Re: [PATCH 0/8] Drop host_lock around LLD SHT->queuecommand() caller

[Jeff Garzik <jeff@garzik.org>]

On 09/16/2010 09:31 PM, Nicholas A. Bellinger wrote:
> On Thu, 2010-09-16 at 18:29 -0700, Tim Chen wrote:
>> On Thu, 2010-09-16 at 15:35 -0700, Nicholas A. Bellinger wrote:
>>> From: Nicholas Bellinger<nab@linux-iscsi.org>
>>>
>>> Greetings all,
>>>
>>> This series contains the first round of a whole-sale conversion for droping
>>> struct Scsi_Host->host_lock around struct Scsi_Host->queuecommand() within
>>> scsi_dispatch_cmd().  So with this first patch the only part of scsi_dispatch_cmd()
>>> that is protected by host_lock is scsi_cmd_get_serial)_.
>>>
>>
>> Maybe we can change the host->cmd_serial_number to atomic and totally
>> avoid the need to take host_lock.
>
> Hmmmm good point, then we would also need an atomic_t for signaling
> (shost_state == SHOST_DEL) in scsi_dispatch_cmd() to be able to
> completly drop host_lock usage within scsi_dispatch_cmd().
>
> I can take a look at doing this specific part, if you would be willing
> to have a look at the host->cmd_serial_number conversion piece.  ;)

But that raises the familiar tale of:  using multiple atomics (w/ their 
locked instructions) may cost more than a spinlock.

	Jeff