[SCSI] Revert "sg: use rwsem to solve race during exclusive open"

This reverts commit 15b06f9a.

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

@@ -170,11 +170,11 @@ typedef struct sg_fd {		/* holds the state of a file descriptor */
 
 typedef struct sg_device { /* holds the state of each scsi generic device */
 	struct scsi_device *device;
+	wait_queue_head_t o_excl_wait;	/* queue open() when O_EXCL in use */
 	int sg_tablesize;	/* adapter's max scatter-gather table size */
 	u32 index;		/* device index number */
 	/* sfds is protected by sg_index_lock */
 	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 */
@@ -265,6 +265,7 @@ sg_open(struct inode *inode, struct file *filp)
 	struct request_queue *q;
 	Sg_device *sdp;
 	Sg_fd *sfp;
+	int res;
 	int retval;
 
 	nonseekable_open(inode, filp);
@@ -293,35 +294,35 @@ sg_open(struct inode *inode, struct file *filp)
 		goto error_out;
 	}
 
-	if ((flags & O_EXCL) && (O_RDONLY == (flags & O_ACCMODE))) {
+	if (flags & O_EXCL) {
+		if (O_RDONLY == (flags & O_ACCMODE)) {
 			retval = -EPERM; /* Can't lock it with read only access */
 			goto error_out;
 		}
-	if (flags & O_NONBLOCK) {
-		if (flags & O_EXCL) {
-			if (!down_write_trylock(&sdp->o_sem)) {
+		if (!sfds_list_empty(sdp) && (flags & O_NONBLOCK)) {
 			retval = -EBUSY;
 			goto error_out;
 		}
-		} else {
-			if (!down_read_trylock(&sdp->o_sem)) {
+		res = wait_event_interruptible(sdp->o_excl_wait,
+					   ((!sfds_list_empty(sdp) || get_exclude(sdp)) ? 0 : set_exclude(sdp, 1)));
+		if (res) {
+			retval = res;	/* -ERESTARTSYS because signal hit process */
+			goto error_out;
+		}
+	} else if (get_exclude(sdp)) {	/* some other fd has an exclusive lock on dev */
+		if (flags & O_NONBLOCK) {
 			retval = -EBUSY;
 			goto error_out;
 		}
+		res = wait_event_interruptible(sdp->o_excl_wait, !get_exclude(sdp));
+		if (res) {
+			retval = res;	/* -ERESTARTSYS because signal hit process */
+			goto error_out;
 		}
-	} else {
-		if (flags & O_EXCL)
-			down_write(&sdp->o_sem);
-		else
-			down_read(&sdp->o_sem);
 	}
-	/* Since write lock is held, no need to check sfd_list */
-	if (flags & O_EXCL)
-		set_exclude(sdp, 1);
-
 	if (sdp->detached) {
 		retval = -ENODEV;
-		goto sem_out;
+		goto error_out;
 	}
 	if (sfds_list_empty(sdp)) {	/* no existing opens on this device */
 		sdp->sgdebug = 0;
@@ -330,18 +331,17 @@ sg_open(struct inode *inode, struct file *filp)
 	}
 	if ((sfp = sg_add_sfp(sdp, dev)))
 		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:
 		if (flags & O_EXCL) {
 			set_exclude(sdp, 0);	/* undo if error */
-			up_write(&sdp->o_sem);
-		} else
-			up_read(&sdp->o_sem);
+			wake_up_interruptible(&sdp->o_excl_wait);
+		}
+		retval = -ENOMEM;
+		goto error_out;
+	}
+	retval = 0;
 error_out:
+	if (retval) {
 		scsi_autopm_put_device(sdp->device);
 sdp_put:
 		scsi_device_put(sdp->device);
@@ -358,18 +358,13 @@ sg_release(struct inode *inode, struct file *filp)
 {
 	Sg_device *sdp;
 	Sg_fd *sfp;
-	int excl;
 
 	if ((!(sfp = (Sg_fd *) filp->private_data)) || (!(sdp = sfp->parentdp)))
 		return -ENXIO;
 	SCSI_LOG_TIMEOUT(3, printk("sg_release: %s\n", sdp->disk->disk_name));
 
-	excl = get_exclude(sdp);
 	set_exclude(sdp, 0);
-	if (excl)
-		up_write(&sdp->o_sem);
-	else
-		up_read(&sdp->o_sem);
+	wake_up_interruptible(&sdp->o_excl_wait);
 
 	scsi_autopm_put_device(sdp->device);
 	kref_put(&sfp->f_ref, sg_remove_sfp);
@@ -1421,7 +1416,7 @@ static Sg_device *sg_alloc(struct gendisk *disk, struct scsi_device *scsidp)
 	sdp->disk = disk;
 	sdp->device = scsidp;
 	INIT_LIST_HEAD(&sdp->sfds);
-	init_rwsem(&sdp->o_sem);
+	init_waitqueue_head(&sdp->o_excl_wait);
 	sdp->sg_tablesize = queue_max_segments(q);
 	sdp->index = k;
 	kref_init(&sdp->d_ref);
@@ -2132,11 +2127,13 @@ static void sg_remove_sfp_usercontext(struct work_struct *work)
 static void sg_remove_sfp(struct kref *kref)
 {
 	struct sg_fd *sfp = container_of(kref, struct sg_fd, f_ref);
+	struct sg_device *sdp = sfp->parentdp;
 	unsigned long iflags;
 
 	write_lock_irqsave(&sg_index_lock, iflags);
 	list_del(&sfp->sfd_siblings);
 	write_unlock_irqrestore(&sg_index_lock, iflags);
+	wake_up_interruptible(&sdp->o_excl_wait);
 
 	INIT_WORK(&sfp->ew.work, sg_remove_sfp_usercontext);
 	schedule_work(&sfp->ew.work);