Re: [PATCH] sd: retry read_capacity on UNIT_ATTENTION

[Hannes Reinecke <hare@suse.de>]

James Bottomley wrote:
> On Thu, 2010-04-01 at 15:44 +0200, Hannes Reinecke wrote:
>> Hazard testing uncovered yet another bug in sd. Under heavy
>> reset activity the retry counter might be exhausted and
>> the command will be returned with sense UNIT_ATTENTION/0x29/00
>> (POWER ON, RESET, OR BUS DEVICE RESET OCCURRED). In those
>> cases we should just increase the retry counter again,
>> retrying one more to clear up this Unit Attention state.
>>
>> Signed-off-by: Hannes Reinecke <hare@suse.de>
>>
>> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
>> index 1962bea..7d75a21 100644
>> --- a/drivers/scsi/sd.c
>> +++ b/drivers/scsi/sd.c
>> @@ -1454,8 +1454,15 @@ static int read_capacity_10(struct scsi_disk *sdkp, struct scsi_device *sdp,
>>  		if (media_not_present(sdkp, &sshdr))
>>  			return -ENODEV;
>>  
>> -		if (the_result)
>> +		if (the_result) {
>>  			sense_valid = scsi_sense_valid(&sshdr);
>> +			if (sense_valid &&
>> +			    sshdr.sense_key == UNIT_ATTENTION &&
>> +			    sshdr.asc = 0x29 && sshdr.asq == 0x00)
>                                       ^^^^
> should be ==
> 
>> +			    /* Device reset might occur several times,
>> +			     * give it one more chance */
>> +			    retries++;
>> +		}
> 
> Firstly, not even compile checked:
> 
> drivers/scsi/sd.c: In function ‘read_capacity_10’:
> drivers/scsi/sd.c:1558: error: ‘struct scsi_sense_hdr’ has no member named ‘asq’
> 
D'oh.

> Secondly, we can't quite do this.  Some devices (only broken ones in my
> experience) will reply UNIT_ATTENTION I was RESET forever, leading to a
> loop here.  Additionally, a massive reset storm on a shared bus would
> DoS the code here, so there must be a give up point after a reasonable
> number of retries.
> 
Hmm. yes.

> The third problem is that if this is happening to a large device, we
> only catch it in RC10 ... so we'll report undersize if the device is >
> SPC2
> 
Okay. In the best of all worlds we would have a module parameter which
would us to adjust this parameter, as I fear the actual number of retries
will depend on the number of devices connected.

But if you fell that's overkill it's fine by me, too.

> How about this instead?
> 
Yes, that's better. Thanks.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: Markus Rex, HRB 16746 (AG Nürnberg)
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