[PATCH v2] xfs: re-enable xfsaild idle mode and fix associated races

[Brian Foster]

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