3 files changed, 15 insertions, 1 deletions

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 030012e1710c..72df63b0c799 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1928,12 +1928,24 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
         if (file->private_data)
                 btrfs_ioctl_trans_end(file);
 
+        /*
+         * We use start here because we will need to wait on the IO to complete
+         * in btrfs_sync_log, which could require joining a transaction (for
+         * example checking cross references in the nocow path).  If we use join
+         * here we could get into a situation where we're waiting on IO to
+         * happen that is blocked on a transaction trying to commit.  With start
+         * we inc the extwriter counter, so we wait for all extwriters to exit
+         * before we start blocking join'ers.  This comment is to keep somebody
+         * from thinking they are super smart and changing this to
+         * btrfs_join_transaction *cough*Josef*cough*.
+         */
         trans = btrfs_start_transaction(root, 0);
         if (IS_ERR(trans)) {
                 ret = PTR_ERR(trans);
                 mutex_unlock(&inode->i_mutex);
                 goto out;
         }
+        trans->sync = true;
 
         ret = btrfs_log_dentry_safe(trans, root, dentry);
         if (ret < 0) {
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index da2ac4c6d78b..b16352ce0f73 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -474,6 +474,7 @@ again:
         h->type = type;
         h->allocating_chunk = false;
         h->reloc_reserved = false;
+        h->sync = false;
         INIT_LIST_HEAD(&h->qgroup_ref_list);
         INIT_LIST_HEAD(&h->new_bgs);
 
@@ -713,7 +714,7 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
                 btrfs_create_pending_block_groups(trans, root);
 
         trans->delayed_ref_updates = 0;
-        if (btrfs_should_throttle_delayed_refs(trans, root)) {
+        if (!trans->sync && btrfs_should_throttle_delayed_refs(trans, root)) {
                 cur = max_t(unsigned long, cur, 1);
                 trans->delayed_ref_updates = 0;
                 btrfs_run_delayed_refs(trans, root, cur);
diff --git a/fs/btrfs/transaction.h b/fs/btrfs/transaction.h
index d05b6013fea8..6ac037e9f9f0 100644
--- a/fs/btrfs/transaction.h
+++ b/fs/btrfs/transaction.h
@@ -93,6 +93,7 @@ struct btrfs_trans_handle {
         short adding_csums;
         bool allocating_chunk;
         bool reloc_reserved;
+        bool sync;
         unsigned int type;
         /*
          * this root is only needed to validate that the root passed to