Re: freeze vs freezer

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Monday, 26 of November 2007, David Chinner wrote:
> On Sat, Nov 24, 2007 at 12:47:21AM +0100, Rafael J. Wysocki wrote:
> > On Thursday, 22 of November 2007, Jeremy Fitzhardinge wrote:
> > > It seems that a process blocked in a write to an xfs filesystem due to
> > > xfs_freeze cannot be frozen by the freezer.
> > 
> > The freezer doesn't handle tasks in TASK_UNINTERRUPTIBLE and I don't know how
> > to make it handle them without at least partially defeating its purpose.
> 
> So how do you handle threads that are blocked on I/O or a lock during
> the system freeze process, then?

We wait until they can continue.

> > > I see this if I suspend my laptop while doing something xfs-filesystem
> > > intensive, like a kernel build.  My suspend scripts freeze the XFS
> > > filesystem (as Dave said I should), which presumably blocks some writer,
> > > and then the freezer times out and fails to complete.
> > > 
> > > Here's part of the process dump the freezer does when it times out:
> > > 
> > > cc1           D 00000000     0 18138  18137
> > >        dd5f1e24 00200082 00000002 00000000 ecdeeb00 ecdeec64 c200f280 00000001 
> > >        009c09a0 dd5f1e0c dd5f1e0c 0000000f 00000000 00000000 00000000 dd5f1e74 
> > >        c7beb480 dd5f1e88 dd5f1ea8 c0228d97 e8889540 dd5f1e38 c015b75d dd5f1e44 
> > > Call Trace:
> > >  [<c0228d97>] xfs_write+0xf4/0x6d9
> > >  [<c0226038>] xfs_file_aio_write+0x53/0x5b
> > >  [<c0171c15>] do_sync_write+0xae/0xec
> > >  [<c0172343>] vfs_write+0xa4/0x120
> > >  [<c01728d7>] sys_write+0x3b/0x60
> > >  [<c0106fae>] sysenter_past_esp+0x6b/0xa1
> > >  =======================
> > > 
> > > 
> > > I haven't looked at how to fix this yet.  I only just worked out why I
> > > was getting suspend failures.
> > 
> > Well, you can add freezer_do_not_count()/freezer_count() annotations to
> > xfs_write() (and whatever else is blocked as a result of the XFS being frozen).
> 
> May as well annotate the whole VFS, then, because once the transaction
> subsystem is frozen any operation that modifies the filesystem will get
> blocked like this.

Well, I don't know how this mechanism actually works, so I can't comment.

Is there a mutex on which tasks block if the filesystem is frozen?

Greetings,
Rafael
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