[PATCH net 2/5] net/smc: fix incorrect barrier usage

["D. Wythe" <alibuda@linux.alibaba.com>]

From: "D. Wythe" <alibuda@linux.alibaba.com>

This patch add explicit CPU barrier to ensure memory
consistency rather than compiler barrier.

Besides, the atomicity between READ_ONCE and cmpxhcg cannot
be guaranteed, so we need to use atomic ops. The simple way
is to replace READ_ONCE with xchg.

Fixes: 475f9ff63ee8 ("net/smc: fix application data exception")
Co-developed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>
Links: https://lore.kernel.org/netdev/1b7c95be-d3d9-53c3-3152-cd835314d37c@linux.ibm.com/T/
---
 net/smc/smc_core.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
index d520ee6..cc7d72e 100644
--- a/net/smc/smc_core.c
+++ b/net/smc/smc_core.c
@@ -1133,9 +1133,10 @@ static void smcr_buf_unuse(struct smc_buf_desc *buf_desc, bool is_rmb,
 
 		smc_buf_free(lgr, is_rmb, buf_desc);
 	} else {
-		/* memzero_explicit provides potential memory barrier semantics */
-		memzero_explicit(buf_desc->cpu_addr, buf_desc->len);
-		WRITE_ONCE(buf_desc->used, 0);
+		memset(buf_desc->cpu_addr, 0, buf_desc->len);
+		/* make sure buf_desc->used not be reordered ahead */
+		smp_mb__before_atomic();
+		xchg(&buf_desc->used, 0);
 	}
 }
 
@@ -1146,17 +1147,21 @@ static void smc_buf_unuse(struct smc_connection *conn,
 		if (!lgr->is_smcd && conn->sndbuf_desc->is_vm) {
 			smcr_buf_unuse(conn->sndbuf_desc, false, lgr);
 		} else {
-			memzero_explicit(conn->sndbuf_desc->cpu_addr, conn->sndbuf_desc->len);
-			WRITE_ONCE(conn->sndbuf_desc->used, 0);
+			memset(conn->sndbuf_desc->cpu_addr, 0, conn->sndbuf_desc->len);
+			/* make sure buf_desc->used not be reordered ahead */
+			smp_mb__before_atomic();
+			xchg(&conn->sndbuf_desc->used, 0);
 		}
 	}
 	if (conn->rmb_desc) {
 		if (!lgr->is_smcd) {
 			smcr_buf_unuse(conn->rmb_desc, true, lgr);
 		} else {
-			memzero_explicit(conn->rmb_desc->cpu_addr,
-					 conn->rmb_desc->len + sizeof(struct smcd_cdc_msg));
-			WRITE_ONCE(conn->rmb_desc->used, 0);
+			memset(conn->rmb_desc->cpu_addr, 0,
+			       conn->rmb_desc->len + sizeof(struct smcd_cdc_msg));
+			/* make sure buf_desc->used not be reordered ahead */
+			smp_mb__before_atomic();
+			xchg(&conn->rmb_desc->used, 0);
 		}
 	}
 }
-- 
1.8.3.1