Commit b0192296 authored by Uros Bizjak's avatar Uros Bizjak Committed by Andrew Morton
Browse files

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: default avatarUros Bizjak <ubizjak@gmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 693fc06e

fs/buffer.c

+5 −9
Original line number Diff line number Diff line
@@ -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);

}