Re: [PATCHv2 1/1] SCSI: update hosts module to use idr index management

[Hannes Reinecke <hare@suse.com>]

On 10/06/2015 07:17 PM, James Bottomley wrote:
> On Tue, 2015-10-06 at 10:14 -0700, Lee Duncan wrote:
>> On 10/06/2015 02:40 AM, Christoph Hellwig wrote:
>>>>   struct Scsi_Host *scsi_host_lookup(unsigned short hostnum)
>>>>   {
>>>> -	struct device *cdev;
>>>> -	struct Scsi_Host *shost = NULL;
>>>> -
>>>> -	cdev = class_find_device(&shost_class, NULL, &hostnum,
>>>> -				 __scsi_host_match);
>>>> -	if (cdev) {
>>>> -		shost = scsi_host_get(class_to_shost(cdev));
>>>> -		put_device(cdev);
>>>> -	}
>>>> +	struct Scsi_Host *shost;
>>>> +
>>>> +	spin_lock(&host_index_lock);
>>>> +	shost = idr_find(&host_index_idr, hostnum);
>>>> +	spin_unlock(&host_index_lock);
>>>> +
>>>>   	return shost;
>>>
>>> How does this actually grab a reference to the host?
>>
>> Good catch -- I should have noticed that.
>>
>> I will resubmit the patch.
>
> I'll wait to see what you produce, but I don't think, using a separate
> idr array, you can close the race window between lookup and get.  One of
> the nice things about using the cdev iterator is that the get is part of
> the lookup process.
>
Hmm. Should be possible if you free up the IDR in scsi_remove_host(), 
just after setting the state to SHOST_DEL.

Cheers,

Hannes