btrfs: merge blocking_writers branches in btrfs_tree_read_lock

There are two ifs that use eb::blocking_writers. As this is a variable
modified inside and outside of locks, we could minimize number of
accesses to avoid problems with getting different results at different
times.

The access here is locked so this can only race with btrfs_tree_unlock
that sets blocking_writers to 0 without lock and unsets the lock owner.

The first branch is taken only if the same thread already holds the
lock, the second if checks for blocking writers. Here we'd either unlock
and wait, or proceed. Both are valid states of the locking protocol.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index 93146b4952769b9d994e7ec9d8a46d65198c52d9..c84c650e56c77b0ca78d2f2d70fc587ad1fc8b57 100644 (file)
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -128,20 +128,21 @@ again:
        read_lock(&eb->lock);
        BUG_ON(eb->blocking_writers == 0 &&
               current->pid == eb->lock_owner);
-       if (eb->blocking_writers && current->pid == eb->lock_owner) {
-               /*
-                * This extent is already write-locked by our thread. We allow
-                * an additional read lock to be added because it's for the same
-                * thread. btrfs_find_all_roots() depends on this as it may be
-                * called on a partly (write-)locked tree.
-                */
-               BUG_ON(eb->lock_nested);
-               eb->lock_nested = true;
-               read_unlock(&eb->lock);
-               trace_btrfs_tree_read_lock(eb, start_ns);
-               return;
-       }
        if (eb->blocking_writers) {
+               if (current->pid == eb->lock_owner) {
+                       /*
+                        * This extent is already write-locked by our thread.
+                        * We allow an additional read lock to be added because
+                        * it's for the same thread. btrfs_find_all_roots()
+                        * depends on this as it may be called on a partly
+                        * (write-)locked tree.
+                        */
+                       BUG_ON(eb->lock_nested);
+                       eb->lock_nested = true;
+                       read_unlock(&eb->lock);
+                       trace_btrfs_tree_read_lock(eb, start_ns);
+                       return;
+               }
                read_unlock(&eb->lock);
                wait_event(eb->write_lock_wq,
                           eb->blocking_writers == 0);