Date:	Fri, 24 Sep 2010 08:32:36 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
CC:	Mike Christie <michaelc@...wisc.edu>,
	Joe Eykholt <jeykholt@...co.com>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Vasu Dev <vasu.dev@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	James Bottomley <James.Bottomley@...e.de>,
	James Smart <james.smart@...lex.com>,
	Andrew Vasquez <andrew.vasquez@...gic.com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Hannes Reinecke <hare@...e.de>, Christoph Hellwig <hch@....de>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v2 01/11] scsi: Convert struct  Scsi_Host->cmd_serial_number
 to atomic_t

On 2010-09-23 23:46, Nicholas A. Bellinger wrote:
> On Sat, 2010-09-18 at 12:58 -0700, Nicholas A. Bellinger wrote:
>> On Fri, 2010-09-17 at 21:45 -0500, Mike Christie wrote:
>>> On 09/17/2010 02:03 PM, Joe Eykholt wrote:
>>>>
>>>> How about instead of adding use_serial_number, let's just have the
>>>> drivers that want a serial number call scsi_cmd_get_serial()
>>>
>>> I think this sounds better.
>>>
>>
>> In that case I will go ahead and add explict scsi_cmd_get_serial() calls
>> to the LLDs that use struct scsi_cmnd->serial_number in anything beyond
>> an obvious and simple informational purpose.
>>
>>> You could also convert drivers to the host tagging if you needed a 
>>> unique id for each command sent to a host.
>>
>> Hmmm, what does this entail again..?
>>
>>>
>>>> and stop calling it from scsi_dispatch_cmd()?   AFAICT, it's only
>>>> used in debug messages in some drivers.  I didn't find other usages
>>>> but didn't do an exhaustive search.
>>>
>>> The comments for serial_number say that it is only supposed to be used 
>>> for debugging printks and most drivers use it for that.
>>
>> So I would suppose it would be OK for those drivers to continue to
>> printk serial_number to show the internal serial_number allocation is
>> now disabled by default.
>>
>>> However, it looks like mpt and dpt_i2o are using it for error handling
>>> and/or lookup type of operations. I think the mpt* uses are not needed
>>> in the abort checks.
>>>
>>
>> In that case, then adding an explict scsi_cmd_get_serial() call in mpt*
>> and dpt_i20 ->queuecommand() callers would be a good first step.
>>
>>> And eata is using it for ordering and tracking or something. It could 
>>> probably be converted to the host tagging if or what you suggested if it 
>>> needs the uniqueue id.
>>
>> Adding an explict scsi_cmd_get_serial() for eata for now, and we can
>> consider LLD canidates for host tag conversion as a future step.
>>
>>>
>>> zfcp looks like it copies it. It does not look like the driver needs it.
>>>
>>
>> Ok, I will look at removing it's usage in zfcp or if necessary add an
>> the explict scsi_cmd_get_serial() call.
>>
>>> scsi_error.c uses it in scsi_try_to_abort_cmd to check if a command has 
>>> completed, but I think that can be done by checking if REQ_ATOM_COMPLETE 
>>> is set.
>>
>> Hmmm, good catch here.  Jejb and hch, does this item work for you..?
>>
>> If so then I will take another peek for any ML uses of a struct
>> scsi_cmnd->serial_number that need to be addressed, and include Joe's
>> and Mike's recommendations into a v3 series.
>>
> 
> Greetings Mike and Co,
> 
> I was doing some followup on these items for a v3 series and started
> with a patch following mnc's recommendations for dropping the
> scsi_error.c codes depending upon struct scsi_cmnd->serial_number:
> 
> diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
> index 1de30eb..f35c127 100644
> --- a/drivers/scsi/scsi_error.c
> +++ b/drivers/scsi/scsi_error.c
> @@ -644,11 +644,7 @@ static int __scsi_try_to_abort_cmd(struct scsi_cmnd *scmd)
>   */
>  static int scsi_try_to_abort_cmd(struct scsi_cmnd *scmd)
>  {
> -       /*
> -        * scsi_done was called just after the command timed out and before
> -        * we had a chance to process it. (db)
> -        */
> -       if (scmd->serial_number == 0)
> +       if (test_bit(REQ_ATOM_COMPLETE, &scmd->request->atomic_flags))
>                 return SUCCESS;
>         return __scsi_try_to_abort_cmd(scmd);
>  }
> 
> and while building I noticed that the simple single enum
> REQ_ATOM_COMPLETE=0 is

> currently located in block/blk.h and along with blk_mark_rq_complete()
> and blk_clear_rq_complete() for setting this bit within struct
> request->atomic_flags.
> 
> jens, hch, tejun, and co, do you guys have a preference how this
> should be handled so that scsi_try_to_abort_cmd() can use proper
> atomic struct request bits here and we can get rid of this (racy..?)
> method of using struct scsi_cmnd->serial_number for anything wrt to
> per struct scsi_cmnd context timeout handling.

Just add a

static inline int blk_test_rq_complete(struct request *rq)
{
        return test_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);
}

in block/blk.h

I need this too for some lockless completion patches I am playing with.

-- 
Jens Axboe

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