[PATCH v3 3/8] fs: aio: Use acquire/release for ring->tail publication

[Jinjie Ruan <ruanjinjie@huawei.com>]

Replace the smp_wmb() + WRITE_ONCE(ring->tail) and READ_ONCE(ring->tail)
+ smp_rmb() barrier pair with smp_store_release()/smp_load_acquire()
on `ring->tail`.

This expresses the publish/subscribe pattern more clearly and allows
architectures with native acquire/release instructions (e.g. arm64's
STLR/LDAR) to avoid the cost of full one-way barriers (DMB ISHST/ISHLD).

The release ensures event data written before updating ring->tail is
visible to readers that observe the new tail value via acquire, which
is exactly the ordering the barrier pair provided.

No functional change intended.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Assisted-by: DeepSeek:DeepSeek-V3
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
---
 fs/aio.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index d78acc69f487..a0279e059e97 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1206,13 +1206,12 @@ static void aio_complete(struct aio_kiocb *iocb)
 	/* after flagging the request as done, we
 	 * must never even look at it again
 	 */
-	smp_wmb();	/* make event visible before updating tail */
-
 	ctx->tail = tail;
 
 	ring = folio_address(ctx->ring_folios[0]);
 	head = ring->head;
-	ring->tail = tail;
+	/* Make event visible before updating tail */
+	smp_store_release(&ring->tail, tail);
 	flush_dcache_folio(ctx->ring_folios[0]);
 
 	ctx->completed_events++;
@@ -1288,13 +1287,12 @@ static long aio_read_events_ring(struct kioctx *ctx,
 	/* Access to ->ring_folios here is protected by ctx->ring_lock. */
 	ring = folio_address(ctx->ring_folios[0]);
 	head = ring->head;
-	tail = ring->tail;
-
 	/*
 	 * Ensure that once we've read the current tail pointer, that
 	 * we also see the events that were stored up to the tail.
+	 * Pairs with smp_store_release() in aio_complete().
 	 */
-	smp_rmb();
+	tail = smp_load_acquire(&ring->tail);
 
 	pr_debug("h%u t%u m%u\n", head, tail, ctx->nr_events);
 
-- 
2.34.1