Re: [PATCH] scsi, mptsas : drop scsi_host lock when calling mptsas_qcmd

["Nicholas A. Bellinger" <nab@linux-iscsi.org>]

On Thu, 2010-09-16 at 23:25 +0200, Andi Kleen wrote:
> > I asked James about getting Vasu's unlocked_qcmds=1 patch merged, but he
> > convinced me that doing conditional locking while is very simple, is not
> > the proper way for getting this resolved in mainline code.  I think in
> > the end this will require a longer sit down to do a wholesale conversion
> > of all existing SCSI LLD drivers, and identifing the broken ones that
> > still need a struct Scsi_Host->host_lock'ed SHT->queuecommand() for
> > whatever strange & legacy reasons.
> 
> The standard way to do that would be to first move the lock down
> into the drivers (similar to how it has been done with the BKL).
> This would be a fairly mechanic mindless patch. Lots of typing,
> but not really a lot of real code review needed.
> 
> Then next step the drivers who know they don't want it can remove it.

Well, the main bit is finding those drives that actually call some form
of struct Scsi_Host->host_lock unlock() -> do_work() -> lock() from
within their own ->queuecomamnd() caller, and avoid running into the big
ugly deadlock that would happen here..

I know that Open-iSCSi and my own iSCSI Initiator do this
"optimization", but I am not sure which other LLDs also do this as
well..

Best,

--nab