Re: [patch 00/15] timers: Provide timer_shutdown[_sync]()

[Guenter Roeck]

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@xxxxxxxxxxxxxxxxxx
> 
> 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@xxxxxxxxxxx
> 
> 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@xxxxxxxxxxxx>

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

Thanks,
Guenter