linux-kernel - Re: [RFC PATCHv2 1/2] drivers: power: Add watchdog timer to catch drivers which lockup during suspend/resume.

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Date:	Tue, 28 May 2013 11:26:09 -0700
From:	Zoran Markovic <zoran.markovic@...aro.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Colin Cross <ccross@...roid.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Benoit Goby <benoit@...roid.com>,
	Android Kernel Team <kernel-team@...roid.com>,
	Todd Poynor <toddpoynor@...gle.com>,
	San Mehat <san@...gle.com>,
	John Stultz <john.stultz@...aro.org>,
	Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [RFC PATCHv2 1/2] drivers: power: Add watchdog timer to catch
 drivers which lockup during suspend/resume.

> What about this:
>  - Add one more list_head to struct dev_pm_info.
>  - Make dpm_prepare() create a new list for the next steps instead of moving
>    devices out of dpm_list.
>  - Start an async work to carry out dpm_suspend() and make the main thread
>    do wait_for_completion_timeout() for every device in dpm_list (in the
>    reverse order).
>  - If it times out, mark the device in question as unusable, possibly resume
>    the already suspended devices (except for descendants of the failed one)
>    and abort the suspend.  Return a specific error code to user space so that
>    it knows what happened.  [You can make this step configurable to BUG()
>    instead of doing all those things if you think that will be more useful for
>    platforms you care about.]
>  - Disable future suspends.
> And analogously for resume.
>
> That should allow people to investigate what happened on a system that
> (hopefully) is not completely dead and you still can have your "reboot if
> suspend hangs" feature if you like.

I looked into implementing this. The problem that I encountered is
that there is no reliable way of canceling an async task, and hence
the asynchronous __device_suspend() would be left racing with a
recovery from a suspend timeout. We could do cancel_work_sync() as a
recovery, but that call blocks until the running async task is
flushed, which might never happen. So doing a panic() is pretty much
the only option for recovering.
- Zoran
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