netdev - Re: [patch V3 12/17] timers: Silently ignore timers with a NULL function

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Message-ID: <644695b9-f343-7fb7-ed8e-763e5fe3d158@linutronix.de>
Date:   Thu, 24 Nov 2022 08:37:27 +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,
        Jacob Keller <jacob.e.keller@...el.com>
Subject: Re: [patch V3 12/17] timers: Silently ignore timers with a NULL
 function

On Wed, 23 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 is 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 paths
> with checks for timer->function == NULL. If the pointer is NULLL, then

s/NULLL/NULL

> 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.
> 
> The warning of debug objects does warn if timer->function == NULL.  It

does NOT warn

> warns when timer was not initialized using timer_setup[_on_stack]() or via
> DEFINE_TIMER(). 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. Same for initializing a timer with a NULL function.
> 
> 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>
> Reviewed-by: Jacob Keller <jacob.e.keller@...el.com>
> 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)
> V3: Changelog and comment updates (Anna-Maria)
> ---
>  kernel/time/timer.c |   57 +++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 52 insertions(+), 5 deletions(-)
> 
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1128,8 +1144,12 @@ static inline int
>   * mod_timer_pending() is the same for pending timers as mod_timer(), but
>   * will not activate inactive timers.
>   *
> + * If @timer->function == NULL then the start operation is silently
> + * discarded.
> + *
>   * Return:
> - * * %0 - The timer was inactive and not modified
> + * * %0 - The timer was inactive and not modified or was is in
> + *	  shutdown state and the operation was discarded

You forgot to update this "was is" mistake. All other places are fine.

>   * * %1 - The timer was active and requeued to expire at @expires
>   */
>  int mod_timer_pending(struct timer_list *timer, unsigned long expires)

Thanks,

	Anna-Maria