Re: [patch] scsi: use lock per host instead of per device for shared queue tag host

[Jens Axboe]

On Thu, Jan 25 2007, Jens Axboe wrote:
> On Wed, Jan 24 2007, Ed Lin wrote:
> > 
> > 
> > > -----Original Message-----
> > > From: David Somayajulu [mailto:david.somayajulu@xxxxxxxxxx] 
> > > Sent: Wednesday, January 24, 2007 5:03 PM
> > > To: Ed Lin; Michael Reed
> > > Cc: linux-scsi; linux-kernel; james.Bottomley; jeff; 
> > > Promise_Linux; Jens Axboe
> > > Subject: RE: [patch] scsi: use lock per host instead of per 
> > > device for shared queue tag host
> > > 
> > > 
> > > > It seems another driver(qla4xxx) is also using shared queue tag.
> > > > It is natural to imagine there might be same symptom in that
> > > > driver. But I don't know the driver and have no hardware so I
> > > > can not say anything certain about it.
> > > 
> > > qla4xxx implements slightly differently, in the sense we 
> > > don't have the
> > > equivalent of         
> > > struct st_ccb ccb[MU_MAX_REQUEST]; 
> > > which is in struct st_hba. In other words we don't have a local array
> > > which like stex to keep track of the outstanding commands to the hba.
> > > 
> > > We had a discussion on this one while implementing block-layer tagging
> > > in qla4xxx and Jens Axboe added the test_and_set_bit() in the 
> > > following
> > > code in blk_queue_start_tag() to take care of it.
> > > 	do {
> > > 		tag = find_first_zero_bit(bqt->tag_map, bqt->max_depth);
> > > 		if (tag >= bqt->max_depth)
> > > 			return 1;
> > > 	} while (test_and_set_bit(tag, bqt->tag_map));
> > > Please see the following link for the discussion
> > > http://marc.theaimsgroup.com/?l=linux-scsi&m=115886351206726&w=2
> > > 
> > > Cheers
> > > David Somayajulu
> > > QLogic Corporation
> > >
> > 
> > Yes, this piece of code of allocating tag, in itself, is safe.
> > But the following
> > 
> > 	if (unlikely(!__test_and_clear_bit(tag, bqt->tag_map))) {
> > 		printk(KERN_ERR "%s: attempt to clear non-busy tag
> > (%d)\n",
> > 		       __FUNCTION__, tag);
> > 		return;
> > 	}
> > 
> > code of freeing tag (in blk_queue_end_tag())seems to be using
> > unsafe __test_and_clear_bit instead of test_and_clear_bit.
> > I once changed it to test_and_clear_bit and thought it was fixed.
> > But the panic happened thereafter nonetheless(using gcc 3.4.6.
> > gcc 4.1.0 is better but still with kernel errors). bqt also needs
> > to be protected in this case. Replacing queue lock per device with
> > a host lock is a simple but logical fix for it. To introduce a
> > more refined lock is possible, but seems too tedious and elaborate
> > for this issue, since a queue lock is already out there, and a
> > hostwide lock is needed anyway.
> 
> Does this fix it? There really should be no need to add extra locking
> for this, it would be a shame.
> 
> diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
> index fb67897..e752e5d 100644
> --- a/block/ll_rw_blk.c
> +++ b/block/ll_rw_blk.c
> @@ -1072,12 +1072,16 @@ void blk_queue_end_tag(request_queue_t *q, struct request *rq)
>  		 */
>  		return;
>  
> -	if (unlikely(!__test_and_clear_bit(tag, bqt->tag_map))) {
> +	smp_mb__before_clear_bit();
> +
> +	if (unlikely(!test_and_clear_bit(tag, bqt->tag_map))) {
>  		printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
>  		       __FUNCTION__, tag);
>  		return;
>  	}
>  
> +	smp_mb__after_clear_bit();
> +
>  	list_del_init(&rq->queuelist);
>  	rq->cmd_flags &= ~REQ_QUEUED;
>  	rq->tag = -1;
> 

Double checking the actual implementation, the smp_mb__* should not be
needed with the test_and_*_bit operations. The __test_and_clear_bit()
change is needed, though. What kind of crash did you see when you did
that? It should not crash, but you could see the "attempt to clear
non-busy tag" error though.

-- 
Jens Axboe

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