netdev - [PATCH 3/3] staging: qlge: add comment explaining memory barrier

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Date:   Mon, 31 Oct 2022 10:25:16 -0400
From:   drake@...ketalley.com
To:     Manish Chopra <manishc@...vell.com>, GR-Linux-NIC-Dev@...vell.com,
        Coiby Xu <coiby.xu@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        netdev@...r.kernel.org, linux-staging@...ts.linux.dev
Cc:     linux-kernel@...r.kernel.org, Drake Talley <drake@...ketalley.com>
Subject: [PATCH 3/3] staging: qlge: add comment explaining memory barrier

From: Drake Talley <drake@...ketalley.com>

codestyle change that fixes the following report from checkpatch:

> WARNING: memory barrier without comment
> #2101: FILE: drivers/staging/qlge/qlge_main.c:2101:

The added comment identifies the next item from the circular
buffer (rx_ring->curr_entry) and its handling/unmapping as the two
operations that must not be reordered.  Based on the kernel
documentation for memory barriers in circular buffers
(https://www.kernel.org/doc/Documentation/circular-buffers.txt) and
the presence of atomic operations in the current context I'm assuming
this usage of the memory barrier is akin to what is explained in the
linked doc.

There are a couple of other uncommented usages of memory barriers in
the current file.  If this comment is adequate I can add similar
comments to the others.

Signed-off-by: Drake Talley <drake@...ketalley.com>
---
 drivers/staging/qlge/qlge_main.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/staging/qlge/qlge_main.c b/drivers/staging/qlge/qlge_main.c
index c8403dbb5bad..f70390bce6d8 100644
--- a/drivers/staging/qlge/qlge_main.c
+++ b/drivers/staging/qlge/qlge_main.c
@@ -2098,6 +2098,12 @@ static int qlge_clean_outbound_rx_ring(struct rx_ring *rx_ring)
 			     rx_ring->cq_id, prod, rx_ring->cnsmr_idx);
 
 		net_rsp = (struct qlge_ob_mac_iocb_rsp *)rx_ring->curr_entry;
+		/*
+		 * Ensure that the next item from the ring buffer is loaded
+		 * before being processed.
+		 * Adding rmb() prevents the compiler from reordering the read
+		 * and subsequent handling of the outbound completion pointer.
+		 */
 		rmb();
 		switch (net_rsp->opcode) {
 		case OPCODE_OB_MAC_TSO_IOCB:
-- 
2.34.1