linux-kernel - [PATCH v2] fs/dcache: optimize start_dir

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Date:   Thu, 29 Oct 2020 12:07:48 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Dave Chinner <david@...morbit.com>
Cc:     viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        peterz@...radead.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: [PATCH v2] fs/dcache: optimize start_dir_add()

Considering both end_dir_add() and d_alloc_parallel(), the
dir->i_dir_seq wants acquire/release semantics, therefore
micro-optimize for ll/sc archs. Also add READ_ONCE around
the variable mostly for documentation purposes - either
the successful cmpxchg or the pause will avoid the tearing).

Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
---
 fs/dcache.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index ea0485861d93..9177f0d08a5a 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2504,8 +2504,8 @@ static inline unsigned start_dir_add(struct inode *dir)
 {

	for (;;) {
-		unsigned n = dir->i_dir_seq;
-		if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
+		unsigned n = READ_ONCE(dir->i_dir_seq);
+		if (!(n & 1) && cmpxchg_acquire(&dir->i_dir_seq, n, n + 1) == n)
			return n;
		cpu_relax();
	}
--
2.26.2