Re: [linux-pm] Is it supposed to be ok to call del_gendisk whileuserspace is frozen?

From: Alan Stern
Date: Thu Mar 04 2010 - 14:36:16 EST

On Thu, 4 Mar 2010, Rafael J. Wysocki wrote:

> On Thursday 04 March 2010, Alan Stern wrote:
> > On Thu, 4 Mar 2010, Rafael J. Wysocki wrote:
> >
> > > > Very well. Then we still need a solution to the original problem:
> > > > Devices sometimes need to be unregistered during resume, but
> > > > del_gendisk() blocks on the writeback thread, which is frozen until
> > > > after the resume finishes. How do you suggest this be fixed?
> > >
> > > I thought about thawing the writeback thread earlier in such cases.
> > >
> > > Would that makes sense / is it doable at all?
> >
> > My thought exactly. This is the only approach that also solves the
> > following race:
> >
> > A driver is unloaded at the same time as a suspend starts.
> >
> > The writeback thread gets frozen.
> >
> > Then before the rmmod thread is frozen, it calls del_gendisk.
> >
> > Delaying things by means of a workqueue (or the equivalent) might also
> > work, but it doesn't seem as safe. For example, some important
> > writebacks might end up getting delayed until too late.
> OK, so what exactly should trigger the thawing of the writeback thread?
> Should we do that unconditionally at one point or wait for a specific situation
> to happen, and how to recognize that situation in the latter case?

My thought was that del_gendisk would do this unconditionally. It
would make the task unfreezable and then thaw it, to prevent a race.

This assumes that del_gendisk never gets called in the middle of a
sleep transition unless the device is really gone. If that's not the
case, Pavel's suggestion (make the resume routine do the device removal
asynchronously) might be better.

Alan Stern

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