Btrfs: fix race in WAIT_SYNC ioctl

We check whether transid is already committed via last_trans_committed and
then search through trans_list for pending transactions.  If
last_trans_committed is updated by btrfs_commit_transaction after we check
it (there is no locking), we will fail to find the committed transaction
and return EINVAL to the caller.  This has been observed occasionally by
ceph-osd (which uses this ioctl heavily).

Fix by rechecking whether the provided transid <= last_trans_committed
after the search fails, and if so return 0.

Signed-off-by: Sage Weil <sage@redhat.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index a47b1000a6e5..86ef8d79c19f 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -609,7 +609,6 @@ int btrfs_wait_for_commit(struct btrfs_root *root, u64 transid)
                 if (transid <= root->fs_info->last_trans_committed)
                         goto out;
 
-                ret = -EINVAL;
                 /* find specified transaction */
                 spin_lock(&root->fs_info->trans_lock);
                 list_for_each_entry(t, &root->fs_info->trans_list, list) {
@@ -625,9 +624,16 @@ int btrfs_wait_for_commit(struct btrfs_root *root, u64 transid)
                         }
                 }
                 spin_unlock(&root->fs_info->trans_lock);
-                /* The specified transaction doesn't exist */
-                if (!cur_trans)
+
+                /*
+                 * The specified transaction doesn't exist, or we
+                 * raced with btrfs_commit_transaction
+                 */
+                if (!cur_trans) {
+                        if (transid > root->fs_info->last_trans_committed)
+                                ret = -EINVAL;
                         goto out;
+                }
         } else {
                 /* find newest transaction that is committing | committed */
                 spin_lock(&root->fs_info->trans_lock);