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

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Date:	Thu, 9 Feb 2012 00:47:03 +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 00:27:05, Jan Kara wrote:
> 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.
  Attached is a fixed version.

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

View attachment "0001-vfs-Provide-function-to-get-superblock-and-wait-for-.patch" of type "text/x-patch" (3555 bytes)