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From: Guenter Roeck <linux@roeck-us.net>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linuxfoundation.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Anna-Maria Behnsen <anna-maria@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Stephen Boyd <sboyd@kernel.org>,
	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
Subject: Re: [patch 00/15] timers: Provide timer_shutdown[_sync]()
Date: Thu, 17 Nov 2022 06:10:39 -0800	[thread overview]
Message-ID: <20221117141039.GA664755@roeck-us.net> (raw)
In-Reply-To: <20221115195802.415956561@linutronix.de>

On Tue, Nov 15, 2022 at 09:28:32PM +0100, Thomas Gleixner wrote:
> Tearing down timers can be tedious when there are circular dependencies to
> other things which need to be torn down. A prime example is timer and
> workqueue where the timer schedules work and the work arms the timer.
> 
> Steven and the Google Chromebook team ran into such an issue in the
> Bluetooth HCI code.
> 
> Steven suggested to create a new function del_timer_free() which marks the
> timer as shutdown. Rearm attempts of shutdown timers are discarded and he
> wanted to emit a warning for that case:
> 
>    https://lore.kernel.org/all/20220407161745.7d6754b3@gandalf.local.home
> 
> This resulted in a lengthy discussion and suggestions how this should be
> implemented. The patch series went through several iterations and during
> the review of the last version it turned out that this approach is
> suboptimal:
> 
>    https://lore.kernel.org/all/20221110064101.429013735@goodmis.org
> 
> The warning is not really helpful because it's entirely unclear how it
> should be acted upon. The only way to address such a case is to add 'if
> (in_shutdown)' conditionals all over the place. This is error prone and in
> most cases of teardown like the HCI one which started this discussion not
> required all.
> 
> What needs to prevented is that pending work which is drained via
> destroy_workqueue() does not rearm the previously shutdown timer. Nothing
> in that shutdown sequence relies on the timer being functional.
> 
> The conclusion was that the semantics of timer_shutdown_sync() should be:
> 
>     - timer is not enqueued
>     - timer callback is not running
>     - timer cannot be rearmed
> 
> Preventing the rearming of shutdown timers is done by discarding rearm
> attempts silently.
> 
> As Steven is short of cycles, I made some spare cycles available and
> reworked the patch series to follow the new semantics and plugged the races
> which were discovered during review.
> 
> The patches have been split up into small pieces to make review easier and
> I took the liberty to throw a bunch of overdue cleanups into the picture
> instead of proliferating the existing state further.
> 
> The last patch in the series addresses the HCI teardown issue for real.
> 

I applied the series to the top of v6.1-rc5, and also applied the result of
running the coccinelle script to auto-convert simple cases. Running this
set of patches through my testbed showed no build errors, runtime
failures, or warnings. I also backported the series to chromeos-5.15,
again applied the coccinelle generated patches, and ran it through a
regression test. No failures either.

With that, for the series,

Tested-by: Guenter Roeck <linux@roeck-us.net>

Let me know if I should send individual tags for each patch in the series.

Thanks,
Guenter

  parent reply	other threads:[~2022-11-17 14:10 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-11-15 20:28 [patch 00/15] timers: Provide timer_shutdown[_sync]() Thomas Gleixner
2022-11-15 20:28 ` [patch 01/15] ARM: spear: Do not use timer namespace for timer_shutdown() function Thomas Gleixner
2022-11-15 21:09   ` timers: Provide timer_shutdown[_sync]() bluez.test.bot
2022-11-18  4:19   ` bluez.test.bot
2022-11-18  4:38   ` bluez.test.bot
2022-11-15 20:28 ` [patch 02/15] clocksource/drivers/arm_arch_timer: Do not use timer namespace for timer_shutdown() function Thomas Gleixner
2022-11-15 20:28 ` [patch 03/15] clocksource/drivers/sp804: " Thomas Gleixner
2022-11-15 20:28 ` [patch 04/15] timers: Get rid of del_singleshot_timer_sync() Thomas Gleixner
2022-11-21 20:08   ` Steven Rostedt
2022-11-15 20:28 ` [patch 05/15] timers: Replace BUG_ON()s Thomas Gleixner
2022-11-21 20:18   ` Steven Rostedt
2022-11-15 20:28 ` [patch 06/15] timers: Update kernel-doc for various functions Thomas Gleixner
2022-11-21 20:43   ` Steven Rostedt
2022-11-22  0:09     ` Randy Dunlap
2022-11-22  0:21       ` Steven Rostedt
2022-11-22 15:18     ` Thomas Gleixner
2022-11-22 15:38       ` Steven Rostedt
2022-11-22 15:41       ` Steven Rostedt
2022-11-22 16:42         ` Thomas Gleixner
2022-11-15 20:28 ` [patch 07/15] timers: Use del_timer_sync() even on UP Thomas Gleixner
2022-11-15 20:28 ` [patch 08/15] timers: Rename del_timer_sync() to timer_delete_sync() Thomas Gleixner
2022-11-21 20:52   ` Steven Rostedt
2022-11-15 20:28 ` [patch 09/15] timers: Rename del_timer() to timer_delete() Thomas Gleixner
2022-11-21 21:08   ` Steven Rostedt
2022-11-15 20:28 ` [patch 10/15] timers: Silently ignore timers with a NULL function Thomas Gleixner
2022-11-21 21:35   ` Steven Rostedt
2022-11-21 21:46     ` Thomas Gleixner
2022-11-15 20:28 ` [patch 11/15] timers: Split [try_to_]del_timer[_sync]() to prepare for shutdown mode Thomas Gleixner
2022-11-15 20:28 ` [patch 12/15] timers: Add shutdown mechanism to the internal functions Thomas Gleixner
2022-11-21 22:18   ` Steven Rostedt
2022-11-15 20:28 ` [patch 13/15] timers: Provide timer_shutdown[_sync]() Thomas Gleixner
2022-11-21 22:21   ` Steven Rostedt
2022-11-15 20:28 ` [patch 14/15] timers: Update the documentation to reflect on the new timer_shutdown() API Thomas Gleixner
2022-11-15 20:28 ` [patch 15/15] Bluetooth: hci_qca: Fix the teardown problem for real Thomas Gleixner
2022-11-15 21:29   ` Luiz Augusto von Dentz
2022-11-17 14:10 ` Guenter Roeck [this message]
2022-11-21 15:15 ` [patch 00/15] timers: Provide timer_shutdown[_sync]() Thomas Gleixner
2022-11-21 15:26   ` Steven Rostedt
2022-11-22  2:38 ` Steven Rostedt

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