[PATCH v2 1/1] [SCSI] sg: fix race condition when do exclusive open

[vaughan <vaughan.cao@oracle.com>]

Rewrite the last patch.
Add a new field 'toopen' in sg_device to count ongoing sg_open's. By checking both 'toopen' and 'exclude' marks when do exclusive open, old race conditions can be avoided.
Replace global sg_open_exclusive_lock with a per device lock - sfd_lock. Since sfds list is now protected by the lock owned by the same sg_device, sg_index_lock becomes a real global lock to only protect sg devices lookup.
Also did some cleanup, such as remove get_exclude() and rename set_exclude() to clear_exclude().

Signed-off-by: Vaughan Cao <vaughan.cao@oracle.com>
---
 drivers/scsi/sg.c | 152 ++++++++++++++++++++++++++++++++----------------------
 1 file changed, 90 insertions(+), 62 deletions(-)

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 25b5455..b0ea73f 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -103,11 +103,8 @@ 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 */
+static DEFINE_RWLOCK(sg_index_lock);	/* protect sg index */
 
 static struct class_interface sg_interface = {
 	.add_dev	= sg_add,
@@ -144,7 +141,7 @@ typedef struct sg_request {	/* SG_MAX_QUEUE requests outstanding per file */
 } Sg_request;
 
 typedef struct sg_fd {		/* holds the state of a file descriptor */
-	/* sfd_siblings is protected by sg_index_lock */
+	/* sfd_siblings is protected by sfd_lock of sg_device */
 	struct list_head sfd_siblings;
 	struct sg_device *parentdp;	/* owning device */
 	wait_queue_head_t read_wait;	/* queue read until command done */
@@ -171,10 +168,10 @@ typedef struct sg_device { /* holds the state of each scsi generic 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 */
+	spinlock_t sfd_lock;	/* protect sfds, exclude, toopen */
 	struct list_head sfds;
+	int toopen;		/* number of who are ready to open sg */
 	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;
@@ -224,25 +221,44 @@ static int sg_allow_access(struct file *filp, unsigned char *cmd)
 	return blk_verify_command(q, cmd, filp->f_mode & FMODE_WRITE);
 }
 
-static int get_exclude(Sg_device *sdp)
+static void clear_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);
+	spin_lock_irqsave(&sdp->sfd_lock, flags);
+	sdp->exclude = 0;
+	spin_unlock_irqrestore(&sdp->sfd_lock, flags);
+	return;
+}
+
+/* we can add exclusively only when no other addition is going on */
+static int try_add_exclude(Sg_device *sdp)
+{
+	unsigned long flags;
+	int ret = 0;
+
+	spin_lock_irqsave(&sdp->sfd_lock, flags);
+	if (list_empty(&sdp->sfds) && !sdp->toopen) {
+		sdp->exclude = 1;
+		sdp->toopen++;
+		ret = 1;
+	}
+	spin_unlock_irqrestore(&sdp->sfd_lock, flags);
 	return ret;
 }
 
-static int set_exclude(Sg_device *sdp, char val)
+static int try_add_shareable(Sg_device *sdp)
 {
 	unsigned long flags;
+	int ret = 0;
 
-	spin_lock_irqsave(&sg_open_exclusive_lock, flags);
-	sdp->exclude = val;
-	spin_unlock_irqrestore(&sg_open_exclusive_lock, flags);
-	return val;
+	spin_lock_irqsave(&sdp->sfd_lock, flags);
+	if (!sdp->exclude && sdp->toopen != INT_MAX) {
+		sdp->toopen++;
+		ret = 1;
+	}
+	spin_unlock_irqrestore(&sdp->sfd_lock, flags);
+	return ret;
 }
 
 static int sfds_list_empty(Sg_device *sdp)
@@ -250,9 +266,9 @@ static int sfds_list_empty(Sg_device *sdp)
 	unsigned long flags;
 	int ret;
 
-	read_lock_irqsave(&sg_index_lock, flags);
+	spin_lock_irqsave(&sdp->sfd_lock, flags);
 	ret = list_empty(&sdp->sfds);
-	read_unlock_irqrestore(&sg_index_lock, flags);
+	spin_unlock_irqrestore(&sdp->sfd_lock, flags);
 	return ret;
 }
 
