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

From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Zoran Markovic <zoran.markovic@linaro.org>
Cc: Colin Cross <ccross@android.com>,
	lkml <linux-kernel@vger.kernel.org>,
	Linux PM list <linux-pm@vger.kernel.org>,
	Benoit Goby <benoit@android.com>,
	Android Kernel Team <kernel-team@android.com>,
	Todd Poynor <toddpoynor@google.com>, San Mehat <san@google.com>,
	John Stultz <john.stultz@linaro.org>, Pavel Machek <pavel@ucw.cz>,
	Len Brown <len.brown@intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [RFC PATCHv2 1/2] drivers: power: Add watchdog timer to catch drivers which lockup during suspend/resume.
Date: Tue, 28 May 2013 22:49:26 +0200	[thread overview]
Message-ID: <1515063.BZy7p4GtyV@vostro.rjw.lan> (raw)
In-Reply-To: <CAME+o4kJYD2K_aL4xNi-jNP1Q8R+seDNZDwsGBhvU00ArR0Skg@mail.gmail.com>

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.