Btrfs: fix race between fsync and lockless direct IO writes

An fsync, using the fast path, can race with a concurrent lockless direct
IO write and end up logging a file extent item that points to an extent
that wasn't written to yet. This is because the fast fsync path collects
ordered extents into a local list and then collects all the new extent
maps to log file extent items based on them, while the direct IO write
path creates the new extent map before it creates the corresponding
ordered extent (and submitting the respective bio(s)).

So fix this by making the direct IO write path create ordered extents
before the extent maps and make the fast fsync path collect any new
ordered extents after it collects the extent maps.
Note that making the fsync handler call inode_dio_wait() (after acquiring
the inode's i_mutex) would not work and lead to a deadlock when doing
AIO, as through AIO we end up in a path where the fsync handler is called
(through dio_aio_complete_work() -> dio_complete() -> vfs_fsync_range())
before the inode's dio counter is decremented (inode_dio_wait() waits
for this counter to have a value of zero).

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 11 insertions, 3 deletions

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 323e12cc9d2f..978c3a810893 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -4127,7 +4127,9 @@ static int btrfs_log_changed_extents(struct btrfs_trans_handle *trans,
                                      struct inode *inode,
                                      struct btrfs_path *path,
                                      struct list_head *logged_list,
-                                     struct btrfs_log_ctx *ctx)
+                                     struct btrfs_log_ctx *ctx,
+                                     const u64 start,
+                                     const u64 end)
 {
         struct extent_map *em, *n;
         struct list_head extents;
@@ -4166,7 +4168,13 @@ static int btrfs_log_changed_extents(struct btrfs_trans_handle *trans,
         }
 
         list_sort(NULL, &extents, extent_cmp);
-
+        /*
+         * Collect any new ordered extents within the range. This is to
+         * prevent logging file extent items without waiting for the disk
+         * location they point to being written. We do this only to deal
+         * with races against concurrent lockless direct IO writes.
+         */
+        btrfs_get_logged_extents(inode, logged_list, start, end);
 process:
         while (!list_empty(&extents)) {
                 em = list_entry(extents.next, struct extent_map, list);
@@ -4701,7 +4709,7 @@ log_extents:
                         goto out_unlock;
                 }
                 ret = btrfs_log_changed_extents(trans, root, inode, dst_path,
-                                                &logged_list, ctx);
+                                                &logged_list, ctx, start, end);
                 if (ret) {
                         err = ret;
                         goto out_unlock;