bcachefs: Use try_cmpxchg() family of functions instead of cmpxchg()

Use try_cmpxchg() family of functions instead of
cmpxchg (*ptr, old, new) == old. x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg
(and related move instruction in front of cmpxchg).

Also, try_cmpxchg() implicitly assigns old *ptr value to "old" when
cmpxchg fails. There is no need to re-read the value in the loop.

No functional change intended.

Signed-off-by: Uros Bizjak <[email protected]>
Cc: Kent Overstreet <[email protected]>
Cc: Brian Foster <[email protected]>
Signed-off-by: Kent Overstreet <[email protected]>

1 files changed, 4 insertions, 4 deletions

diff --git a/fs/bcachefs/btree_trans_commit.c b/fs/bcachefs/btree_trans_commit.c
index 74e1ff225674..5e67dcb30f33 100644
--- a/fs/bcachefs/btree_trans_commit.c
+++ b/fs/bcachefs/btree_trans_commit.c
@@ -228,14 +228,14 @@ static int __btree_node_flush(struct journal *j, struct journal_entry_pin *pin,
 	struct btree_write *w = container_of(pin, struct btree_write, journal);
 	struct btree *b = container_of(w, struct btree, writes[i]);
 	struct btree_trans *trans = bch2_trans_get(c);
-	unsigned long old, new, v;
+	unsigned long old, new;
 	unsigned idx = w - b->writes;
 
 	btree_node_lock_nopath_nofail(trans, &b->c, SIX_LOCK_read);
-	v = READ_ONCE(b->flags);
 
+	old = READ_ONCE(b->flags);
 	do {
-		old = new = v;
+		new = old;
 
 		if (!(old & (1 << BTREE_NODE_dirty)) ||
 		    !!(old & (1 << BTREE_NODE_write_idx)) != idx ||
@@ -245,7 +245,7 @@ static int __btree_node_flush(struct journal *j, struct journal_entry_pin *pin,
 		new &= ~BTREE_WRITE_TYPE_MASK;
 		new |= BTREE_WRITE_journal_reclaim;
 		new |= 1 << BTREE_NODE_need_write;
-	} while ((v = cmpxchg(&b->flags, old, new)) != old);
+	} while (!try_cmpxchg(&b->flags, &old, new));
 
 	btree_node_write_if_need(c, b, SIX_LOCK_read);
 	six_unlock_read(&b->c.lock);