[PATCH] Fix selection of command serial numbers and pids

This patch fixes one of Christroph's fixme comments in the SCSI midlayer.

The selection of the serial number and pid for commands was done
by a global variable and not SMP safe. The race was not very
serious because it was only used for error handling, but it's
still better to fix it.

I audited all the drivers and none seemed to use them
for anything interesting, so I just made it a per host counter
protected by host lock.

(in fact they could be probably removed because they only see
to be used as a flag in exception handling and for debugging.
The patch would be unfortunately quite big because a lot
of driver printks use them)

This should be slight faster on SMP too because the cache line of
the counter won't bounce around the machine.
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>

drivers/scsi/scsi.c

@@ -87,12 +87,6 @@
 #define CDB_SIZE(cmd)	(((((cmd)->cmnd[0] >> 5) & 7) < 6) ? \
 				COMMAND_SIZE((cmd)->cmnd[0]) : (cmd)->cmd_len)
 
-/*
- * Data declarations.
- */
-unsigned long scsi_pid;
-static unsigned long serial_number;
-
 /*
  * Note - the initial logging level can be set here to log events at boot time.
  * After the system is up, you may enable logging via the /proc interface.
@@ -510,6 +504,21 @@ void scsi_log_completion(struct scsi_cmnd *cmd, int disposition)
 }
 #endif
 
+/* 
+ * Assign a serial number and pid to the 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)
+{
+	cmd->serial_number = host->cmd_serial_number++;
+	if (cmd->serial_number == 0) 
+		cmd->serial_number = host->cmd_serial_number++;
+	
+	cmd->pid = host->cmd_pid++;
+	if (cmd->pid == 0)
+		cmd->pid = host->cmd_pid++;
+}
+
 /*
  * Function:    scsi_dispatch_command
  *
@@ -557,13 +566,6 @@ int scsi_dispatch_cmd(struct scsi_cmnd *cmd)
 		goto out;
 	}
 
-	/* Assign a unique nonzero serial_number. */
-	/* XXX(hch): this is racy */
-	if (++serial_number == 0)
-		serial_number = 1;
-	cmd->serial_number = serial_number;
-	cmd->pid = scsi_pid++;
-
 	/* 
 	 * If SCSI-2 or lower, store the LUN value in cmnd.
 	 */
@@ -594,6 +596,10 @@ int scsi_dispatch_cmd(struct scsi_cmnd *cmd)
 		host->resetting = 0;
 	}
 
+	/* 
+	 * AK: unlikely race here: for some reason the timer could
+	 * expire before the serial number is set up below.
+	 */
 	scsi_add_timer(cmd, cmd->timeout_per_command, scsi_times_out);
 
 	scsi_log_send(cmd);
@@ -622,6 +628,8 @@ int scsi_dispatch_cmd(struct scsi_cmnd *cmd)
 	}
 
 	spin_lock_irqsave(host->host_lock, flags);
+	scsi_cmd_get_serial(host, cmd); 
+
 	if (unlikely(test_bit(SHOST_CANCEL, &host->shost_state))) {
 		cmd->result = (DID_NO_CONNECT << 16);
 		scsi_done(cmd);

include/scsi/scsi_cmnd.h

@@ -54,6 +54,7 @@ struct scsi_cmnd {
 	 * completed and the SCSI Command structure has already being reused
 	 * for another command, so that we can avoid incorrectly aborting or
 	 * resetting the new command.
+	 * The serial number is only unique per host.
 	 */
 	unsigned long serial_number;
 	unsigned long serial_number_at_timeout;
@@ -139,7 +140,7 @@ struct scsi_cmnd {
 	int result;		/* Status code from lower level driver */
 
 	unsigned char tag;	/* SCSI-II queued command tag */
-	unsigned long pid;	/* Process ID, starts at 0 */
+	unsigned long pid;	/* Process ID, starts at 0. Unique per host. */
 };
 
 /*

include/scsi/scsi_host.h

@@ -483,7 +483,12 @@ struct Scsi_Host {
 	short unsigned int sg_tablesize;
 	short unsigned int max_sectors;
 	unsigned long dma_boundary;
-
+	/* 
+	 * Used to assign serial numbers to the cmds.
+	 * Protected by the host lock.
+	 */
+	unsigned long cmd_serial_number, cmd_pid; 
+	
 	unsigned unchecked_isa_dma:1;
 	unsigned use_clustering:1;
 	unsigned use_blk_tcq:1;