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, 4 insertions, 3 deletions

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 2c8f7b204617..aaa12c1eb348 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -20,6 +20,7 @@
 #include "volumes.h"
 #include "check-integrity.h"
 #include "locking.h"
+#include "rcu-string.h"
 
 static struct kmem_cache *extent_state_cache;
 static struct kmem_cache *extent_buffer_cache;
@@ -1917,9 +1918,9 @@ int repair_io_failure(struct btrfs_mapping_tree *map_tree, u64 start,
                 return -EIO;
         }
 
-        printk(KERN_INFO "btrfs read error corrected: ino %lu off %llu (dev %s "
-                        "sector %llu)\n", page->mapping->host->i_ino, start,
-                        dev->name, sector);
+        printk_in_rcu(KERN_INFO "btrfs read error corrected: ino %lu off %llu "
+                      "(dev %s sector %llu)\n", page->mapping->host->i_ino,
+                      start, rcu_str_deref(dev->name), sector);
 
         bio_put(bio);
         return 0;