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 22:49:26 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Zoran Markovic <zoran.markovic@...aro.org>
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.

On Tuesday, May 28, 2013 11:26:09 AM Zoran Markovic wrote:
> > 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.

Why exactly would it be racing?  We wouldn't call device_resume() for
the device that timed out (and its descendants).

> 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.

Well, its usefulness is quite limited, then.  That said I'm still not convinced
that this actually is the case.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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