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

[Hannes Reinecke <hare@suse.de>]

On 06/11/2013 08:57 PM, James Bottomley wrote:
> On Mon, 2013-06-10 at 01:20 -0700, 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.
> 
> Actually, I think we can dump the workqueue altogether.  The only reason
> we need it is because the current abort handlers wait for the command
> and return the completion state.  However, all LLDs are capable of
> emitting TMFs at interrupt level, so if we separated the emit from the
> wait, we could simply do this sequence:
> 
> on timeout, fire the abort from interrupt and mark the command as having
> an abort issued (possibly by adding a pointer to the abort task), return
> BLK_EH_RESET_TIMER.
> 
> Now if the timeout fires again, assume the abort was unsucessful and
> escalate to LUN reset.
> 
> This is fully asynchronous, fully tracked and doesn't rely on work
> queues.
> 
Hehe. Been there, done that, doesn't work :-(
That was my original idea, too.

But some LLDDs send TMFs in a non-interrupt-safe way, so the only
way to make it work was to use a workqueue.
(Eg qla2xxx has a parameter to send TMFs async, but this doesn't
work on older firmware).

> The necessary additions for something like this are the from interrupt
> issue abort and LUN reset, which could just be additional callbacks in
> the host template.
> 
However, we could make that optional, so that LLDDs not capable of
sending TMFs in an interrupt-safe manner will continue to use the
original framework.

Ok, let's see whether this flies.

Cheers,

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