From b0192296b45232872720d969366449c001ab1f4a Mon Sep 17 00:00:00 2001 From: Uros Bizjak Date: Thu, 14 Jul 2022 19:16:53 +0200 Subject: [PATCH] buffer: use try_cmpxchg in discard_buffer Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in discard_buffer. 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, enabling further code simplifications. Note that the value from *ptr should be read using READ_ONCE to prevent the compiler from merging, refetching or reordering the read. No functional change intended. Link: https://lkml.kernel.org/r/20220714171653.12128-1-ubizjak@gmail.com Signed-off-by: Uros Bizjak Cc: Alexander Viro Signed-off-by: Andrew Morton --- fs/buffer.c | 14 +++++--------- 1 file changed, 5 insertions(+), 9 deletions(-) diff --git a/fs/buffer.c b/fs/buffer.c index 55e762a58eb6..2fd98bb7d74e 100644 --- a/fs/buffer.c +++ b/fs/buffer.c @@ -1464,19 +1464,15 @@ EXPORT_SYMBOL(set_bh_page); static void discard_buffer(struct buffer_head * bh) { - unsigned long b_state, b_state_old; + unsigned long b_state; lock_buffer(bh); clear_buffer_dirty(bh); bh->b_bdev = NULL; - b_state = bh->b_state; - for (;;) { - b_state_old = cmpxchg(&bh->b_state, b_state, - (b_state & ~BUFFER_FLAGS_DISCARD)); - if (b_state_old == b_state) - break; - b_state = b_state_old; - } + b_state = READ_ONCE(bh->b_state); + do { + } while (!try_cmpxchg(&bh->b_state, &b_state, + b_state & ~BUFFER_FLAGS_DISCARD)); unlock_buffer(bh); } -- 2.39.5