netdev - Re: [patch 05/15] timers: Replace BUG

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Date:   Mon, 21 Nov 2022 15:18:50 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Julia Lawall <Julia.Lawall@...ia.fr>,
        Arnd Bergmann <arnd@...db.de>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Marc Zyngier <maz@...nel.org>,
        Marcel Holtmann <marcel@...tmann.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        linux-bluetooth@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: Re: [patch 05/15] timers: Replace BUG_ON()s

On Tue, 15 Nov 2022 21:28:41 +0100 (CET)
Thomas Gleixner <tglx@...utronix.de> wrote:

> The timer code still has a few BUG_ON()s left which are crashing the kernel
> in situations where it still can recover or simply refuse to take an
> action.
> 
> Remove the one in the hotplug callback which checks for the CPU being
> offline. If that happens then the whole hotplug machinery will explode in
> colourful ways.
> 
> Replace the rest with WARN_ON_ONCE() and conditional returns where
> appropriate.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  kernel/time/timer.c |   11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1193,7 +1193,8 @@ EXPORT_SYMBOL(timer_reduce);
>   */
>  void add_timer(struct timer_list *timer)
>  {
> -	BUG_ON(timer_pending(timer));
> +	if (WARN_ON_ONCE(timer_pending(timer)))
> +		return;
>  	__mod_timer(timer, timer->expires, MOD_TIMER_NOTPENDING);
>  }
>  EXPORT_SYMBOL(add_timer);
> @@ -1210,7 +1211,8 @@ void add_timer_on(struct timer_list *tim
>  	struct timer_base *new_base, *base;
>  	unsigned long flags;
>  
> -	BUG_ON(timer_pending(timer) || !timer->function);
> +	if (WARN_ON_ONCE(timer_pending(timer) || !timer->function))
> +		return;
>  
>  	new_base = get_timer_cpu_base(timer->flags, cpu);
>  

I wonder if this patch should be split up into two patches, as the above is
trivial and the below is a bit more "complex". Although it's probably moot
as none of these should ever trigger.

> @@ -2017,8 +2019,6 @@ int timers_dead_cpu(unsigned int cpu)
>  	struct timer_base *new_base;
>  	int b, i;
>  
> -	BUG_ON(cpu_online(cpu));
> -
>  	for (b = 0; b < NR_BASES; b++) {
>  		old_base = per_cpu_ptr(&timer_bases[b], cpu);
>  		new_base = get_cpu_ptr(&timer_bases[b]);
> @@ -2035,7 +2035,8 @@ int timers_dead_cpu(unsigned int cpu)
>  		 */
>  		forward_timer_base(new_base);
>  
> -		BUG_ON(old_base->running_timer);
> +		WARN_ON_ONCE(old_base->running_timer);
> +		old_base->running_timer = NULL;

I guess interesting things could happen if running_timer was not NULL, but
again, WARN_ON_ONCE() should never be triggered in a correctly running
kernel.

Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>

-- Steve

>  
>  		for (i = 0; i < WHEEL_SIZE; i++)
>  			migrate_timer_list(new_base, old_base->vectors + i);