2017-09-06 16:36:44

by Haakon Bugge

Subject: [PATCH net v2] rds: Fix incorrect statistics counting

In rds_send_xmit() there is logic to batch the sends. However, if
another thread has acquired the lock and has incremented the send_gen,
it is considered a race and we yield. The code incrementing the
s_send_lock_queue_raced statistics counter did not count this event
correctly.

This commit counts the race condition correctly.

Changes from v1:
- Removed check for *someone_on_xmit()*
- Fixed incorrect indentation

Signed-off-by: Håkon Bugge <[email protected]>
Reviewed-by: Knut Omang <[email protected]>
---
net/rds/send.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/rds/send.c b/net/rds/send.c
index 058a407..b52cdc8 100644
--- a/net/rds/send.c
+++ b/net/rds/send.c
@@ -428,14 +428,18 @@ int rds_send_xmit(struct rds_conn_path *cp)
* some work and we will skip our goto
*/
if (ret == 0) {
+ bool raced;
+
smp_mb();
+ raced = send_gen != READ_ONCE(cp->cp_send_gen);
+
if ((test_bit(0, &conn->c_map_queued) ||
- !list_empty(&cp->cp_send_queue)) &&
- send_gen == READ_ONCE(cp->cp_send_gen)) {
- rds_stats_inc(s_send_lock_queue_raced);
+ !list_empty(&cp->cp_send_queue)) && !raced) {
if (batch_count < send_batch_count)
goto restart;
queue_delayed_work(rds_wq, &cp->cp_send_w, 1);
+ } else if (raced) {
+ rds_stats_inc(s_send_lock_queue_raced);
}
}
out:
--
2.9.3

2017-09-06 16:48:50

by Santosh Shilimkar

Subject: Re: [PATCH net v2] rds: Fix incorrect statistics counting

On 9/6/2017 9:35 AM, Håkon Bugge wrote:
> In rds_send_xmit() there is logic to batch the sends. However, if
> another thread has acquired the lock and has incremented the send_gen,
> it is considered a race and we yield. The code incrementing the
> s_send_lock_queue_raced statistics counter did not count this event
> correctly.
>
> This commit counts the race condition correctly.
>
> Changes from v1:
> - Removed check for *someone_on_xmit()*
> - Fixed incorrect indentation
>
> Signed-off-by: Håkon Bugge <[email protected]>
> Reviewed-by: Knut Omang <[email protected]>
> ---

Thanks for the update.
Acked-by: Santosh Shilimkar <[email protected]>

2017-09-08 03:07:35

by David Miller

Subject: Re: [PATCH net v2] rds: Fix incorrect statistics counting

From: H?kon Bugge <[email protected]>
Date: Wed, 6 Sep 2017 18:35:51 +0200

> In rds_send_xmit() there is logic to batch the sends. However, if
> another thread has acquired the lock and has incremented the send_gen,
> it is considered a race and we yield. The code incrementing the
> s_send_lock_queue_raced statistics counter did not count this event
> correctly.
>
> This commit counts the race condition correctly.
>
> Changes from v1:
> - Removed check for *someone_on_xmit()*
> - Fixed incorrect indentation
>
> Signed-off-by: H?kon Bugge <[email protected]>
> Reviewed-by: Knut Omang <[email protected]>

Applied.