Re: [patch 13/15] timers: Provide timer_shutdown[_sync]()

[Steven Rostedt]

On Tue, 15 Nov 2022 21:28:54 +0100 (CET)
Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:

> +/**
> + * timer_shutdown_sync - Shutdown a timer and prevent rearming
> + * @timer: The timer to be shutdown
> + *
> + * When the function returns it is guaranteed that:
> + *   - @timer is not queued
> + *   - The callback function of @timer is not running
> + *   - @timer cannot be enqueued again. Any attempt to rearm
> + *     @timer is silently ignored.
> + *
> + * See timer_delete_sync() for synchronization rules.

 "See timer_delete_sync() for synchronization and context rules."

As where it can be executed is as important as the synchronization that is
needed.

-- Steve


> + *
> + * This function is useful for final teardown of an infrastructure where
> + * the timer is subject to a circular dependency problem.
> + *
> + * A common pattern for this is a timer and a workqueue where the timer can
> + * schedule work and work can arm the timer. On shutdown the workqueue must
> + * be destroyed and the timer must be prevented from rearming. Unless the
> + * code has conditionals like 'if (mything->in_shutdown)' to prevent that
> + * there is no way to get this correct with timer_delete_sync().
> + *
> + * timer_shutdown_sync() is solving the problem. The correct ordering of
> + * calls in this case is:
> + *
> + *	timer_shutdown_sync(&mything->timer);
> + *	workqueue_destroy(&mything->workqueue);
> + *
> + * After this 'mything' can be safely freed.
> + *
> + * This obviously requires that the timer is not required to be functional
> + * for the rest of the shutdown operation.
> + *
> + * Return:
> + * * %0 - The timer was not pending
> + * * %1 - The timer was pending
> + */
> +int timer_shutdown_sync(struct timer_list *timer)
> +{
> +	return __timer_delete_sync(timer, true);
> +}
> +EXPORT_SYMBOL_GPL(timer_shutdown_sync);
> +