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

[Brian Foster <bfoster@xxxxxxxxxx>]

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