Re: [RFC][PATCH] scsi-misc-2.5 software enqueue when can_queue reached

[James Bottomley <James.Bottomley@steeleye.com>]

On Thu, 2003-03-06 at 11:41, Patrick Mansfield wrote:
> Such a solution complicates moving to a per-device queue lock - we need a
> per-host lock while pulling off the restart list, and we need a
> per-queue-lock prior to calling __blk_run_queue(); but when starving in
> the scsi_request_fn(), we have a per-queue-lock and need the per-host lock
> to add to the restart list. So we have to allow dropping a lock in both
> cases without leading to a hung requeset queue.

Actually, I don't think it does. Any driver that can truly have a per
device lock can't have a fixed size pool of resources that device
requests are competing for (otherwise it would need a host lock to
access these resources), thus can_queue should always be set to a number
much greater than the aggregate of the local device queue depths (we'd
need a comment in the code to this effect).

Do we actually have any LLDs in the tree today that can benefit from a
device based locking approach?  (i.e. they must manage their resource
pools on a device basis and also have a mailbox scheme that doesn't
require host based locking).

James