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. 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).
> Several solutions have been posted, none of which really solve the problem:
>
> a) Let's tag fuse server processes and freeze them later. This is
> basically impossible, because many processes could be interoperating
> and there's no way to know which is depending on which (example:
> sshfs uses ssh for communication, which possibly relies on openvpn
> process for packet transmission).
>
> b) While waiting for replies to fuse request, allow process to
> freeze. Does not fully solve the problem, as VFS might be holding
> locks, and other processes waiting for those locks will not be
> freezable.
I agree that these solutions won't work.
Regards,
Nigel
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