linux-kernel - [PATCH 4/4] fs/buffer: optimize discard

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Message-Id: <20250515173925.147823-5-dave@stgolabs.net>
Date: Thu, 15 May 2025 10:39:25 -0700
From: Davidlohr Bueso <dave@...olabs.net>
To: brauner@...nel.org
Cc: jack@...e.cz,
	viro@...iv.linux.org.uk,
	mcgrof@...nel.org,
	dave@...olabs.net,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 4/4] fs/buffer: optimize discard_buffer()

While invalidating, the clearing of the bits in discard_buffer()
is done in one fully ordered CAS operation. In the past this was
done via individual clear_bit(), until e7470ee89f0 (fs: buffer:
do not use unnecessary atomic operations when discarding buffers).
This implies that there were never strong ordering requirements
outside of being serialized by the buffer lock.

As such relax the ordering for archs that can benefit. Further,
the implied ordering in buffer_unlock() makes current cmpxchg
implied barrier redundant due to release semantics. And while in
theory the unlock could be part of the bulk clearing, it is
best to leave it explicit, but without the double barriers.

Signed-off-by: Davidlohr Bueso <dave@...olabs.net>
---
 fs/buffer.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 210b43574a10..f0fc78910abf 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1616,8 +1616,8 @@ static void discard_buffer(struct buffer_head * bh)
 	bh->b_bdev = NULL;
 	b_state = READ_ONCE(bh->b_state);
 	do {
-	} while (!try_cmpxchg(&bh->b_state, &b_state,
-			      b_state & ~BUFFER_FLAGS_DISCARD));
+	} while (!try_cmpxchg_relaxed(&bh->b_state, &b_state,
+				      b_state & ~BUFFER_FLAGS_DISCARD));
 	unlock_buffer(bh);
 }

-- 
2.39.5