Re: [PATCH] Preserve task state in reentrant calls to ___wait_event

Rafael J. Wysocki
2015-11-06T23:06:46Z

On Friday, November 06, 2015 08:44:08 PM Chris Bainbridge wrote:
> In the ACPI SBS initialisation, a reentrant call to wait_event_timeout()
> causes an intermittent boot stall of several minutes usually following
> the "Switching to clocksource tsc" message. This stall is caused by:
>
> 1. drivers/acpi/sbshc.c wait_transaction_complete() calls
> wait_event_timeout():
>
> if (wait_event_timeout(hc->wait, smb_check_done(hc),
> msecs_to_jiffies(timeout)))
>
> 2. ___wait_event sets task state to uninterruptible
>
> 3. ___wait_event calls the "condition" smb_check_done()
>
> 4. smb_check_done (sbshc.c) calls through to ec_read() in
> drivers/acpi/ec.c
>
> 5. ec_guard() is reached which calls wait_event_timeout()
>
> if (wait_event_timeout(ec->wait,
> ec_transaction_completed(ec),
> guard))
>
> ie. wait_event_timeout() is being called again inside evaluation of
> the previous wait_event_timeout() condition

Hmm.

This doesn't look quite right to me.

> 5. The EC IRQ handler calls wake_up() and wakes up the sleeping task in
> ec_guard()
>
> 6. The task is now in state running even though the wait "condition" is
> still being evaluated
>
> 7. The "condition" check returns false so ___wait_event calls
> schedule_timeout()
>
> 8. Since the task state is running, the scheduler immediately schedules
> it again
>
> 9. This loop repeats for around 250 seconds event though the original
> wait_event_timeout was only 1000ms.
>
> This happens because each the call to schedule_timeout() usually
> returns immediately, taking less than 1ms, so the jiffies timeout
> counter is not decremented. The task is now stuck in a running
> state, and so is highly likely to get rescheduled immediately, which
> takes less than a jiffy.
>
> The root problem is that wait_event_timeout() does not preserve the task
> state when called by tasks that are not running. We fix this by
> preserving and restoring the task state in ___wait_event().
>
> Signed-off-by: Chris Bainbridge <chris.bainbridge@gmail.com>
> ---
> I am assuming here that wait_event_timeout() is supposed to support reentrant
> calls. If not, perhaps it should BUG_ON when called with a non-running task
> state, and the SBS HC / ACPI EC code needs to be fixed to stop doing this.

OK, so I'm not sure about that too. Peter?

I'd fix the SBS HC / ACPI EC code to stop doing this regardless.

> If reentrant calls are supposed to work, then this patch will preserve the task
> state (there may be a more appropriate way to support reentrant calls than this
> exact patch, suggestions/alternatives are welcome, but this does work).

Thanks,
Rafael