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Message-ID: <3779da12-6da5-8f6b-ec93-f8d52e38a40@linutronix.de>
Date: Wed, 23 Nov 2022 13:02:27 +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
Subject: Re: [patch V2 15/17] timers: Provide timer_shutdown[_sync]()
On Tue, 22 Nov 2022, Thomas Gleixner wrote:
> @@ -1605,6 +1629,48 @@ int timer_delete_sync(struct timer_list
> }
> EXPORT_SYMBOL(timer_delete_sync);
>
> +/**
> + * 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.
> + *
> + * 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.
NIT... Maybe the first requires could be replaced by
assumes/expects/presupposes to prevent double use of required?
Thanks,
Anna-Maria
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