@@ -266,6 +282,7 @@ sg_open(struct inode *inode, struct file *filp)
 	Sg_fd *sfp;
 	int res;
 	int retval;
+	unsigned long iflags;
 
 	nonseekable_open(inode, filp);
 	SCSI_LOG_TIMEOUT(3, printk("sg_open: dev=%d, flags=0x%x\n", dev, flags));
@@ -293,27 +310,26 @@ sg_open(struct inode *inode, struct file *filp)
 		goto error_out;
 	}
 
-	if (flags & O_EXCL) {
-		if (O_RDONLY == (flags & O_ACCMODE)) {
-			retval = -EPERM; /* Can't lock it with read only access */
-			goto error_out;
-		}
-		if (!sfds_list_empty(sdp) && (flags & O_NONBLOCK)) {
-			retval = -EBUSY;
-			goto error_out;
-		}
-		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) {
+	if ((flags & O_EXCL) && (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)
+			res = try_add_exclude(sdp);
+		else
+			res = try_add_shareable(sdp);
+		if (!res) {
 			retval = -EBUSY;
 			goto error_out;
 		}
-		res = wait_event_interruptible(sdp->o_excl_wait, !get_exclude(sdp));
+	} else {
+		if (flags & O_EXCL)
+			res = wait_event_interruptible(sdp->o_excl_wait,
+				try_add_exclude(sdp));
+		else
+			res = wait_event_interruptible(sdp->o_excl_wait,
+				try_add_shareable(sdp));
 		if (res) {
 			retval = res;	/* -ERESTARTSYS because signal hit process */
 			goto error_out;
@@ -331,10 +347,12 @@ sg_open(struct inode *inode, struct file *filp)
 	if ((sfp = sg_add_sfp(sdp, dev)))
 		filp->private_data = sfp;
 	else {
-		if (flags & O_EXCL) {
-			set_exclude(sdp, 0);	/* undo if error */
-			wake_up_interruptible(&sdp->o_excl_wait);
-		}
+		spin_lock_irqsave(&sdp->sfd_lock, iflags);
+		sdp->toopen--;
+		if (flags & O_EXCL)
+			sdp->exclude = 0;	/* undo if error */
+		spin_unlock_irqrestore(&sdp->sfd_lock, iflags);
+		wake_up_interruptible(&sdp->o_excl_wait);
 		retval = -ENOMEM;
 		goto error_out;
 	}
@@ -362,7 +380,7 @@ sg_release(struct inode *inode, struct file *filp)
 		return -ENXIO;
 	SCSI_LOG_TIMEOUT(3, printk("sg_release: %s\n", sdp->disk->disk_name));
 
-	set_exclude(sdp, 0);
+	clear_exclude(sdp);
 	wake_up_interruptible(&sdp->o_excl_wait);
 
 	scsi_autopm_put_device(sdp->device);
@@ -1414,6 +1432,7 @@ static Sg_device *sg_alloc(struct gendisk *disk, struct scsi_device *scsidp)
 	disk->first_minor = k;
 	sdp->disk = disk;
 	sdp->device = scsidp;
+	spin_lock_init(&sdp->sfd_lock);
 	INIT_LIST_HEAD(&sdp->sfds);
 	init_waitqueue_head(&sdp->o_excl_wait);
 	sdp->sg_tablesize = queue_max_segments(q);
@@ -1557,10 +1576,12 @@ static void sg_remove(struct device *cl_dev, struct class_interface *cl_intf)
 	/* Need a write lock to set sdp->detached. */
 	write_lock_irqsave(&sg_index_lock, iflags);
 	sdp->detached = 1;
+	spin_lock(&sdp->sfd_lock);
 	list_for_each_entry(sfp, &sdp->sfds, sfd_siblings) {
 		wake_up_interruptible(&sfp->read_wait);
 		kill_fasync(&sfp->async_qp, SIGPOLL, POLL_HUP);
 	}
+	spin_unlock(&sdp->sfd_lock);
 	write_unlock_irqrestore(&sg_index_lock, iflags);
 
