The patch titled
Subject: buffer: use try_cmpxchg in discard_buffer
has been added to the -mm mm-nonmm-unstable branch. Its filename is
buffer-use-try_cmpxchg-in-discard_buffer.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/buffer-use-try_cmpxchg-in-discard_buffer.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Uros Bizjak <ubizjak@xxxxxxxxx>
Subject: buffer: use try_cmpxchg in discard_buffer
Date: Thu, 14 Jul 2022 19:16:53 +0200
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@xxxxxxxxx
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/buffer.c | 14 +++++---------
1 file changed, 5 insertions(+), 9 deletions(-)
--- a/fs/buffer.c~buffer-use-try_cmpxchg-in-discard_buffer
+++ a/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);
}
_
Patches currently in -mm which might be from ubizjak@xxxxxxxxx are
llist-use-try_cmpxchg-in-llist_add_batch-and-llist_del_first.patch
epoll-use-try_cmpxchg-in-list_add_tail_lockless.patch
buffer-use-try_cmpxchg-in-discard_buffer.patch
