[SCSI] sg: checking sdp->detached isn't protected when open

@detached is set under the protection of sg_index_lock. Without getting the
lock, new sfp will be added during sg removal and there is no chance for it
to be picked out. So check with sg_index_lock held in sg_add_sfp().
Signed-off-by: Vaughan Cao <vaughan.cao@oracle.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/scsi/sg.c

@@ -295,23 +295,20 @@ sg_open(struct inode *inode, struct file *filp)
 	if (flags & O_EXCL)
 		sdp->exclude = 1;	/* used by release lock */
 
-	if (sdp->detached) {
-		retval = -ENODEV;
-		goto sem_out;
-	}
 	if (sfds_list_empty(sdp)) {	/* no existing opens on this device */
 		sdp->sgdebug = 0;
 		q = sdp->device->request_queue;
 		sdp->sg_tablesize = queue_max_segments(q);
 	}
-	if ((sfp = sg_add_sfp(sdp, dev)))
+	sfp = sg_add_sfp(sdp, dev);
+	if (!IS_ERR(sfp))
 		filp->private_data = sfp;
 		/* retval is already provably zero at this point because of the
 		 * check after retval = scsi_autopm_get_device(sdp->device))
 		 */
 	else {
-		retval = -ENOMEM;
-sem_out:
+		retval = PTR_ERR(sfp);
+
 		if (flags & O_EXCL) {
 			sdp->exclude = 0;	/* undo if error */
 			up_write(&sdp->o_sem);
@@ -2045,7 +2042,7 @@ sg_add_sfp(Sg_device * sdp, int dev)
 
 	sfp = kzalloc(sizeof(*sfp), GFP_ATOMIC | __GFP_NOWARN);
 	if (!sfp)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	init_waitqueue_head(&sfp->read_wait);
 	rwlock_init(&sfp->rq_list_lock);
@@ -2060,6 +2057,10 @@ sg_add_sfp(Sg_device * sdp, int dev)
 	sfp->keep_orphan = SG_DEF_KEEP_ORPHAN;
 	sfp->parentdp = sdp;
 	write_lock_irqsave(&sg_index_lock, iflags);
+	if (sdp->detached) {
+		write_unlock_irqrestore(&sg_index_lock, iflags);
+		return ERR_PTR(-ENODEV);
+	}
 	list_add_tail(&sfp->sfd_siblings, &sdp->sfds);
 	write_unlock_irqrestore(&sg_index_lock, iflags);
 	SCSI_LOG_TIMEOUT(3, printk("sg_add_sfp: sfp=0x%p\n", sfp));