[PATCH AUTOSEL 5.4 05/19] net/smc: Transfer remaining wait queue entries during fallback

From: Sasha Levin
Date: Thu Nov 25 2021 - 21:40:43 EST


From: Wen Gu <guwen@xxxxxxxxxxxxxxxxx>

[ Upstream commit 2153bd1e3d3dbf6a3403572084ef6ed31c53c5f0 ]

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@xxxxxxxxxxxxxxxxx>
Reviewed-by: Tony Lu <tonylu@xxxxxxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
net/smc/af_smc.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)

diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
index 6b0f09c5b195f..1fad05d6dd9ad 100644
--- a/net/smc/af_smc.c
+++ b/net/smc/af_smc.c
@@ -467,12 +467,26 @@ static void smc_link_save_peer_info(struct smc_link *link,

static void smc_switch_to_fallback(struct smc_sock *smc)
{
+ 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;
if (smc->sk.sk_socket && smc->sk.sk_socket->file) {
smc->clcsock->file = smc->sk.sk_socket->file;
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);
}
}

--
2.33.0