 
> Ironically the point of the commit was to *avoid* extra splats. ;)
> 
> Fix it.
> 
> Fixes: 5a5d7e9badd2 ("cpuidle: lib/bug: Disable rcu_is_watching() during WARN/BUG")
> Signed-off-by: Lukas Wunner <lukas@...ner.de>
> Cc: stable@...r.kernel.org # v6.1+
> Cc: Peter Zijlstra <peterz@...radead.org>
> ---
> The original commit went in through the tip tree, hence submitting to
> tip maintainers.  The commit was backported to v6.1-stable (even though
> it wasn't tagged for stable), hence this fix needs a stable designation.
> 
>  kernel/panic.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 07239d4..ffa037fa 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -697,6 +697,7 @@ void warn_slowpath_fmt(const char *file, int line, unsigned taint,
>  	if (!fmt) {
>  		__warn(file, line, __builtin_return_address(0), taint,
>  		       NULL, NULL);
> +		warn_rcu_exit(rcu);
>  		return;
>  	}

Urgh, thanks!