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>

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index c9265a603488cea14574dd1c8e95c679b650ab13..a4fe5494d01bdd55c7a07cf47f33f5de8c206c6c 100644 (file)
--- 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);