[PATCH net-next] RDS: fix rds-ping deadlock over TCP transport

[PATCH net-next] RDS: fix rds-ping deadlock over TCP transport

[Santosh Shilimkar]

Sowmini found hang with rds-ping while testing RDS over TCP. Its
a corner case and doesn't happen always. The issue is not reproducible
with IB transport. Its clear from below dump why we see it with RDS TCP.

 [<ffffffff8153b7e5>] do_tcp_setsockopt+0xb5/0x740
 [<ffffffff8153bec4>] tcp_setsockopt+0x24/0x30
 [<ffffffff814d57d4>] sock_common_setsockopt+0x14/0x20
 [<ffffffffa096071d>] rds_tcp_xmit_prepare+0x5d/0x70 [rds_tcp]
 [<ffffffffa093b5f7>] rds_send_xmit+0xd7/0x740 [rds]
 [<ffffffffa093bda2>] rds_send_pong+0x142/0x180 [rds]
 [<ffffffffa0939d34>] rds_recv_incoming+0x274/0x330 [rds]
 [<ffffffff810815ae>] ? ttwu_queue+0x11e/0x130
 [<ffffffff814dcacd>] ? skb_copy_bits+0x6d/0x2c0
 [<ffffffffa0960350>] rds_tcp_data_recv+0x2f0/0x3d0 [rds_tcp]
 [<ffffffff8153d836>] tcp_read_sock+0x96/0x1c0
 [<ffffffffa0960060>] ? rds_tcp_recv_init+0x40/0x40 [rds_tcp]
 [<ffffffff814d6a90>] ? sock_def_write_space+0xa0/0xa0
 [<ffffffffa09604d1>] rds_tcp_data_ready+0xa1/0xf0 [rds_tcp]
 [<ffffffff81545249>] tcp_data_queue+0x379/0x5b0
 [<ffffffffa0960cdb>] ? rds_tcp_write_space+0xbb/0x110 [rds_tcp]
 [<ffffffff81547fd2>] tcp_rcv_established+0x2e2/0x6e0
 [<ffffffff81552602>] tcp_v4_do_rcv+0x122/0x220
 [<ffffffff81553627>] tcp_v4_rcv+0x867/0x880
 [<ffffffff8152e0b3>] ip_local_deliver_finish+0xa3/0x220

This happens because rds_send_xmit() chain wants to take
sock_lock which is already taken by tcp_v4_rcv() on its
way to rds_tcp_data_ready(). Commit db6526dcb51b ("RDS: use
rds_send_xmit() state instead of RDS_LL_SEND_FULL") which
was trying to opportunistically finish the send request
in same thread context.

But because of above recursive lock hang with RDS TCP,
the send work from rds_send_pong() needs to deferred to
worker to avoid lock up. Given RDS ping is more of connectivity
test than performance critical path, its should be ok even
for transport like IB.

Reported-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
---
 net/rds/send.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/net/rds/send.c b/net/rds/send.c
index ee49c25..7a377a1 100644
--- a/net/rds/send.c
+++ b/net/rds/send.c
@@ -1182,9 +1182,8 @@ rds_send_pong(struct rds_connection *conn, __be16 dport)
 	rds_stats_inc(s_send_queued);
 	rds_stats_inc(s_send_pong);
 
-	ret = rds_send_xmit(conn);
-	if (ret == -ENOMEM || ret == -EAGAIN)
-		queue_delayed_work(rds_wq, &conn->c_send_w, 1);
+	if (!test_bit(RDS_LL_SEND_FULL, &conn->c_flags))
+		queue_delayed_work(rds_wq, &conn->c_send_w, 0);
 
 	rds_message_put(rm);
 	return 0;
-- 
1.9.1

Re: [PATCH net-next] RDS: fix rds-ping deadlock over TCP transport

[Sowmini Varadhan]

On (10/16/15 20:26), Santosh Shilimkar wrote:
> 
> diff --git a/net/rds/send.c b/net/rds/send.c
> +	if (!test_bit(RDS_LL_SEND_FULL, &conn->c_flags))
> +		queue_delayed_work(rds_wq, &conn->c_send_w, 0);

A minor note- it would help to add some comments here explaining 
that the pong has already been added to the sendq earlier.. 
in the case of IB, if RDS_LL_SEND_FULL has been set, it takes some
head-scratching to figure out how the pong gets sent, and a  few
comments could help clarify that.

but other than that, the contents look good to me, thus

Acked-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>

Re: [PATCH net-next] RDS: fix rds-ping deadlock over TCP transport

[santosh shilimkar]

On 10/16/2015 6:45 PM, Sowmini Varadhan wrote:
> On (10/16/15 20:26), Santosh Shilimkar wrote:
>>
>> diff --git a/net/rds/send.c b/net/rds/send.c
>> +	if (!test_bit(RDS_LL_SEND_FULL, &conn->c_flags))
>> +		queue_delayed_work(rds_wq, &conn->c_send_w, 0);
>
> A minor note- it would help to add some comments here explaining
> that the pong has already been added to the sendq earlier..
> in the case of IB, if RDS_LL_SEND_FULL has been set, it takes some
> head-scratching to figure out how the pong gets sent, and a  few
> comments could help clarify that.
>
Right. The check confused you. I will update the patch and drop
the check all together and add a jiffies for the queuing which
should take care of it. Will post v2 with that update.

> but other than that, the contents look good to me, thus
>
> Acked-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
>
Thanks !!

regards,
Santosh