Make zvol minor functionality more robust

Close the race window in zvol_open() to prevent removal of
zvol_state in the 'first open' code path. Move the call to
check_disk_change() under zvol_state_lock to make sure the
zvol_media_changed() and zvol_revalidate_disk() called by
check_disk_change() are invoked with positive zv_open_count.

Skip opened zvols when removing minors and set private_data
to NULL for zvols that are not in use whose minors are being
removed, to indicate to zvol_open() that the state is gone.
Skip opened zvols when renaming minors to avoid modifying
zv_name that might be in use, e.g. in zvol_ioctl().

Drop zvol_state_lock before calling add_disk() when creating
minors to avoid deadlocks with zvol_open().

Wrap dmu_objset_find() with spl_fstran_mark()/unmark().

Signed-off-by: Boris Protopopov <boris.protopopov@actifio.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Closes #4344

module/zfs/zvol.c

@@ -35,6 +35,7 @@
  * needs to be run before opening and using a device.
  *
  * Copyright 2014 Nexenta Systems, Inc.  All rights reserved.
+ * Copyright (c) 2016 Actifio, Inc. All rights reserved.
  */
 
 #include <sys/dbuf.h>
@@ -607,6 +608,8 @@ zvol_write(zvol_state_t *zv, uio_t *uio, boolean_t sync)
 	rl_t *rl;
 	int error = 0;
 
+	ASSERT(zv && zv->zv_open_count > 0);
+
 	rl = zfs_range_lock(&zv->zv_znode, uio->uio_loffset, uio->uio_resid,
 	    RL_WRITER);
 
@@ -675,6 +678,8 @@ zvol_discard(struct bio *bio)
 	rl_t *rl;
 	dmu_tx_t *tx;
 
+	ASSERT(zv && zv->zv_open_count > 0);
+
 	if (end > zv->zv_volsize)
 		return (SET_ERROR(EIO));
 
@@ -722,6 +727,8 @@ zvol_read(zvol_state_t *zv, uio_t *uio)
 	rl_t *rl;
 	int error = 0;
 
+	ASSERT(zv && zv->zv_open_count > 0);
+
 	rl = zfs_range_lock(&zv->zv_znode, uio->uio_loffset, uio->uio_resid,
 	    RL_READER);
 	while (uio->uio_resid > 0 && uio->uio_loffset < volsize) {
@@ -1020,7 +1027,7 @@ zvol_last_close(zvol_state_t *zv)
 static int
 zvol_open(struct block_device *bdev, fmode_t flag)
 {
-	zvol_state_t *zv = bdev->bd_disk->private_data;
+	zvol_state_t *zv;
 	int error = 0, drop_mutex = 0;
 
 	/*
@@ -1034,7 +1041,17 @@ zvol_open(struct block_device *bdev, fmode_t flag)
 		drop_mutex = 1;
 	}
 
-	ASSERT3P(zv, !=, NULL);
+	/*
+	 * Obtain a copy of private_data under the lock to make sure
+	 * that either the result of zvol_freeg() setting
+	 * bdev->bd_disk->private_data to NULL is observed, or zvol_free()
+	 * is not called on this zv because of the positive zv_open_count.
+	 */
+	zv = bdev->bd_disk->private_data;
+	if (zv == NULL) {
+		error = -ENXIO;
+		goto out_mutex;
+	}
 
 	if (zv->zv_open_count == 0) {
 		error = zvol_first_open(zv);
@@ -1049,6 +1066,8 @@ zvol_open(struct block_device *bdev, fmode_t flag)
 
 	zv->zv_open_count++;
 
+	check_disk_change(bdev);
+
 out_open_count:
 	if (zv->zv_open_count == 0)
 		zvol_last_close(zv);
@@ -1057,8 +1076,6 @@ out_mutex:
 	if (drop_mutex)
 		mutex_exit(&zvol_state_lock);
 
-	check_disk_change(bdev);
-
 	return (SET_ERROR(error));
 }
 
@@ -1072,16 +1089,16 @@ zvol_release(struct gendisk *disk, fmode_t mode)
 	zvol_state_t *zv = disk->private_data;
 	int drop_mutex = 0;
 
+	ASSERT(zv && zv->zv_open_count > 0);
+
 	if (!mutex_owned(&zvol_state_lock)) {
 		mutex_enter(&zvol_state_lock);
 		drop_mutex = 1;
 	}
 
-	if (zv->zv_open_count > 0) {
 	zv->zv_open_count--;
 	if (zv->zv_open_count == 0)
 		zvol_last_close(zv);
-	}
 
 	if (drop_mutex)
 		mutex_exit(&zvol_state_lock);
@@ -1098,8 +1115,7 @@ zvol_ioctl(struct block_device *bdev, fmode_t mode,
 	zvol_state_t *zv = bdev->bd_disk->private_data;
 	int error = 0;
 
