scsi: convert host_busy to atomic_t

Avoid taking the host-wide host_lock to check the per-host queue limit.
Instead we do an atomic_inc_return early on to grab our slot in the queue,
and if necessary decrement it after finishing all checks.

Signed-off-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Martin K. Petersen <[e-mail]>
Reviewed-by: Hannes Reinecke <[e-mail]>
Reviewed-by: Webb Scales <[e-mail]>
Acked-by: Jens Axboe <[e-mail]>
Tested-by: Bart Van Assche <[e-mail]>
Tested-by: Robert Elliott <[e-mail]>

drivers/scsi/advansys.c

···
 
 	printk("Scsi_Host at addr 0x%p, device %s\n", s, dev_name(boardp->dev));
 	printk(" host_busy %u, host_no %d,\n",
-	       s->host_busy, s->host_no);
+	       atomic_read(&s->host_busy), s->host_no);
 
 	printk(" base 0x%lx, io_port 0x%lx, irq %d,\n",
 	       (ulong)s->base, (ulong)s->io_port, boardp->irq);
···
 
 	seq_printf(m,
 		   " host_busy %u, max_id %u, max_lun %llu, max_channel %u\n",
-		   shost->host_busy, shost->max_id,
+		   atomic_read(&shost->host_busy), shost->max_id,
 		   shost->max_lun, shost->max_channel);
 
 	seq_printf(m,

drivers/scsi/libiscsi.c

···
 	 */
 	for (;;) {
 		spin_lock_irqsave(session->host->host_lock, flags);
-		if (!session->host->host_busy) { /* OK for ERL == 0 */
+		if (!atomic_read(&session->host->host_busy)) { /* OK for ERL == 0 */
 			spin_unlock_irqrestore(session->host->host_lock, flags);
 			break;
 		}
···
 		msleep_interruptible(500);
 		iscsi_conn_printk(KERN_INFO, conn, "iscsi conn_destroy(): "
 				  "host_busy %d host_failed %d\n",
-				  session->host->host_busy,
+				  atomic_read(&session->host->host_busy),
 				  session->host->host_failed);
 		/*
 		 * force eh_abort() to unblock

drivers/scsi/libsas/sas_scsi_host.c

···
 	spin_unlock_irq(shost->host_lock);
 
 	SAS_DPRINTK("Enter %s busy: %d failed: %d\n",
-		    __func__, shost->host_busy, shost->host_failed);
+		    __func__, atomic_read(&shost->host_busy), shost->host_failed);
 	/*
 	 * Deal with commands that still have SAS tasks (i.e. they didn't
 	 * complete via the normal sas_task completion mechanism),
···
 		goto retry;
 
 	SAS_DPRINTK("--- Exit %s: busy: %d failed: %d tries: %d\n",
-		    __func__, shost->host_busy, shost->host_failed, tries);
+		    __func__, atomic_read(&shost->host_busy),
+		    shost->host_failed, tries);
 }
 
 enum blk_eh_timer_return sas_scsi_timed_out(struct scsi_cmnd *cmd)

drivers/scsi/qlogicpti.c

···
 	/* Temporary workaround until bug is found and fixed (one bug has been found
 	   already, but fixing it makes things even worse) -jj */
 	int num_free = QLOGICPTI_REQ_QUEUE_LEN - REQ_QUEUE_DEPTH(in_ptr, out_ptr) - 64;
-	host->can_queue = host->host_busy + num_free;
+	host->can_queue = atomic_read(&host->host_busy) + num_free;
 	host->sg_tablesize = QLOGICPTI_MAX_SG(num_free);
 }
 

drivers/scsi/scsi.c

···
 			if (level > 3)
 				scmd_printk(KERN_INFO, cmd,
 					    "scsi host busy %d failed %d\n",
-					    cmd->device->host->host_busy,
+					    atomic_read(&cmd->device->host->host_busy),
 					    cmd->device->host->host_failed);
 		}
 	}

drivers/scsi/scsi_error.c

···
 /* called with shost->host_lock held */
 void scsi_eh_wakeup(struct Scsi_Host *shost)
 {
-	if (shost->host_busy == shost->host_failed) {
+	if (atomic_read(&shost->host_busy) == shost->host_failed) {
 		trace_scsi_eh_wakeup(shost);
 		wake_up_process(shost->ehandler);
 		SCSI_LOG_ERROR_RECOVERY(5, shost_printk(KERN_INFO, shost,
···
 	while (!kthread_should_stop()) {
 		set_current_state(TASK_INTERRUPTIBLE);
 		if ((shost->host_failed == 0 && shost->host_eh_scheduled == 0) ||
-		    shost->host_failed != shost->host_busy) {
+		    shost->host_failed != atomic_read(&shost->host_busy)) {
 			SCSI_LOG_ERROR_RECOVERY(1,
 				shost_printk(KERN_INFO, shost,
 					     "scsi_eh_%d: sleeping\n",
···
 			shost_printk(KERN_INFO, shost,
 				     "scsi_eh_%d: waking up %d/%d/%d\n",
 				     shost->host_no, shost->host_eh_scheduled,
-				     shost->host_failed, shost->host_busy));
+				     shost->host_failed,
+				     atomic_read(&shost->host_busy)));
 
 		/*
 		 * We have a host that is failing for some reason.  Figure out

drivers/scsi/scsi_lib.c

···
 	struct scsi_target *starget = scsi_target(sdev);
 	unsigned long flags;
 
