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

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Date:   Fri, 15 Sep 2023 12:35:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org, 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>
Subject: Re: [tip: core/urgent] panic: Reenable preemption in WARN slowpath

On Fri, Sep 15, 2023 at 11:45:02AM +0200, Ingo Molnar wrote:
> 
> * 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?

nah, it's a warn, if that triggers you get to keep the pieces. Also
preempt_enable_no_resched() isn't exported because its a horribly
dangerous function.