 	sysfs_remove_link(&scsidp->sdev_gendev.kobj, "generic");
@@ -2085,9 +2106,12 @@ sg_add_sfp(Sg_device * sdp, int dev)
 	sfp->cmd_q = SG_DEF_COMMAND_Q;
 	sfp->keep_orphan = SG_DEF_KEEP_ORPHAN;
 	sfp->parentdp = sdp;
-	write_lock_irqsave(&sg_index_lock, iflags);
+	spin_lock_irqsave(&sdp->sfd_lock, iflags);
 	list_add_tail(&sfp->sfd_siblings, &sdp->sfds);
-	write_unlock_irqrestore(&sg_index_lock, iflags);
+	sdp->toopen--; /* ongoing open is complete */
+	if (sdp->toopen == INT_MAX-1)
+		wake_up_interruptible(&sdp->o_excl_wait);
+	spin_unlock_irqrestore(&sdp->sfd_lock, iflags);
 	SCSI_LOG_TIMEOUT(3, printk("sg_add_sfp: sfp=0x%p\n", sfp));
 	if (unlikely(sg_big_buff != def_reserved_size))
 		sg_big_buff = def_reserved_size;
@@ -2137,9 +2161,9 @@ static void sg_remove_sfp(struct kref *kref)
 	struct sg_device *sdp = sfp->parentdp;
 	unsigned long iflags;
 
-	write_lock_irqsave(&sg_index_lock, iflags);
+	spin_lock_irqsave(&sdp->sfd_lock, iflags);
 	list_del(&sfp->sfd_siblings);
-	write_unlock_irqrestore(&sg_index_lock, iflags);
+	spin_unlock_irqrestore(&sdp->sfd_lock, iflags);
 	wake_up_interruptible(&sdp->o_excl_wait);
 
 	INIT_WORK(&sfp->ew.work, sg_remove_sfp_usercontext);
@@ -2531,7 +2555,7 @@ static int sg_proc_seq_show_devstrs(struct seq_file *s, void *v)
 	return 0;
 }
 
-/* must be called while holding sg_index_lock */
+/* must be called while holding sg_index_lock and sfd_lock */
 static void sg_proc_debug_helper(struct seq_file *s, Sg_device * sdp)
 {
 	int k, m, new_interface, blen, usg;
@@ -2616,22 +2640,26 @@ static int sg_proc_seq_show_debug(struct seq_file *s, void *v)
 
 	read_lock_irqsave(&sg_index_lock, iflags);
 	sdp = it ? sg_lookup_dev(it->index) : NULL;
-	if (sdp && !list_empty(&sdp->sfds)) {
-		struct scsi_device *scsidp = sdp->device;
+	if (sdp) {
+		spin_lock(&sdp->sfd_lock);
+		if (!list_empty(&sdp->sfds)) {
+			struct scsi_device *scsidp = sdp->device;
 
-		seq_printf(s, " >>> device=%s ", sdp->disk->disk_name);
-		if (sdp->detached)
-			seq_printf(s, "detached pending close ");
-		else
-			seq_printf
-			    (s, "scsi%d chan=%d id=%d lun=%d   em=%d",
-			     scsidp->host->host_no,
-			     scsidp->channel, scsidp->id,
-			     scsidp->lun,
-			     scsidp->host->hostt->emulated);
-		seq_printf(s, " sg_tablesize=%d excl=%d\n",
-			   sdp->sg_tablesize, get_exclude(sdp));
-		sg_proc_debug_helper(s, sdp);
+			seq_printf(s, " >>> device=%s ", sdp->disk->disk_name);
+			if (sdp->detached)
+				seq_printf(s, "detached pending close ");
+			else
+				seq_printf
+					(s, "scsi%d chan=%d id=%d lun=%d   em=%d",
+					 scsidp->host->host_no,
+					 scsidp->channel, scsidp->id,
+					 scsidp->lun,
+					 scsidp->host->hostt->emulated);
+			seq_printf(s, " sg_tablesize=%d excl=%d\n",
+					sdp->sg_tablesize, sdp->exclude);
+			sg_proc_debug_helper(s, sdp);
+		}
+		spin_unlock(&sdp->sfd_lock);
 	}
 	read_unlock_irqrestore(&sg_index_lock, iflags);
 	return 0;
-- 
1.7.11.7