linux-kernel - Re: [PATCH 1/2] vfs: Provide function to get superblock and wait for it to thaw

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Message-ID: <20120208232705.GF1696@quack.suse.cz>
Date:	Thu, 9 Feb 2012 00:27:05 +0100
From:	Jan Kara <jack@...e.cz>
To:	Dave Chinner <david@...morbit.com>
Cc:	Jan Kara <jack@...e.cz>, Al Viro <viro@...IV.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	mpatocka@...hat.com, sandeen@...deen.net
Subject: Re: [PATCH 1/2] vfs: Provide function to get superblock and wait
 for it to thaw

On Thu 09-02-12 10:20:49, Dave Chinner wrote:
> On Tue, Feb 07, 2012 at 11:37:46PM +0100, Jan Kara wrote:
> > In quota code we need to find a superblock corresponding to a device and wait
> > for superblock to be unfrozen. However this waiting has to happen without
> > s_umount semaphore because that is required for superblock to thaw. So provide
> > a function in VFS for this to keep dances with s_umount where they belong.
> > 
> > Signed-off-by: Jan Kara <jack@...e.cz>
> > ---
> >  fs/super.c         |   47 +++++++++++++++++++++++++++++++++++------------
> >  include/linux/fs.h |    1 +
> >  2 files changed, 36 insertions(+), 12 deletions(-)
> > 
> > diff --git a/fs/super.c b/fs/super.c
> > index 6015c02..e320239 100644
> > --- a/fs/super.c
> > +++ b/fs/super.c
> > @@ -593,15 +593,7 @@ void iterate_supers_type(struct file_system_type *type,
> >  
> >  EXPORT_SYMBOL(iterate_supers_type);
> >  
> > -/**
> > - *	get_super - get the superblock of a device
> > - *	@bdev: device to get the superblock for
> > - *	
> > - *	Scans the superblock list and finds the superblock of the file system
> > - *	mounted on the device given. %NULL is returned if no match is found.
> > - */
> > -
> > -struct super_block *get_super(struct block_device *bdev)
> > +static struct super_block *__get_super(struct block_device *bdev, bool thawed)
> >  {
> >  	struct super_block *sb;
> >  
> > @@ -618,9 +610,13 @@ rescan:
> >  			spin_unlock(&sb_lock);
> >  			down_read(&sb->s_umount);
> >  			/* still alive? */
> > -			if (sb->s_root && (sb->s_flags & MS_BORN))
> > -				return sb;
> > -			up_read(&sb->s_umount);
> > +			if (sb->s_root && (sb->s_flags & MS_BORN)) {
> > +				if (sb->s_frozen == SB_UNFROZEN)
> > +					return sb;
> > +				up_read(&sb->s_umount);
> > +				vfs_check_frozen(sb, SB_FREEZE_WRITE);
> > +			} else
> > +				up_read(&sb->s_umount);
> >  			/* nope, got unmounted */
> >  			spin_lock(&sb_lock);
> >  			__put_super(sb);
> > @@ -631,9 +627,36 @@ rescan:
> >  	return NULL;
> >  }
> 
> The thawed parameter appears to be unused and the waiting for frozen
> filesystems appears to happen for all callers. Is this intentional?
  Ah, sorry. Obviously that's a bug. Thanks for spotting it. I'll fix it
up.

								Honza

-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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