Btrfs: fix race during transaction joins

While we're allocating ram for a new transaction, we drop our spinlock.
When we get the lock back, we do check to see if a transaction started
while we slept, but we don't check to make sure it isn't blocked
because a commit has already started.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 8 insertions, 5 deletions

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 29f782cc2cc9..960835eaf4da 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -55,6 +55,7 @@ static noinline int join_transaction(struct btrfs_root *root, int nofail)
         struct btrfs_transaction *cur_trans;
 
         spin_lock(&root->fs_info->trans_lock);
+loop:
         if (root->fs_info->trans_no_join) {
                 if (!nofail) {
                         spin_unlock(&root->fs_info->trans_lock);
@@ -75,16 +76,18 @@ static noinline int join_transaction(struct btrfs_root *root, int nofail)
         cur_trans = kmem_cache_alloc(btrfs_transaction_cachep, GFP_NOFS);
         if (!cur_trans)
                 return -ENOMEM;
+
         spin_lock(&root->fs_info->trans_lock);
         if (root->fs_info->running_transaction) {
+                /*
+                 * someone started a transaction after we unlocked.  Make sure
+                 * to redo the trans_no_join checks above
+                 */
                 kmem_cache_free(btrfs_transaction_cachep, cur_trans);
                 cur_trans = root->fs_info->running_transaction;
-                atomic_inc(&cur_trans->use_count);
-                atomic_inc(&cur_trans->num_writers);
-                cur_trans->num_joined++;
-                spin_unlock(&root->fs_info->trans_lock);
-                return 0;
+                goto loop;
         }
+
         atomic_set(&cur_trans->num_writers, 1);
         cur_trans->num_joined = 0;
         init_waitqueue_head(&cur_trans->writer_wait);