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From: Nigel Cunningham <ncunningham@crca.org.au>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: rjw@sisk.pl, linux-pm@lists.linux-foundation.org,
	linux-kernel@vger.kernel.org
Subject: Re: [linux-pm] Freezer: Don't count threads waiting for frozen filesystems.
Date: Tue, 28 Oct 2008 07:59:28 +1100	[thread overview]
Message-ID: <1225141168.26724.23.camel@nigel-laptop> (raw)
In-Reply-To: <E1KuRN4-0002gS-Hq@pomaz-ex.szeredi.hu>

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


  reply	other threads:[~2008-10-27 20:59 UTC|newest]

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <1224886068.6478.21.camel@nigel-laptop>
2008-10-26 20:01 ` Rafael J. Wysocki
2008-10-27 11:12   ` Miklos Szeredi
2008-10-27 11:20     ` Nigel Cunningham
2008-10-27 11:37       ` Rafael J. Wysocki
2008-10-27 11:40         ` Nigel Cunningham
2008-10-27 12:38           ` Miklos Szeredi
2008-10-27 20:59             ` Nigel Cunningham [this message]
2008-10-27 21:09               ` Miklos Szeredi
2008-10-27 22:13                 ` Nigel Cunningham
2008-10-28 20:25                   ` Miklos Szeredi
2008-10-28 21:29                     ` Nigel Cunningham
2008-10-28 21:51                       ` Miklos Szeredi
2008-10-28 22:00                         ` Rafael J. Wysocki
2008-10-28 22:02                           ` Miklos Szeredi
2008-10-28 22:21                             ` Rafael J. Wysocki
2008-10-28 23:21                               ` Miklos Szeredi
2008-10-28 23:59                                 ` Rafael J. Wysocki
2008-10-29  8:10                                   ` Miklos Szeredi
2008-10-29 13:51                                     ` Alan Stern
2008-10-29 14:50                                       ` Miklos Szeredi
2008-10-29 15:28                                         ` Alan Stern
2008-10-29 15:50                                           ` Miklos Szeredi
2008-10-29 16:17                                             ` Alan Stern
2008-10-29 16:10                                           ` Miklos Szeredi
2008-10-29 20:37                                             ` Alan Stern
2008-10-29 21:11                                               ` Rafael J. Wysocki
2008-10-29 21:45                                                 ` Nigel Cunningham
2008-10-29 22:07                                                   ` Rafael J. Wysocki
2008-10-29 23:53                                                     ` Miklos Szeredi
2008-11-09 13:44                                                       ` Pavel Machek
2008-10-29 23:48                                                   ` Miklos Szeredi
2008-10-30 13:04                                                     ` Nigel Cunningham
2008-10-30 13:56                                                       ` Alan Stern
2008-10-30 21:44                                                         ` Nigel Cunningham
2008-10-31  8:49                                                           ` Miklos Szeredi
2008-10-31  9:10                                                             ` Nigel Cunningham
2008-10-31  9:16                                                               ` Miklos Szeredi
2008-10-31 11:28                                                                 ` Nigel Cunningham
2008-10-31 12:44                                                                   ` Miklos Szeredi
2008-10-31 21:11                                                                     ` Nigel Cunningham
2008-10-29 23:43                                               ` Miklos Szeredi
2008-10-30 13:54                                                 ` Alan Stern
2008-10-30 14:39                                                   ` Miklos Szeredi
2008-10-30 17:07                                                     ` Alan Stern
2008-10-30 17:43                                                       ` Miklos Szeredi
2008-10-30 20:17                                                     ` Rafael J. Wysocki
2008-11-15 16:58                                                     ` Pavel Machek
2008-10-29 23:56                                               ` Matthew Garrett
2008-10-28 22:03                         ` Nigel Cunningham
2008-10-28 23:04                     ` Nigel Cunningham
2008-10-28 23:12                       ` Miklos Szeredi
2008-10-28 23:17                         ` Nigel Cunningham
2008-10-28 23:24                           ` Miklos Szeredi
2008-10-28 23:41                             ` Nigel Cunningham
2008-10-28 23:45                               ` Miklos Szeredi
2008-10-28 23:50                                 ` Miklos Szeredi
2008-10-28 23:58                                   ` Nigel Cunningham
2008-10-28 23:54                                 ` Nigel Cunningham
2008-10-27 11:37       ` Miklos Szeredi

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