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Date: Mon, 21 Nov 2022 17:18:03 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
Linus Torvalds <torvalds@...uxfoundation.org>,
Anna-Maria Behnsen <anna-maria@...utronix.de>,
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 12/15] timers: Add shutdown mechanism to the internal
functions
On Tue, 15 Nov 2022 21:28:52 +0100 (CET)
Thomas Gleixner <tglx@...utronix.de> 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.
>
> Add a shutdown argument to the relevant internal functions which makes the
> actual deactivation code set timer->function to NULL which in turn prevents
> rearming of the timer.
>
> Co-developed-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Link: https://lore.kernel.org/all/20220407161745.7d6754b3@gandalf.local.home
> Link: https://lore.kernel.org/all/20221110064101.429013735@goodmis.org
> ---
> kernel/time/timer.c | 64 ++++++++++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 55 insertions(+), 9 deletions(-)
>
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1293,14 +1293,21 @@ void add_timer_on(struct timer_list *tim
> EXPORT_SYMBOL_GPL(add_timer_on);
>
> /**
> - * __timer_delete - Internal function: Deactivate a timer.
> + * __timer_delete - Internal function: Deactivate a timer
> * @timer: The timer to be deactivated
> + * @shutdown: If true this indicates that the timer is about to be
Nit, needs a common (or "then") after true.
> + * shutdown permanently.
> + *
> + * If @shutdown is true then @timer->function is set to NULL under the
> + * timer base lock which prevents further rearming of the time. In that
> + * case any attempt to rearm @timer after this function returns will be
> + * silently ignored.
> *
> * Return:
> * * %0 - The timer was not pending
> * * %1 - The timer was pending and deactivated
> */
> -static int __timer_delete(struct timer_list *timer)
> +static int __timer_delete(struct timer_list *timer, bool shutdown)
> {
> struct timer_base *base;
> unsigned long flags;
> @@ -1308,9 +1315,22 @@ static int __timer_delete(struct timer_l
>
> debug_assert_init(timer);
>
> - if (timer_pending(timer)) {
> + /*
> + * If @shutdown is set then the lock has to be taken whether the
> + * timer is pending or not to protect against a concurrent rearm
> + * which might hit between the lockless pending check and the lock
> + * aquisition. By taking the lock it is ensured that such a newly
> + * enqueued timer is dequeued and cannot end up with
> + * timer->function == NULL in the expiry code.
> + *
> + * If timer->function is currently executed, then this makes sure
> + * that the callback cannot requeue the timer.
> + */
> + if (timer_pending(timer) || shutdown) {
> base = lock_timer_base(timer, &flags);
> ret = detach_if_pending(timer, base, true);
> + if (shutdown)
> + timer->function = NULL;
> raw_spin_unlock_irqrestore(&base->lock, flags);
> }
>
> @@ -1332,20 +1352,31 @@ static int __timer_delete(struct timer_l
> */
> int timer_delete(struct timer_list *timer)
> {
> - return __timer_delete(timer);
> + return __timer_delete(timer, false);
> }
> EXPORT_SYMBOL(timer_delete);
>
> /**
> * __try_to_del_timer_sync - Internal function: Try to deactivate a timer
> * @timer: Timer to deactivate
> + * @shutdown: If true this indicates that the timer is about to be
Same here.
> + * shutdown permanently.
> + *
> + * If @shutdown is true then @timer->function is set to NULL under the
> + * timer base lock which prevents further rearming of the timer. Any
> + * attempt to rearm @timer after this function returns will be silently
> + * ignored.
> + *
> + * This function cannot guarantee that the timer cannot be rearmed
> + * right after dropping the base lock if @shutdown is false. That
> + * needs to be prevented by the calling code if necessary.
> *
> * Return:
> * * %0 - The timer was not pending
> * * %1 - The timer was pending and deactivated
> * * %-1 - The timer callback function is running on a different CPU
> */
> -static int __try_to_del_timer_sync(struct timer_list *timer)
> +static int __try_to_del_timer_sync(struct timer_list *timer, bool shutdown)
> {
> struct timer_base *base;
> unsigned long flags;
> @@ -1357,6 +1388,8 @@ static int __try_to_del_timer_sync(struc
>
> if (base->running_timer != timer)
> ret = detach_if_pending(timer, base, true);
> + if (shutdown)
> + timer->function = NULL;
>
> raw_spin_unlock_irqrestore(&base->lock, flags);
>
> @@ -1379,7 +1412,7 @@ static int __try_to_del_timer_sync(struc
> */
> int try_to_del_timer_sync(struct timer_list *timer)
> {
> - return __try_to_del_timer_sync(timer);
> + return __try_to_del_timer_sync(timer, false);
> }
> EXPORT_SYMBOL(try_to_del_timer_sync);
>
> @@ -1460,12 +1493,25 @@ static inline void del_timer_wait_runnin
> * __timer_delete_sync - Internal function: Deactivate a timer and wait
> * for the handler to finish.
> * @timer: The timer to be deactivated
> + * @shutdown: If true @timer->function will be set to NULL under the
Here too.
-- Steve
> + * timer base lock which prevents rearming of @timer
> + *
> + * If @shutdown is not set the timer can be rearmed later. If the timer can
> + * be rearmed concurrently, i.e. after dropping the base lock then the
> + * return value is meaningless.
> + *
> + * If @shutdown is set then @timer->function is set to NULL under timer
> + * base lock which prevents rearming of the timer. Any attempt to rearm
> + * a shutdown timer is silently ignored.
> + *
> + * If the timer should be reused after shutdown it has to be initialized
> + * again.
> *
> * Return:
> * * %0 - The timer was not pending
> * * %1 - The timer was pending and deactivated
> */
> -static int __timer_delete_sync(struct timer_list *timer)
> +static int __timer_delete_sync(struct timer_list *timer, bool shutdown)
> {
> int ret;
>
> @@ -1495,7 +1541,7 @@ static int __timer_delete_sync(struct ti
> lockdep_assert_preemption_enabled();
>
> do {
> - ret = __try_to_del_timer_sync(timer);
> + ret = __try_to_del_timer_sync(timer, shutdown);
>
> if (unlikely(ret < 0)) {
> del_timer_wait_running(timer);
> @@ -1547,7 +1593,7 @@ static int __timer_delete_sync(struct ti
> */
> int timer_delete_sync(struct timer_list *timer)
> {
> - return __timer_delete_sync(timer);
> + return __timer_delete_sync(timer, false);
> }
> EXPORT_SYMBOL(timer_delete_sync);
>
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