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

From: Anna-Maria Behnsen <anna-maria@linutronix.de>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linuxfoundation.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Stephen Boyd <sboyd@kernel.org>,
	Guenter Roeck <linux@roeck-us.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Julia Lawall <Julia.Lawall@inria.fr>,
	Arnd Bergmann <arnd@arndb.de>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Marc Zyngier <maz@kernel.org>,
	Marcel Holtmann <marcel@holtmann.org>,
	Johan Hedberg <johan.hedberg@gmail.com>,
	Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
	linux-bluetooth@vger.kernel.org,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	netdev@vger.kernel.org, Jacob Keller <jacob.e.keller@intel.com>
Subject: Re: [patch V3 12/17] timers: Silently ignore timers with a NULL function
Date: Thu, 24 Nov 2022 08:37:27 +0100 (CET)	[thread overview]
Message-ID: <644695b9-f343-7fb7-ed8e-763e5fe3d158@linutronix.de> (raw)
In-Reply-To: <20221123201625.135055320@linutronix.de>

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@goodmis.org>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Tested-by: Guenter Roeck <linux@roeck-us.net>
> Reviewed-by: Jacob Keller <jacob.e.keller@intel.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