linux-kernel - Re: [patch V2 12/17] timers: Silently ignore timers with a NULL function

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Message-ID: <1f8f4b0-f9fe-c9c6-7620-862deff6b9d8@linutronix.de>
Date:   Wed, 23 Nov 2022 11:39:00 +0100 (CET)
From:   Anna-Maria Behnsen <anna-maria@...utronix.de>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        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 V2 12/17] timers: Silently ignore timers with a NULL
 function

On Tue, 22 Nov 2022, Thomas Gleixner wrote:

> Tearing down timers which have circular dependencies to other
> functionality, e.g. workqueues, where the timer can schedule work and work
> can arm timers is not trivial.
> 
> In those cases it is desired to shutdown the timer in a way which prevents
> rearming of the timer. The mechanism to do so it to set timer->function to
> NULL and use this as an indicator for the timer arming functions to ignore
> the (re)arm request.
> 
> In preparation for that replace the warnings in the relevant code pathes
> with checks for timer->function == NULL and discard the rearm request
> silently.
> 
> Add debug_assert_init() instead of the WARN_ON_ONCE(!timer->function)
> checks so that debug objects can warn about non-initialized timers.
> 
> If developers fail to enable debug objects and then waste lots of time to
> figure out why their non-initialized timer is not firing, they deserve it.
> 
> Co-developed-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Tested-by: Guenter Roeck <linux@...ck-us.net>
> Link: https://lore.kernel.org/all/20220407161745.7d6754b3@gandalf.local.home
> Link: https://lore.kernel.org/all/20221110064101.429013735@goodmis.org
> ---
> V2: Use continue instead of return and amend the return value docs (Steven)
> ---
>  kernel/time/timer.c |   60 +++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 55 insertions(+), 5 deletions(-)
> 
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1202,6 +1230,9 @@ EXPORT_SYMBOL(timer_reduce);
>   *
>   * If @timer->expires is already in the past @timer will be queued to
>   * expire at the next timer tick.
> + *
> + * If @timer->function == NULL then the start operation is silently
> + * discarded.
>   */
>  void add_timer(struct timer_list *timer)
>  {

Could you move the new paragraph after the paragraph where is is mentioned,
that timer->function has to be set prior calling add_timer()?

Thanks,

       	Anna-Maria