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[PATCH net v2] rds: Fix incorrect statistics counting

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  [PATCH net v2] rds: Fix incorrect statistics counting Håkon Bugge <Haakon.Bugge@oracle.com> - 2017-09-06 18:40 +0200
    Re: [PATCH net v2] rds: Fix incorrect statistics counting Santosh Shilimkar <santosh.shilimkar@oracle.com> - 2017-09-06 18:50 +0200
    Re: [PATCH net v2] rds: Fix incorrect statistics counting David Miller <davem@davemloft.net> - 2017-09-08 05:10 +0200

#1727644 — [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 <haakon.bugge@oracle.com>
Reviewed-by: Knut Omang <knut.omang@oracle.com>
---
 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

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#1727646

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 <haakon.bugge@oracle.com>
> Reviewed-by: Knut Omang <knut.omang@oracle.com>
> ---

Thanks for the update.
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>

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#1728554

From: Håkon Bugge <Haakon.Bugge@oracle.com>
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: Håkon Bugge <haakon.bugge@oracle.com>
> Reviewed-by: Knut Omang <knut.omang@oracle.com>

Applied.

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