-	if (zv == NULL)
+	ASSERT(zv && zv->zv_open_count > 0);
-		return (SET_ERROR(-ENXIO));
 
 	switch (cmd) {
 	case BLKFLSBUF:
@@ -1133,6 +1149,8 @@ static int zvol_media_changed(struct gendisk *disk)
 {
 	zvol_state_t *zv = disk->private_data;
 
+	ASSERT(zv && zv->zv_open_count > 0);
+
 	return (zv->zv_changed);
 }
 
@@ -1140,6 +1158,8 @@ static int zvol_revalidate_disk(struct gendisk *disk)
 {
 	zvol_state_t *zv = disk->private_data;
 
+	ASSERT(zv && zv->zv_open_count > 0);
+
 	zv->zv_changed = 0;
 	set_capacity(zv->zv_disk, zv->zv_volsize >> 9);
 
@@ -1156,7 +1176,11 @@ static int
 zvol_getgeo(struct block_device *bdev, struct hd_geometry *geo)
 {
 	zvol_state_t *zv = bdev->bd_disk->private_data;
-	sector_t sectors = get_capacity(zv->zv_disk);
+	sector_t sectors;
+
+	ASSERT(zv && zv->zv_open_count > 0);
+
+	sectors = get_capacity(zv->zv_disk);
 
 	if (sectors > 2048) {
 		geo->heads = 16;
@@ -1312,9 +1336,14 @@ out_kmem:
 static void
 zvol_free(zvol_state_t *zv)
 {
+	ASSERT(MUTEX_HELD(&zvol_state_lock));
+	ASSERT(zv->zv_open_count == 0);
+
 	avl_destroy(&zv->zv_znode.z_range_avl);
 	mutex_destroy(&zv->zv_znode.z_range_lock);
 
+	zv->zv_disk->private_data = NULL;
+
 	del_gendisk(zv->zv_disk);
 	blk_cleanup_queue(zv->zv_queue);
 	put_disk(zv->zv_disk);
@@ -1448,7 +1477,15 @@ out:
 
 	if (error == 0) {
 		zvol_insert(zv);
+		/*
+		 * Drop the lock to prevent deadlock with sys_open() ->
+		 * zvol_open(), which first takes bd_disk->bd_mutex and then
+		 * takes zvol_state_lock, whereas this code path first takes
+		 * zvol_state_lock, and then takes bd_disk->bd_mutex.
+		 */
+		mutex_exit(&zvol_state_lock);
 		add_disk(zv->zv_disk);
+		mutex_enter(&zvol_state_lock);
 	}
 
 	return (SET_ERROR(error));
@@ -1545,10 +1582,15 @@ int
 zvol_create_minors(const char *name)
 {
 	int error = 0;
+	fstrans_cookie_t cookie;
 
-	if (!zvol_inhibit_dev)
+	if (zvol_inhibit_dev)
+		return (0);
+
+	cookie = spl_fstrans_mark();
 	error = dmu_objset_find((char *)name, zvol_create_minors_cb,
 	    NULL, DS_FIND_CHILDREN | DS_FIND_SNAPSHOTS);
+	spl_fstrans_unmark(cookie);
 
 	return (SET_ERROR(error));
 }
@@ -1572,7 +1614,13 @@ zvol_remove_minors(const char *name)
 
 		if (name == NULL || strcmp(zv->zv_name, name) == 0 ||
 		    (strncmp(zv->zv_name, name, namelen) == 0 &&
-		    zv->zv_name[namelen] == '/')) {
+		    (zv->zv_name[namelen] == '/' ||
+		    zv->zv_name[namelen] == '@'))) {
+
+			/* If in use, leave alone */
+			if (zv->zv_open_count > 0)
+				continue;
+
 			zvol_remove(zv);
 			zvol_free(zv);
 		}
@@ -1603,6 +1651,10 @@ zvol_rename_minors(const char *oldname, const char *newname)
 	for (zv = list_head(&zvol_state_list); zv != NULL; zv = zv_next) {
 		zv_next = list_next(&zvol_state_list, zv);
 
+		/* If in use, leave alone */
+		if (zv->zv_open_count > 0)
+			continue;
+
 		if (strcmp(zv->zv_name, oldname) == 0) {
 			__zvol_rename_minor(zv, newname);
 		} else if (strncmp(zv->zv_name, oldname, oldnamelen) == 0 &&
@@ -1643,8 +1695,17 @@ snapdev_snapshot_changed_cb(const char *dsname, void *arg) {
 
 int
 zvol_set_snapdev(const char *dsname, uint64_t snapdev) {
+	fstrans_cookie_t cookie;
+
+	if (zvol_inhibit_dev)
+		/* caller should continue to modify snapdev property */
+		return (-1);
+
+	cookie = spl_fstrans_mark();
 	(void) dmu_objset_find((char *) dsname, snapdev_snapshot_changed_cb,
 		&snapdev, DS_FIND_SNAPSHOTS | DS_FIND_CHILDREN);
+	spl_fstrans_unmark(cookie);
+
 	/* caller should continue to modify snapdev property */
 	return (-1);
 }