Btrfs: use rcu to protect device->name

Al pointed out that we can just toss out the old name on a device and add a
new one arbitrarily, so anybody who uses device->name in printk could
possibly use free'd memory.  Instead of adding locking around all of this he
suggested doing it with RCU, so I've introduced a struct rcu_string that
does just that and have gone through and protected all accesses to
device->name that aren't under the uuid_mutex with rcu_read_lock().  This
protects us and I will use it for dealing with removing the device that we
used to mount the file system in a later patch.  Thanks,

Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <josef@redhat.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 5a3bf323e2bd..1c9664b16cd1 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -44,6 +44,7 @@
 #include "free-space-cache.h"
 #include "inode-map.h"
 #include "check-integrity.h"
+#include "rcu-string.h"
 
 static struct extent_io_ops btree_extent_io_ops;
 static void end_workqueue_fn(struct btrfs_work *work);
@@ -2575,8 +2576,9 @@ static void btrfs_end_buffer_write_sync(struct buffer_head *bh, int uptodate)
                 struct btrfs_device *device = (struct btrfs_device *)
                         bh->b_private;
 
-                printk_ratelimited(KERN_WARNING "lost page write due to "
-                                   "I/O error on %s\n", device->name);
+                printk_ratelimited_in_rcu(KERN_WARNING "lost page write due to "
+                                          "I/O error on %s\n",
+                                          rcu_str_deref(device->name));
                 /* note, we dont' set_buffer_write_io_error because we have
                  * our own ways of dealing with the IO errors
                  */
@@ -2749,8 +2751,8 @@ static int write_dev_flush(struct btrfs_device *device, int wait)
                 wait_for_completion(&device->flush_wait);
 
                 if (bio_flagged(bio, BIO_EOPNOTSUPP)) {
-                        printk("btrfs: disabling barriers on dev %s\n",
-                               device->name);
+                        printk_in_rcu("btrfs: disabling barriers on dev %s\n",
+                                      rcu_str_deref(device->name));
                         device->nobarriers = 1;
                 }
                 if (!bio_flagged(bio, BIO_UPTODATE)) {