Oops, I forgot to mention that reqfn is reformatted mostly as
suggested by Chritoph Hellwig. Sorry.
On Tue, Apr 12, 2005 at 07:33:03PM +0900, Tejun Heo wrote:
> 03_scsi_reqfn_reimplementation.patch
>
> This patch rewrites scsi_request_fn(). scsi_dispatch_cmd() is
> merged into scsi_request_fn(). Goals are
>
> * Remove unnecessary operations (host_lock unlocking/locking,
> recursing into scsi_run_queue(), ...)
> * Consolidate defer/kill paths.
> * Be concise.
>
> The following bugs are fixed.
>
> * All killed requests now get fully prep'ed and pass through
> __scsi_done(). This is the only kill path.
> - scsi_cmnd leak in offline-kill path removed
> - unfinished request bug in
> scsi_dispatch_cmd():SDEV_DEL-kill path removed.
> - commands are never terminated directly from blk
> layer unless they are invalid, so no need to supply
> req->end_io callback for special requests.
> * Timer is added while holding host_lock, after all conditions
> are checked and serial number is assigned. This guarantees
> that until host_lock is released, the scsi_cmnd pointed to
> by cmd isn't released. That didn't hold in the original
> code and, theoretically, the original code could access
> already released cmd.
> * For the same reason, if shost->hostt->queuecommand() fails,
> we try to delete the timer before releasing host_lock.
>
> Other changes/notes
>
> * host_lock is acquired and released only once.
> enter (qlocked) -> enter loop -> dev-prep -> switch to hlock -\
> ^---- switch to qlock <- issue <- host-prep <-/
> * unnecessary if () on get_device() removed.
> * loop on elv_next_request() instead of blk_queue_plugged().
> We now explicitly break out of loop when we plug and check if
> the queue has been plugged underneath us at the end of loop.
> * All device/host state checks are done in this function and
> done only once while holding qlock/host_lock respectively.
> * Requests which get deferred during dev-prep are never
> removed from request queue, so deferring is achieved by
> simply breaking out of the loop and returning.
> * Failure of blk_queue_start_tag() on tagged queue is a BUG
> now. This condition should have been catched by
> scsi_dev_queue_ready().
> * req->special == NULL test removed. This just can't happen,
> and even if it ever happens, scsi_request_fn() will
> deterministically oops.
> * Requests which gets deferred during host-prep are requeued
> using blk_requeue_request(). This is the only requeue path.
>
> Note that scsi_kill_requests() still terminates requests using
> blk layer. The path is circular-ref workaround and soon to be
> replaced, so ignore it for now.
--
tejun
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