linux-kernel - Re: [tip: core/urgent] panic: Reenable preemption in WARN slowpath

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Date:   Fri, 15 Sep 2023 11:45:02 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     linux-tip-commits@...r.kernel.org, Lukas Wunner <lukas@...ner.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [tip: core/urgent] panic: Reenable preemption in WARN slowpath


* tip-bot2 for Lukas Wunner <tip-bot2@...utronix.de> wrote:

> The following commit has been merged into the core/urgent branch of tip:
> 
> Commit-ID:     cccd32816506cbac3a4c65d9dff51b3125ef1a03
> Gitweb:        https://git.kernel.org/tip/cccd32816506cbac3a4c65d9dff51b3125ef1a03
> Author:        Lukas Wunner <lukas@...ner.de>
> AuthorDate:    Fri, 15 Sep 2023 09:55:39 +02:00
> Committer:     Ingo Molnar <mingo@...nel.org>
> CommitterDate: Fri, 15 Sep 2023 11:28:08 +02:00
> 
> panic: Reenable preemption in WARN slowpath
> 
> 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.

> diff --git a/kernel/panic.c b/kernel/panic.c
> index 07239d4..ffa037f 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;

BTW., one more thing we might want to consider here is to re-enable 
preemption in warn_rcu_exit() a bit more gently, without forcing a
pending reschedule, ie. preempt_enable_no_resched() or so?

Thanks,

	Ingo