net/smc: Transfer remaining wait queue entries during fallback

The SMC fallback is incomplete currently. There may be some
wait queue entries remaining in smc socket->wq, which should
be removed to clcsocket->wq during the fallback.

For example, in nginx/wrk benchmark, this issue causes an
all-zeros test result:

server: nginx -g 'daemon off;'
client: smc_run wrk -c 1 -t 1 -d 5 http://11.200.15.93/index.html

  Running 5s test @ http://11.200.15.93/index.html
     1 threads and 1 connections
     Thread Stats   Avg      Stdev     Max   ± Stdev
     	Latency     0.00us    0.00us   0.00us    -nan%
	Req/Sec     0.00      0.00     0.00      -nan%
	0 requests in 5.00s, 0.00B read
     Requests/sec:      0.00
     Transfer/sec:       0.00B

The reason for this all-zeros result is that when wrk used SMC
to replace TCP, it added an eppoll_entry into smc socket->wq
and expected to be notified if epoll events like EPOLL_IN/
EPOLL_OUT occurred on the smc socket.

However, once a fallback occurred, wrk switches to use clcsocket.
Now it is clcsocket->wq instead of smc socket->wq which will
be woken up. The eppoll_entry remaining in smc socket->wq does
not work anymore and wrk stops the test.

This patch fixes this issue by removing remaining wait queue
entries from smc socket->wq to clcsocket->wq during the fallback.

Link: https://www.spinics.net/lists/netdev/msg779769.html
Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
Reviewed-by: Tony Lu <tonylu@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: Wen Gu

net/smc/af_smc.c

@@ -566,6 +566,10 @@ static void smc_stat_fallback(struct smc_sock *smc)
 
 static void smc_switch_to_fallback(struct smc_sock *smc, int reason_code)
 {
+	wait_queue_head_t *smc_wait = sk_sleep(&smc->sk);
+	wait_queue_head_t *clc_wait = sk_sleep(smc->clcsock->sk);
+	unsigned long flags;
+
 	smc->use_fallback = true;
 	smc->fallback_rsn = reason_code;
 	smc_stat_fallback(smc);
@@ -575,6 +579,16 @@ static void smc_switch_to_fallback(struct smc_sock *smc, int reason_code)
 		smc->clcsock->file->private_data = smc->clcsock;
 		smc->clcsock->wq.fasync_list =
 			smc->sk.sk_socket->wq.fasync_list;
+
+		/* There may be some entries remaining in
+		 * smc socket->wq, which should be removed
+		 * to clcsocket->wq during the fallback.
+		 */
+		spin_lock_irqsave(&smc_wait->lock, flags);
+		spin_lock(&clc_wait->lock);
+		list_splice_init(&smc_wait->head, &clc_wait->head);
+		spin_unlock(&clc_wait->lock);
+		spin_unlock_irqrestore(&smc_wait->lock, flags);
 	}
 }