Btrfs: fix race with freeze and free space inodes

So we start our freeze, somebody comes in and does an fsync() on a file
where we have to commit a transaction for whatever reason, and we will
deadlock because the freeze is waiting on FS_FREEZE people to stop writing
to the file system, but the transaction is waiting for its free space inodes
to be written out, which are in turn waiting on sb_start_intwrite while
trying to write the file extents.  To fix this we'll just skip the
sb_start_intwrite() if we TRANS_JOIN_NOLOCK since we're being waited on by a
transaction commit so we're safe wrt to freeze and this will keep us from
deadlocking.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>

1 files changed, 11 insertions, 2 deletions

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index c9265a603488..a4fe5494d01b 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -342,7 +342,15 @@ again:
         if (!h)
                 return ERR_PTR(-ENOMEM);
 
-        if (!__sb_start_write(root->fs_info->sb, SB_FREEZE_FS, false)) {
+        /*
+         * If we are JOIN_NOLOCK we're already committing a transaction and
+         * waiting on this guy, so we don't need to do the sb_start_intwrite
+         * because we're already holding a ref.  We need this because we could
+         * have raced in and did an fsync() on a file which can kick a commit
+         * and then we deadlock with somebody doing a freeze.
+         */
+        if (type != TRANS_JOIN_NOLOCK &&
+            !__sb_start_write(root->fs_info->sb, SB_FREEZE_FS, false)) {
                 if (type == TRANS_JOIN_FREEZE)
                         return ERR_PTR(-EPERM);
                 sb_start_intwrite(root->fs_info->sb);
@@ -601,7 +609,8 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
                 }
         }
 
-        sb_end_intwrite(root->fs_info->sb);
+        if (lock)
+                sb_end_intwrite(root->fs_info->sb);
 
         WARN_ON(cur_trans != info->running_transaction);
         WARN_ON(atomic_read(&cur_trans->num_writers) < 1);