linux-kernel - [PATCH] io_uring/rsrc: skip atomic refcount for uncloned buffers

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Message-ID: <20250619143435.3474028-1-csander@purestorage.com>
Date: Thu, 19 Jun 2025 08:34:34 -0600
From: Caleb Sander Mateos <csander@...estorage.com>
To: Jens Axboe <axboe@...nel.dk>
Cc: Caleb Sander Mateos <csander@...estorage.com>,
	io-uring@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] io_uring/rsrc: skip atomic refcount for uncloned buffers

io_buffer_unmap() performs an atomic decrement of the io_mapped_ubuf's
reference count in case it has been cloned into another io_ring_ctx's
registered buffer table. This is an expensive operation and unnecessary
in the common case that the io_mapped_ubuf is only registered once.
Load the reference count first and check whether it's 1. In that case,
skip the atomic decrement and immediately free the io_mapped_ubuf.

Signed-off-by: Caleb Sander Mateos <csander@...estorage.com>
---
 io_uring/rsrc.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index 94a9db030e0e..9a1f24a43035 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -133,12 +133,14 @@ static void io_free_imu(struct io_ring_ctx *ctx, struct io_mapped_ubuf *imu)
 		kvfree(imu);
 }
 
 static void io_buffer_unmap(struct io_ring_ctx *ctx, struct io_mapped_ubuf *imu)
 {
-	if (!refcount_dec_and_test(&imu->refs))
-		return;
+	if (unlikely(refcount_read(&imu->refs) > 1)) {
+		if (!refcount_dec_and_test(&imu->refs))
+			return;
+	}
 
 	if (imu->acct_pages)
 		io_unaccount_mem(ctx, imu->acct_pages);
 	imu->release(imu->priv);
 	io_free_imu(ctx, imu);
-- 
2.45.2