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Re: [PATCH 11/19] xfs: Convert to new freezing code

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 11/19] xfs: Convert to new freezing code
From: Jan Kara <jack@xxxxxxx>
Date: Fri, 9 Mar 2012 09:23:06 +0100
Cc: Jan Kara <jack@xxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, Al Viro <viro@xxxxxxxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, dchinner@xxxxxxxxxx, sandeen@xxxxxxxxxx, Kamal Mostafa <kamal@xxxxxxxxxxxxx>, Ben Myers <bpm@xxxxxxx>, Alex Elder <elder@xxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20120308232041.GU3592@dastard>
References: <1330963277-26336-1-git-send-email-jack@xxxxxxx> <1330963277-26336-12-git-send-email-jack@xxxxxxx> <20120308232041.GU3592@dastard>
User-agent: Mutt/1.5.20 (2009-06-14)
On Fri 09-03-12 10:20:41, Dave Chinner wrote:
> On Mon, Mar 05, 2012 at 05:01:09PM +0100, Jan Kara wrote:
> > Generic code now blocks all writers from standard write paths. So we block 
> > all
> > writers coming from ioctl and replace blocking of transactions on frozen
> > filesystem with a debugging check. As a bonus, we get a protection of ioctl
> > against racing remount read-only. We also convert xfs_file_aio_write() to a
> > non-racy freeze protection.
> ....
> > diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> > index 329b06a..6468a2a 100644
> > --- a/fs/xfs/xfs_trans.c
> > +++ b/fs/xfs/xfs_trans.c
> > @@ -577,7 +577,6 @@ xfs_trans_alloc(
> >     xfs_mount_t     *mp,
> >     uint            type)
> >  {
> > -   xfs_wait_for_freeze(mp, SB_FREEZE_TRANS);
> >     return _xfs_trans_alloc(mp, type, KM_SLEEP);
> >  }
> 
> So what is there to stop internal XFS threads from starting
> transactions when the filesystem is frozen? Previously this
> SB_FREEZE_TRANS would guarantee even internal fucntions would get
> stopped, but now there's nothing?
> 
> I do beleive that ext4 has the same problem (the issue reported with
> the lazy inode init background thread), and I can see that any other
> filesystem that can make modifications via internal triggers will
> see the same problem - freeze doesn't block them any more...
  Yes, I'll have to audit filesystem internal threads. I forgot about these
in the first round. Most of them shouldn't have anything to do because the
filesystem is clean but there are exceptions as ext4 lazyinit has shown.

                                                                Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR

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