net/smc: fix application data exception

There is a certain probability that following
exceptions will occur in the wrk benchmark test:

Running 10s test @ http://11.213.45.6:80
8 threads and 64 connections
Thread Stats Avg Stdev Max +/- Stdev
Latency 3.72ms 13.94ms 245.33ms 94.17%
Req/Sec 1.96k 713.67 5.41k 75.16%
155262 requests in 10.10s, 23.10MB read
Non-2xx or 3xx responses: 3

We will find that the error is HTTP 400 error, which is a serious
exception in our test, which means the application data was
corrupted.

Consider the following scenarios:

CPU0 CPU1

buf_desc->used = 0;
cmpxchg(buf_desc->used, 0, 1)
deal_with(buf_desc)

memset(buf_desc->cpu_addr,0);

This will cause the data received by a victim connection to be cleared,
thus triggering an HTTP 400 error in the server.

This patch exchange the order between clear used and memset, add
barrier to ensure memory consistency.

Fixes: 1c5526968e27 ("net/smc: Clear memory when release and reuse buffer")
Signed-off-by: D. Wythe <[e-mail]>
Reviewed-by: Wenjia Zhang <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/smc/smc_core.c

···
 
 		smc_buf_free(lgr, is_rmb, buf_desc);
 	} else {
-		buf_desc->used = 0;
-		memset(buf_desc->cpu_addr, 0, buf_desc->len);
+		/* memzero_explicit provides potential memory barrier semantics */
+		memzero_explicit(buf_desc->cpu_addr, buf_desc->len);
+		WRITE_ONCE(buf_desc->used, 0);
 	}
 }
 
···
 		if (!lgr->is_smcd && conn->sndbuf_desc->is_vm) {
 			smcr_buf_unuse(conn->sndbuf_desc, false, lgr);
 		} else {
-			conn->sndbuf_desc->used = 0;
-			memset(conn->sndbuf_desc->cpu_addr, 0,
-			       conn->sndbuf_desc->len);
+			memzero_explicit(conn->sndbuf_desc->cpu_addr, conn->sndbuf_desc->len);
+			WRITE_ONCE(conn->sndbuf_desc->used, 0);
 		}
 	}
 	if (conn->rmb_desc) {
 		if (!lgr->is_smcd) {
 			smcr_buf_unuse(conn->rmb_desc, true, lgr);
 		} else {
-			conn->rmb_desc->used = 0;
-			memset(conn->rmb_desc->cpu_addr, 0,
-			       conn->rmb_desc->len +
-			       sizeof(struct smcd_cdc_msg));
+			memzero_explicit(conn->rmb_desc->cpu_addr,
+					 conn->rmb_desc->len + sizeof(struct smcd_cdc_msg));
+			WRITE_ONCE(conn->rmb_desc->used, 0);
 		}
 	}
 }