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buffer: use try_cmpxchg in discard_buffer

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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 <ubizjak@gmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Uros Bizjak 2022-07-14 19:16:53 +02:00 committed by Andrew Morton
parent 693fc06e98
commit b0192296b4
1 changed files with 5 additions and 9 deletions
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14
fs/buffer.c
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@ -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);
}