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

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Date:	Fri, 1 May 2009 07:51:35 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
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 4/6] kill-the-BKL/reiserfs: release the write lock
	inside get_neighbors()


* 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.

No such assumption seems to be present here though.

	Ingo
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