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

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Message-ID: <20090501135118.GH6011@nowhere>
Date:	Fri, 1 May 2009 15:51:19 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Jeff Mahoney <jeffm@...e.com>,
	ReiserFS Development List <reiserfs-devel@...r.kernel.org>,
	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 09:30:51AM -0400, Chris Mason wrote:
> On Fri, 2009-05-01 at 15:19 +0200, Frederic Weisbecker wrote:
> > 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.
> 
> > 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...
> 
> The blocksize should only be changing very early in the mount.  It is
> basically:
> 
> set the block size to something
> read the super block
> read the block size from the super block
> set the block size to the correct value
> use it from then on.
> 
> This should all be done with by the time we read bitmaps.
> 
> -chris


Ok, thanks.

So I guess we can keep it as is without a local variable.

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