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

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Date:	Mon, 27 Oct 2008 22:09:15 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	ncunningham@...a.org.au
CC:	miklos@...redi.hu, 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.

On Tue, 28 Oct 2008, Nigel Cunningham wrote:
> Hi.
> 
> On Mon, 2008-10-27 at 13:38 +0100, Miklos Szeredi wrote:
> > On Mon, 27 Oct 2008, Nigel Cunningham wrote:
> > > Hi.
> > > 
> > > On Mon, 2008-10-27 at 12:37 +0100, Rafael J. Wysocki wrote:
> > > > Well, I guess it's better if you post the entire thing so that we can see
> > > > what the role of the $subject patch is in it, even if this patch finally gets
> > > > merged separately.
> > > 
> > > Ah.. that makes me see how vfs_check_frozen was getting triggered...
> > > (fs/namei.c, below).
> > 
> > Nigel, thanks for the patch, it makes thinks a lot clearer.
> > 
> > >From a quick look through the patch it seems to solve a bunch of cases
> > where new fuse requests during the freezing could cause problems.  But
> > it doesn't do anything with requests that are already under way when
> > the freezing starts, which would still result in all the same
> > problems.
> > 
> > Take this scenario:
> > 
> >  1) process A does rename("/mnt/fuse/a", "/mnt/fuse/b")
> >  2) request goes to process B serving the fuse filesystem
> >  3) filesystems are frozen, no new fuse requests
> >  4) processes are frozen, let's say B first, then A
> >  5) freezing A will fail, since it's still waiting for the request to finish
> 
> I'll have a look at the code again. I deliberately didn't stop existing
> requests, but perhaps that's the wrong behaviour.
> 
> In the above scenario, process B won't see process A's new request until
> post-resume if the filesystem is already frozen, so steps 4 & 5 don't
> happen.

No, I mean the filesystem is only frozen at 3 not before, so B is
already processing the request by the time the filesystem gets frozen.

> Process B will also always be frozen before process A if process
> A is userspace (most likely in the above scenario) or was mounted after
> process B. (I've had this patch distributed as is for almost a year,
> with no problems at all, so I believe I'm right here).

Both A and B are userspace processes.  The order of freezing userspace
processes is basically random, there's no way to make sure that
processes doing work on behalf of a fuse filesystem (B) are frozen
after processes accessing the fuse filesystem (A).

Miklos
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