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SubjectRe: [RFC PATCHv2 1/2] drivers: power: Add watchdog timer to catch drivers which lockup during suspend/resume.
On Wed, 5 Jun 2013, Zoran Markovic wrote:

> > It does block in my environment, AFAICS. Looking a bit further in the
> > code, it looks like dpm_suspend() does an async_synchronize_full()
> > which would wait for all async tasks to complete. This is a
> > show-stopper because (under the circumstances) the assumption that
> > every async suspend routine eventually completes doesn't hold.
> >
> > We could possibly select which async tasks to wait for, but this would
> > add unnecessary complexity to a feature targeted for debugging. It
> > seems that this approach - although sounding reasonable - needs to
> > wait until we have a mechanism to cancel an async task.
> Looks like the implementation of proposal for an async suspend +
> wait_for_completion_timeout is quite complex due to above limitations.
> How do we proceed from here? We have the following options:
> 1. Give up on the idea of having a suspend/resume watchdog.
> 2. Use the timer implementation (with possible modifications).
> 3. Wait for the implementation of (or implement) killing of an already
> running async work.
> Are there any other ideas floating around?

In general, the kernel is not designed to operate when kernel threads
get killed at random times. It's also not designed to operate normally
while in the middle of a system suspend.

This means there is basically no hope of recovering from a hung async
suspend task. (In much the same way, there is no hope of recovering
from any hung kernel thread.) The best you can accomplish is to store
some useful information somewhere and either panic or force a reboot.

Given that the usual storage media may be inaccessible, it may not be
easy to find a place to store the information.

(By the way, what do you do if a _synchronous_ suspend routine hangs?
The two problems are fairly similar.)

Alan Stern

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