[SCSI] Revert "sg: no need sg_open_exclusive_lock"

This reverts commit 00b2d9d6d05b56fc1d77071ff8ccbd2c65b48dec.

This is one of four patches that was causing this bug

[  205.372823] ================================================
[  205.372901] [ BUG: lock held when returning to user space! ]
[  205.372979] 3.12.0-rc6-hw-debug-pagealloc+ #67 Not tainted
[  205.373055] ------------------------------------------------
[  205.373132] megarc.bin/5283 is leaving the kernel with locks still held!
[  205.373212] 1 lock held by megarc.bin/5283:
[  205.373285]  #0:  (&sdp->o_sem){.+.+..}, at: [<ffffffff8161e650>] sg_open+0x3a0/0x4d0

Cc: 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

@@ -105,6 +105,8 @@ static int scatter_elem_sz_prev = SG_SCATTER_SZ;
 static int sg_add(struct device *, struct class_interface *);
 static void sg_remove(struct device *, struct class_interface *);
 
+static DEFINE_SPINLOCK(sg_open_exclusive_lock);
+
 static DEFINE_IDR(sg_index_idr);
 static DEFINE_RWLOCK(sg_index_lock);	/* Also used to lock
 							   file descriptor list for device */
@@ -174,6 +176,7 @@ typedef struct sg_device { /* holds the state of each scsi generic device */
 	struct list_head sfds;
 	struct rw_semaphore o_sem;	/* exclude open should hold this rwsem */
 	volatile char detached;	/* 0->attached, 1->detached pending removal */
+	/* exclude protected by sg_open_exclusive_lock */
 	char exclude;		/* opened for exclusive access */
 	char sgdebug;		/* 0->off, 1->sense, 9->dump dev, 10-> all devs */
 	struct gendisk *disk;
@@ -222,6 +225,27 @@ static int sg_allow_access(struct file *filp, unsigned char *cmd)
 	return blk_verify_command(cmd, filp->f_mode & FMODE_WRITE);
 }
 
+static int get_exclude(Sg_device *sdp)
+{
+	unsigned long flags;
+	int ret;
+
+	spin_lock_irqsave(&sg_open_exclusive_lock, flags);
+	ret = sdp->exclude;
+	spin_unlock_irqrestore(&sg_open_exclusive_lock, flags);
+	return ret;
+}
+
+static int set_exclude(Sg_device *sdp, char val)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&sg_open_exclusive_lock, flags);
+	sdp->exclude = val;
+	spin_unlock_irqrestore(&sg_open_exclusive_lock, flags);
+	return val;
+}
+
 static int sfds_list_empty(Sg_device *sdp)
 {
 	unsigned long flags;
@@ -293,7 +317,7 @@ sg_open(struct inode *inode, struct file *filp)
 	}
 	/* Since write lock is held, no need to check sfd_list */
 	if (flags & O_EXCL)
-		sdp->exclude = 1;	/* used by release lock */
+		set_exclude(sdp, 1);
 
 	if (sdp->detached) {
 		retval = -ENODEV;
@@ -313,7 +337,7 @@ sg_open(struct inode *inode, struct file *filp)
 		retval = -ENOMEM;
 sem_out:
 		if (flags & O_EXCL) {
-			sdp->exclude = 0;	/* undo if error */
+			set_exclude(sdp, 0);	/* undo if error */
 			up_write(&sdp->o_sem);
 		} else
 			up_read(&sdp->o_sem);
@@ -340,8 +364,8 @@ sg_release(struct inode *inode, struct file *filp)
 		return -ENXIO;
 	SCSI_LOG_TIMEOUT(3, printk("sg_release: %s\n", sdp->disk->disk_name));
 
-	excl = sdp->exclude;
-	sdp->exclude = 0;
+	excl = get_exclude(sdp);
+	set_exclude(sdp, 0);
 	if (excl)
 		up_write(&sdp->o_sem);
 	else
@@ -2598,7 +2622,7 @@ static int sg_proc_seq_show_debug(struct seq_file *s, void *v)
 			     scsidp->lun,
 			     scsidp->host->hostt->emulated);
 		seq_printf(s, " sg_tablesize=%d excl=%d\n",
-			   sdp->sg_tablesize, sdp->exclude);
+			   sdp->sg_tablesize, get_exclude(sdp));
 		sg_proc_debug_helper(s, sdp);
 	}
 	read_unlock_irqrestore(&sg_index_lock, iflags);