linux-kernel - Re: [PATCH 5/6] kill-the-BKL/reiserfs: release the write lock inside reiserfs_read_bitmap

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Date:	Fri, 1 May 2009 15:19:55 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	LKML <linux-kernel@...r.kernel.org>, Jeff Mahoney <jeffm@...e.com>,
	ReiserFS Development List <reiserfs-devel@...r.kernel.org>,
	Chris Mason <chris.mason@...cle.com>,
	Alexander Beregalov <a.beregalov@...il.com>,
	Alessio Igor Bogani <abogani@...ware.it>,
	Jonathan Corbet <corbet@....net>,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 5/6] kill-the-BKL/reiserfs: release the write lock
	inside reiserfs_read_bitmap_block()

On Fri, May 01, 2009 at 07:47:34AM +0200, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
> > reiserfs_read_bitmap_block() uses sb_bread() to read the bitmap block. This
> > helper might sleep.
> > 
> > Then, when the bkl was used, it was released at this point. We can then
> > relax the write lock too here.
> > 
> > [ Impact: release the reiserfs write lock when it is not needed ]
> > 
> > Cc: Jeff Mahoney <jeffm@...e.com>
> > Cc: Chris Mason <chris.mason@...cle.com>
> > Cc: Alexander Beregalov <a.beregalov@...il.com>
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > ---
> >  fs/reiserfs/bitmap.c |    2 ++
> >  1 files changed, 2 insertions(+), 0 deletions(-)
> > 
> > diff --git a/fs/reiserfs/bitmap.c b/fs/reiserfs/bitmap.c
> > index 1470334..6854957 100644
> > --- a/fs/reiserfs/bitmap.c
> > +++ b/fs/reiserfs/bitmap.c
> > @@ -1249,7 +1249,9 @@ struct buffer_head *reiserfs_read_bitmap_block(struct super_block *sb,
> >  	else if (bitmap == 0)
> >  		block = (REISERFS_DISK_OFFSET_IN_BYTES >> sb->s_blocksize_bits) + 1;
> >  
> > +	reiserfs_write_unlock(sb);
> >  	bh = sb_bread(sb, block);
> > +	reiserfs_write_lock(sb);
> >  	if (bh == NULL)
> >  		reiserfs_warning(sb, "sh-2029: %s: bitmap block (#%u) "
> >  		                 "reading failed", __func__, block);
> 
> Note, there's a side-effect here: the access to sb->b_blocksize is 
> moved outside of the lock. Previously it was accessed via the BKL. 
> On a mounted filesystem sb->b_blocksize is not supposed to change, 
> so it's probably not an issue - but wanted to mention it.
> 
> 	Ingo


Indeed. Well I guess it can't be dynamically changed.
This is something that can be chosen with mkfs on filesystem creation
but I guess it can't be changed, at least not while the filesystem
is mounted. I hope...

Frederic.

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