Re: Re: freeze_processes questions

Rafael J. Wysocki
2005-04-06T22:41:45Z

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Hi,

On Thursday, 7 of April 2005 00:07, Nigel Cunningham wrote:
> Hi.
>
> On Thu, 2005-04-07 at 07:35, Rafael J. Wysocki wrote:
> > On Wednesday, 6 of April 2005 23:17, Pavel Machek wrote:
> > > > > > I don't think Rafael is suggesting ignoring them. He's suggesting what
> > > > > > I'm already doing:
> > > > > > - Signal so they enter the freezer if they leave the state;
> > > > > > - Don't count them when deciding whether freezing failed;
> > > > > > - Handle the case where they don't leave the state until post resume (I
> > > > > > let them enter the refrigerator, but have code in there to check whether
> > > > > > the freezer is still on).
> > > > > >
> > > > > > In this way, I handle kseriod and anything else uninterruptible without
> > > > > > any problems.
> > > > >
> > > > > What happens if a process owns a lock needed to suspend a device and it is
> > > > > waiting in TASK_UNINTERRUPTIBLE?
> > > >
> > > > Well, we're in trouble. :-)
> > > >
> > > > However, if any process that we have frozen owns such a lock, we're in trouble
> > > > too.
> > >
> > > No, we are not. Processes can't own any locks when they are in
> > > refrigerator... It is not ok to call refrigerator from any context
> > > where you own a lock.
> >
> > I didn't mean a lock in general, but a lock that is needed to suspend a device.
> > If a process holding that lock is frozen, the _suspend() routine for the device
> > won't complete, because it won't be able to get the lock. IOW it will go to
> > sleep and we are single-threaded at that time ... Or am I missing anything?
>
> Whenever this topic comes up, the discussion is always abstract. If
> someone could think of a lock that might actually deadlock suspending,
> we look seriously at the issue.

Well, (having thought about it for a while) why, actually, a device's _suspend()
routine would need such a lock? Devices are suspended on one CPU in
a single thread which can only (potentially) race with interrupt handlers ...

> In the meantime, all I can say, from roughly three years of developing
> and testing, is that it never seems to happen.

Well, I have much less experience, but I don't think it's likely to happen,
too. :-)

Greets,
Rafael

--
- Would you tell me, please, which way I ought to go from here?
- That depends a good deal on where you want to get to.
-- Lewis Carroll "Alice's Adventures in Wonderland"