[PATCH v2 1/5] btrfs: merge blocking_writers branches in btrfs_tree_read_lock

David Sterba
2019-10-30T10:56:55Z

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.

Signed-off-by: David Sterba <dsterba@suse.com>
---
fs/btrfs/locking.c | 27 ++++++++++++++-------------
1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index 93146b495276..c84c650e56c7 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -128,20 +128,21 @@ void btrfs_tree_read_lock(struct extent_buffer *eb)
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);
--
2.23.0