Subject: Re: [RFC] ext3/jbd race: releasing in-use journal_heads
From: "Stephen C. Tweedie" <sct@redhat.com>
To: Andrew Morton <akpm@osdl.org>
Cc: "ext2-devel@lists.sourceforge.net" <ext2-devel@lists.sourceforge.net>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       Stephen Tweedie <sct@redhat.com>
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	 <20050304160451.4c33919c.akpm@osdl.org>
	 <1110213656.15117.193.camel@sisko.sctweedie.blueyonder.co.uk>
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Date: Mon, 07 Mar 2005 17:05:14 +0000
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Hi,

On Mon, 2005-03-07 at 16:40, Stephen C. Tweedie wrote:

> Andrew, can you remember why we ended up with both of those locks in the
> first place?  If we can do it, the efficient way out here is to abandon
> the journal_head_lock and use the bh_state_lock for both.  We already
> hold that over many of the key refile spots, and this would avoid the
> need to take yet another lock in those paths.

Actually, I realised there's an easier way: provide a variant of
__journal_unfile_buffer() which doesn't clear jh->b_transaction.

That's a subtle change in the logic, but everywhere that is calling
this, we already hold various locks --- except for that elusive
bh_journal_head lock.

But since everywhere else is using _other_ locks, the transient state
where we're on the transaction but not on a list won't be visible ---
we'll have refiled before we drop the lock.

I think things should work just fine with this change.
__journal_unfile_buffer() already handles the case where we're called
with b_transaction set but no b_jlist, for example.

--Stephen


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