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

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Date:	Wed, 29 Oct 2008 10:04:14 +1100
From:	Nigel Cunningham <ncunningham@...a.org.au>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	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 again.

On Tue, 2008-10-28 at 21:25 +0100, Miklos Szeredi wrote:
> On Tue, 28 Oct 2008, Nigel Cunningham wrote:
> > The answer is to freeze the fuse filesystems first, stopping new
> > requests (freezing the processes making them) before they are passed on
> > to userspace and allowing existing requests to complete or freeze. Once
> > that is done, the userspace fuse processes will be idle, at least as far
> > as fuse activity is concerned. After fuse activity is stopped, userspace
> > can be frozen without worrying about what processes are fuse and what
> > aren't. This is what my patch implements so far.
> 
> OK.
> 
> > To deal with requests that are already in progress, I'd suggest three
> > possibilities, in the order I think they should be preferred.
> > 
> > 1) Use wait_event_freezeable[_timeout] in fuse code. Probably preferable
> > to #2, but I thought of it later :)
> > 
> > 2) Use freezer_do_not_count as part of FUSE_MIGHT_FREEZE (resetting
> > before exiting the callers, of course). If the request doesn't complete
> > during the freezing period, it must be because the userspace thread was
> > already frozen. If the request does complete, we're counted again during
> > the normal freezing of userspace that follows the freezing of
> > filesystems.
> > 
> > 3) Adding a means to check whether processes being frozen are fuse
> > requests. The code could then wait for fc->num_waiting - (say)
> > fc->num_frozen == 0.
> 
> Yup, these fix the freezing of tasks which have outstanding fuse
> requests.
> 
> However it does not fix the freezing of tasks which are waiting for
> VFS locks (i.e. inode->i_mutex) held by the outstanding fuse requests.
> This is the tricky part...

Okay. Looking back on our conversation brought me back to this message,
which I think needs another reply.

If we take the strategy of holding new requests and allowing existing
ones to complete, then am I right in thinking that we only need to worry
about where inode->i_mutex is taken in fs/fuse/file.c (I don't see it
taken in other fs/fuse/*.c files).

If that's correct, dealing with that issue looks relatively straight
forward: we need some more FUSE_MIGHT_FREEZE calls for those functions,
and something done to the vfs_check_frozen call - I'm a bit confused by
that - inode->i_sb will refer to the fuse filesystem, won't it?

Regards,

Nigel

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