-	spin_lock_irqsave(shost->host_lock, flags);
-	shost->host_busy--;
+	atomic_dec(&shost->host_busy);
 	atomic_dec(&starget->target_busy);
+
 	if (unlikely(scsi_host_in_recovery(shost) &&
-		     (shost->host_failed || shost->host_eh_scheduled)))
+		     (shost->host_failed || shost->host_eh_scheduled))) {
+		spin_lock_irqsave(shost->host_lock, flags);
 		scsi_eh_wakeup(shost);
-	spin_unlock(shost->host_lock);
-	spin_lock(sdev->request_queue->queue_lock);
+		spin_unlock_irqrestore(shost->host_lock, flags);
+	}
+
+	spin_lock_irqsave(sdev->request_queue->queue_lock, flags);
 	sdev->device_busy--;
 	spin_unlock_irqrestore(sdev->request_queue->queue_lock, flags);
 }
···
 
 static inline int scsi_host_is_busy(struct Scsi_Host *shost)
 {
-	if ((shost->can_queue > 0 && shost->host_busy >= shost->can_queue) ||
+	if ((shost->can_queue > 0 &&
+	     atomic_read(&shost->host_busy) >= shost->can_queue) ||
 	    shost->host_blocked || shost->host_self_blocked)
 		return 1;
 
···
 				   struct Scsi_Host *shost,
 				   struct scsi_device *sdev)
 {
-	int ret = 0;
-
-	spin_lock_irq(shost->host_lock);
+	unsigned int busy;
 
 	if (scsi_host_in_recovery(shost))
-		goto out;
-	if (shost->host_busy == 0 && shost->host_blocked) {
+		return 0;
+
+	busy = atomic_inc_return(&shost->host_busy) - 1;
+	if (shost->host_blocked) {
+		if (busy)
+			goto starved;
+
 		/*
 		 * unblock after host_blocked iterates to zero
 		 */
-		if (--shost->host_blocked != 0)
-			goto out;
+		spin_lock_irq(shost->host_lock);
+		if (--shost->host_blocked != 0) {
+			spin_unlock_irq(shost->host_lock);
+			goto out_dec;
+		}
+		spin_unlock_irq(shost->host_lock);
 
 		SCSI_LOG_MLQUEUE(3,
 			shost_printk(KERN_INFO, shost,
 				     "unblocking host at zero depth\n"));
 	}
-	if (scsi_host_is_busy(shost)) {
-		if (list_empty(&sdev->starved_entry))
-			list_add_tail(&sdev->starved_entry, &shost->starved_list);
-		goto out;
-	}
+
+	if (shost->can_queue > 0 && busy >= shost->can_queue)
+		goto starved;
+	if (shost->host_self_blocked)
+		goto starved;
 
 	/* We're OK to process the command, so we can't be starved */
-	if (!list_empty(&sdev->starved_entry))
-		list_del_init(&sdev->starved_entry);
+	if (!list_empty(&sdev->starved_entry)) {
+		spin_lock_irq(shost->host_lock);
+		if (!list_empty(&sdev->starved_entry))
+			list_del_init(&sdev->starved_entry);
+		spin_unlock_irq(shost->host_lock);
+	}
 
-	shost->host_busy++;
-	ret = 1;
-out:
+	return 1;
+
+starved:
+	spin_lock_irq(shost->host_lock);
+	if (list_empty(&sdev->starved_entry))
+		list_add_tail(&sdev->starved_entry, &shost->starved_list);
 	spin_unlock_irq(shost->host_lock);
-	return ret;
+out_dec:
+	atomic_dec(&shost->host_busy);
+	return 0;
 }
 
 /*
···
 	 * with the locks as normal issue path does.
 	 */
 	sdev->device_busy++;
-	spin_unlock(sdev->request_queue->queue_lock);
-	spin_lock(shost->host_lock);
-	shost->host_busy++;
+	atomic_inc(&shost->host_busy);
 	atomic_inc(&starget->target_busy);
-	spin_unlock(shost->host_lock);
-	spin_lock(sdev->request_queue->queue_lock);
 
 	blk_complete_request(req);
 }

drivers/scsi/scsi_sysfs.c

···
 static DEVICE_ATTR(eh_deadline, S_IRUGO | S_IWUSR, show_shost_eh_deadline, store_shost_eh_deadline);
 
 shost_rd_attr(unique_id, "%u\n");
-shost_rd_attr(host_busy, "%hu\n");
 shost_rd_attr(cmd_per_lun, "%hd\n");
 shost_rd_attr(can_queue, "%hd\n");
 shost_rd_attr(sg_tablesize, "%hu\n");
···
 shost_rd_attr(prot_capabilities, "%u\n");
 shost_rd_attr(prot_guard_type, "%hd\n");
 shost_rd_attr2(proc_name, hostt->proc_name, "%s\n");
+
+static ssize_t
+show_host_busy(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	struct Scsi_Host *shost = class_to_shost(dev);
+	return snprintf(buf, 20, "%d\n", atomic_read(&shost->host_busy));
+}
+static DEVICE_ATTR(host_busy, S_IRUGO, show_host_busy, NULL);
 
 static struct attribute *scsi_sysfs_shost_attrs[] = {
 	&dev_attr_unique_id.attr,

include/scsi/scsi_host.h

···
 	 */
 	struct blk_queue_tag	*bqt;
 
-	/*
-	 * The following two fields are protected with host_lock;
-	 * however, eh routines can safely access during eh processing
-	 * without acquiring the lock.
-	 */
-	unsigned int host_busy;		   /* commands actually active on low-level */
-	unsigned int host_failed;	   /* commands that failed. */
+	atomic_t host_busy;		   /* commands actually active on low-level */
+	unsigned int host_failed;	   /* commands that failed.
+					      protected by host_lock */
 	unsigned int host_eh_scheduled;    /* EH scheduled without command */
     
 	unsigned int host_no;  /* Used for IOCTL_GET_IDLUN, /proc/scsi et al. */