linux-kernel - Re: [RFC PATCH 2/2] Add timeout feature

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Date:	Wed, 2 Apr 2008 16:21:47 +1000
From:	David Chinner <dgc@....com>
To:	Takashi Sato <t-sato@...jp.nec.com>
Cc:	David Chinner <dgc@....com>, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, xfs@....sgi.com,
	dm-devel@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] Add timeout feature

On Tue, Apr 01, 2008 at 07:54:42PM +0900, Takashi Sato wrote:
> Hi,
> 
> David Chinner wrote:
> >The timeout is not for the freeze operation - the timeout is
> >only set up once the freeze is complete. i.e:
> >
> >$ time sudo ~/test_src/xfs_io -f -x -c 'gfreeze 10' /mnt/scratch/test
> >freezing with level = 10
> >
> >real    0m23.204s
> >user    0m0.008s
> >sys     0m0.012s
> >
> >The freeze takes 23s, and then the 10s timeout is started. So
> >this timeout does not protect against freeze_bdev() hangs at all.
> >All it does is introduce silent unfreezing of the block device that
> >can not be synchronised with the application that is operating
> >on the frozen device.
> 
> Exactly my timeout feature is only for an application, not for 
> freeze_bdev().
> I think it is needed for the situation we can't unfreeze from userspace.
> (e.g. Freezing the root filesystem)

Ummm - why can't you unfreeze the root fs from userspace? freezing
only prevents modification to the filesystem. A frozen filesystem is
effectively a read-only filesystem...

On XFS:

# xfs_freeze -f /
# echo $?
0
# xfs_freeze -u /
# echo $?
0

The underlying filesystem is broken w.r.t. freezing if you can't
read from it successfully once it's been frozen....

> >FWIW, resetting this timeout from userspace is unreliable - there's
> >no guarantee that under load your userspace process will get to run
> >again inside the timeout to reset it, hence leaving you with a
> >unfrozen filesystem when you really want it frozen...
> 
> The timeout period specified to the reset ioctl should be much larger than
> the interval for calling the reset ioctl repeatedly.
> (e.g timeout period = 2 minutes, calling interval = 5 seconds)

What application developer will ever use this?

> The reset ioctl will work under such setting.
> If a timeout still occurs before a reset, it would imply that an unexpected
> problem (e.g. deadlock) occur in an application.

Right - the application is broken and needs fixing.  We don't need
to supply a crutch in a "new" API to support hypothetically broken
applications that don't actually exist yet.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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