linux-kernel - Re: [PATCH 4/6] kill-the-BKL/reiserfs: release the write lock inside get

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Date:	Fri, 01 May 2009 09:29:03 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Frederic Weisbecker <fweisbec@...il.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 4/6] kill-the-BKL/reiserfs: release the write lock
 inside get_neighbors()

On Fri, 2009-05-01 at 15:25 +0200, Frederic Weisbecker wrote:
> On Fri, May 01, 2009 at 07:51:35AM +0200, Ingo Molnar wrote:
> > 
> > * Frederic Weisbecker <fweisbec@...il.com> wrote:
> > 
> > > get_neighbors() is used to get the left and/or right blocks 
> > > against a given one in order to balance a tree.
> > > 
> > > sb_bread() is used to read the buffer of these neighors blocks and 
> > > while it waits for this operation, it might sleep.
> > > 
> > > The bkl was released at this point, and then we can also release 
> > > the write lock before calling sb_bread().
> > > 
> > > This is safe because if the filesystem is changed after this lock 
> > > release, the function returns REPEAT_SEARCH (aka SCHEDULE_OCCURRED 
> > > in the function header comments) in order to repeat the neighbhor 
> > > research.
> > > 
> > > [ Impact: release the reiserfs write lock when it is not needed ]
> > 
> > This should also be safe because under the BKL we _already_ dropped 
> > the lock when sb_bread() blocked (which it really would in the 
> > normal case).
> > 
> > There's one special case to consider though: sb_read() maps to 
> > __bread() which can return without sleeping if the bh is already 
> > uptodate. So if the filesystem _knows_ that the bh is already 
> > uptodate and holds a reference to it (this is common pattern in 
> > filesystems), it can have a locking assumption on that.
> > 

sb_bread calls __bread which calls __getblk which always calls
might_sleep() before returning.

So, the unlock isn't adding a schedule that wasn't there before.

-chris


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