SCSI host lock push-down

Move the mid-layer's ->queuecommand() invocation from being locked
with the host lock to being unlocked to facilitate speeding up the
critical path for drivers who don't need this lock taken anyway.

The patch below presents a simple SCSI host lock push-down as an
equivalent transformation.  No locking or other behavior should change
with this patch.  All existing bugs and locking orders are preserved.

Additionally, add one parameter to queuecommand,
	struct Scsi_Host *
and remove one parameter from queuecommand,
	void (*done)(struct scsi_cmnd *)

Scsi_Host* is a convenient pointer that most host drivers need anyway,
and 'done' is redundant to struct scsi_cmnd->scsi_done.

Minimal code disturbance was attempted with this change.  Most drivers
needed only two one-line modifications for their host lock push-down.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Acked-by: James Bottomley <James.Bottomley@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 13 deletions

diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index 348fba0a897..2aeb2e9c4d3 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -634,12 +634,13 @@ void scsi_log_completion(struct scsi_cmnd *cmd, int disposition)
  * Description: a serial number identifies a request for error recovery
  * and debugging purposes.  Protected by the Host_Lock of host.
  */
-static inline void scsi_cmd_get_serial(struct Scsi_Host *host, struct scsi_cmnd *cmd)
+void scsi_cmd_get_serial(struct Scsi_Host *host, struct scsi_cmnd *cmd)
 {
 	cmd->serial_number = host->cmd_serial_number++;
 	if (cmd->serial_number == 0) 
 		cmd->serial_number = host->cmd_serial_number++;
 }
+EXPORT_SYMBOL(scsi_cmd_get_serial);
 
 /**
  * scsi_dispatch_command - Dispatch a command to the low-level driver.
@@ -651,7 +652,6 @@ static inline void scsi_cmd_get_serial(struct Scsi_Host *host, struct scsi_cmnd
 int scsi_dispatch_cmd(struct scsi_cmnd *cmd)
 {
 	struct Scsi_Host *host = cmd->device->host;
-	unsigned long flags = 0;
 	unsigned long timeout;
 	int rtn = 0;
 
@@ -737,23 +737,15 @@ int scsi_dispatch_cmd(struct scsi_cmnd *cmd)
 		goto out;
 	}
 
-	spin_lock_irqsave(host->host_lock, flags);
-	/*
-	 * AK: unlikely race here: for some reason the timer could
-	 * expire before the serial number is set up below.
-	 *
-	 * TODO: kill serial or move to blk layer
-	 */
-	scsi_cmd_get_serial(host, cmd); 
-
 	if (unlikely(host->shost_state == SHOST_DEL)) {
 		cmd->result = (DID_NO_CONNECT << 16);
 		scsi_done(cmd);
 	} else {
 		trace_scsi_dispatch_cmd_start(cmd);
-		rtn = host->hostt->queuecommand(cmd, scsi_done);
+		cmd->scsi_done = scsi_done;
+		rtn = host->hostt->queuecommand(host, cmd);
 	}
-	spin_unlock_irqrestore(host->host_lock, flags);
+
 	if (rtn) {
 		trace_scsi_dispatch_cmd_error(cmd, rtn);
 		if (rtn != SCSI_MLQUEUE_DEVICE_BUSY &&