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Date: Wed, 20 Jun 2012 11:59:58 -0400
From: Brian Foster <bfoster@redhat.com>
MIME-Version: 1.0
Subject: Re: [PATCH v2] xfs: re-enable xfsaild idle mode and fix associated
	races
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	<20120620080523.GA26167@infradead.org>
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To: Christoph Hellwig <hch@infradead.org>
Cc: xfs@oss.sgi.com

On 06/20/2012 04:05 AM, Christoph Hellwig wrote:
> On Thu, Jun 07, 2012 at 12:49:53PM -0400, Brian Foster wrote:
[snip]
>>  	spin_lock(&ailp->xa_lock);
>> +
>> +	/* barrier matches the xa_target update in xfs_ail_push() */
>> +	smp_rmb();
>> +	target = ailp->xa_target;
>> +	ailp->xa_target_prev = target;
>> +
>>  	lip = xfs_trans_ail_cursor_first(ailp, &cur, ailp->xa_last_pushed_lsn);
>>  	if (!lip) {
>>  		/*
>> +
>> +		spin_lock(&ailp->xa_lock);
>> +
>> +		/*
>> +		 * Idle if the AIL is empty and we are not racing with a target
>> +		 * update. The barrier matches the xa_target update in
>> +		 * xfs_ail_push().
>> +		 */
>> +		smp_rmb();
> 
> Given that both sides are under xa_lock I can't see any need for
> barriers here.
> 

Actually now that I look at it, it appears xfs_trans_ail_copy_lsn() does
not necessarily acquire the xa_lock...

Brian

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