Re: [PATCHv2 13/22] scsi_dh_alua: Use workqueue for RTPG

[Hannes Reinecke <hare@suse.de>]

On 01/13/2016 11:04 AM, Christoph Hellwig wrote:
> On Wed, Jan 13, 2016 at 08:02:02AM +0100, Hannes Reinecke wrote:
>>>> +	/* Check for existing port_group references */
>>>> +	spin_lock(&h->pg_lock);
>>>> +	if (h->pg) {
>>>> +		old_pg = pg;
>>>> +		/* port_group has changed. Update to new port group */
>>>> +		if (h->pg != pg) {
>>>> +			old_pg = h->pg;
>>>> +			rcu_assign_pointer(h->pg, pg);
>>>> +			pg_updated = true;
>>>> +		}
>>>> +	} else {
>>>> +		rcu_assign_pointer(h->pg, pg);
>>>> +		pg_updated = true;
>>>> +	}
>>>> +	alua_rtpg_queue(h->pg, sdev, NULL);
>>>> +	spin_unlock(&h->pg_lock);
>>>> +
>>>> +	if (pg_updated)
>>>> +		synchronize_rcu();
>>>> +	if (old_pg) {
>>>> +		if (old_pg->rtpg_sdev)
>>>> +			flush_delayed_work(&old_pg->rtpg_work);
>>>> +		kref_put(&old_pg->kref, release_port_group);
>>>> +	}
>>>
>>> The synchronize_rcu() needs to be done in release_port_group, or even
>>> better be replaced by doing a kfree_rcu there instead of a kfree.
>>>
>> Point is that we don't necessarily have an old_pg to call
>> release_port_group() on, but we still need to call synchronize_rcu()
>> to inform everyong that h->pg now has a new value.
>
> No, you don't.  synchronize_rcu() just waits for all previously
> started RCU grace periods to finish.  If old_pg doesn't get freed
> you don't need to wait for the grace period.  synchronize_rcu does
> not notify anyone about anything, it just waits.
>
Ah. Right. Good point. Of course we don't need to call it if there's 
nothing to be freed.

Will be folding it into the next round of patches, or just resending 
this one if the remaining patches meets with approval.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@suse.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton
HRB 21284 (AG Nürnberg)
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