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, 9 insertions, 7 deletions

diff --git a/fs/btrfs/check-integrity.c b/fs/btrfs/check-integrity.c
index 9cebb1fd6a3c..da6e9364a5e3 100644
--- a/fs/btrfs/check-integrity.c
+++ b/fs/btrfs/check-integrity.c
@@ -93,6 +93,7 @@
 #include "print-tree.h"
 #include "locking.h"
 #include "check-integrity.h"
+#include "rcu-string.h"
 
 #define BTRFSIC_BLOCK_HASHTABLE_SIZE 0x10000
 #define BTRFSIC_BLOCK_LINK_HASHTABLE_SIZE 0x10000
@@ -843,13 +844,14 @@ static int btrfsic_process_superblock_dev_mirror(
                 superblock_tmp->never_written = 0;
                 superblock_tmp->mirror_num = 1 + superblock_mirror_num;
                 if (state->print_mask & BTRFSIC_PRINT_MASK_SUPERBLOCK_WRITE)
-                        printk(KERN_INFO "New initial S-block (bdev %p, %s)"
-                               " @%llu (%s/%llu/%d)\n",
-                               superblock_bdev, device->name,
-                               (unsigned long long)dev_bytenr,
-                               dev_state->name,
-                               (unsigned long long)dev_bytenr,
-                               superblock_mirror_num);
+                        printk_in_rcu(KERN_INFO "New initial S-block (bdev %p, %s)"
+                                     " @%llu (%s/%llu/%d)\n",
+                                     superblock_bdev,
+                                     rcu_str_deref(device->name),
+                                     (unsigned long long)dev_bytenr,
+                                     dev_state->name,
+                                     (unsigned long long)dev_bytenr,
+                                     superblock_mirror_num);
                 list_add(&superblock_tmp->all_blocks_node,
                          &state->all_blocks_list);
                 btrfsic_block_hashtable_add(superblock_tmp,