linux-kernel - Re: [linux-pm] Freezer: Don't count threads waiting for frozen filesystems.

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Date:	Mon, 27 Oct 2008 22:20:27 +1100
From:	Nigel Cunningham <ncunningham@...a.org.au>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	rjw@...k.pl, linux-pm@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [linux-pm] Freezer: Don't count threads waiting for frozen
	filesystems.

Hi Miklos.

On Mon, 2008-10-27 at 12:12 +0100, Miklos Szeredi wrote:
> On Sun, 26 Oct 2008, Rafael J. Wysocki wrote:
> > On Saturday, 25 of October 2008, Nigel Cunningham wrote:
> > > While working on freezing fuse filesystems, I found that if a filesystem
> > > is frozen when we try to freeze processes, freezing can fail because
> > > threads are waiting in vfs_check_frozen for the filesystem to be thawed.
> > > We should thus not count such threads.
> > > 
> > > The check will be safe if a filesystem is thawed while we're freezing
> > > processes because filesystem thaws are only invoked from userspace. Any
> > > waiting processes will be woken and frozen prior to us completing the
> > > freezing of userspace (the caller invoking the filesystem thaw will be
> > > freezing) or - in the worst case - together with kernel threads.
> 
> The description is missing some details: why is the filesystem frozen
> before suspend?  AFAICS this can happen when DM calls bdev_freeze() on
> the device before the task freezing begins.  Is this the case?

It doesn't matter why a process is sitting in that wait_event call. What
does matter is that one can be there. In the case where I saw it, I was
working on fuse freezing. I don't remember the details, as it's a year
since I made this patch, but I don't think I wasn't using fuse or DM.

> Also, while the patch might solve some of the symptoms of the fuse
> vs. process freezer interaction, it will not fully fix that problem.
> As such it's just a hack to hide the problem, making it less likely to
> appear.

No, it's part of the solution. I haven't posted the full fuse freezing
patch because I thought this could be profitably merged without the rest
of the patch.

Regards,

Nigel

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