Re: [PATCH 3/9] scsi: improved eh timeout handler

[Hannes Reinecke <hare@suse.de>]

On 06/10/2013 10:20 AM, Christoph Hellwig wrote:
> On Mon, Jun 10, 2013 at 09:40:52AM +0200, Hannes Reinecke wrote:
>> When a command runs into a timeout we need to send an 'ABORT TASK'
>> TMF. This is typically done by the 'eh_abort_handler' LLDD callback.
>>
>> Conceptually, however, this function is a normal SCSI command, so
>> there is no need to enter the error handler.
>>
>> This patch implements a new scsi_abort_command() function which
>> invokes an asynchronous function scsi_eh_abort_handler() to
>> abort the commands via 'eh_abort_handler'.
>>
>> If the 'eh_abort_handler' returns SUCCESS or FAST_IO_FAIL the
>> command will be retried if possible. If no retries are allowed
>> the command will be returned immediately, as we have to assume
>> the TMF succeeded and the command is completed with the LLDD.
>> If the TMF fails the command will be pushed back onto the
>> list of failed commands and the SCSI EH handler will be
>> called immediately for all timed-out commands.
> 
> Why can't we use a work item per command?  Linking things into a list
> just to queue it up to workqueues missed half of the point of the
> workqueue infrastructure.
> 
Hmm. I felt that using a per command workqueue might be a bit excessive.
Also the current semantics call for a synchronous command abort.
So even using a per command workqueue won't buy us anything as the
workqueue item would have to wait for the command abort to complete,
which again is quite a waste.
And concurrency would be hell; you'd have to flush the workqueue
items for all outstanding if a device reset should attempted.
And hope that no completion arrives at the time you're attempting to
flush them. etc.

I've been planning for asynchronous command aborts eventually, where
using a per-command workqueue item comes in useful. Gut for now
using existing callbacks makes life so much easier. And per-command
workqueues will just complicate matters.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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