From d52d7aa8714929fc0f26e605aafa38461213e606 Mon Sep 17 00:00:00 2001
From: Attilio Rao <attilio@FreeBSD.org>
Date: Thu, 21 Mar 2013 19:58:25 +0000
Subject: [PATCH] Fix a bug in UMTX_PROFILING: UMTX_PROFILING should really
 analyze the distribution of locks as they index entries in the umtxq_chains
 hash-table. However, the current implementation does add/dec the length
 counters for *every* thread insert/removal, measuring at all really userland
 contention and not the hash distribution.

Fix this by correctly add/dec the length counters in the points where
it is really needed.

Please note that this bug brought us questioning in the past the quality
of the umtx hash table distribution.
To date with all the benchmarks I could try I was not able to reproduce
any issue about the hash distribution on umtx.

Sponsored by:	EMC / Isilon storage division
Reviewed by:	jeff, davide
MFC after:	2 weeks
---
 sys/kern/kern_umtx.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/sys/kern/kern_umtx.c b/sys/kern/kern_umtx.c
index f50a6f90ac81..6026b8080dbf 100644
--- a/sys/kern/kern_umtx.c
+++ b/sys/kern/kern_umtx.c
@@ -542,19 +542,19 @@ umtxq_insert_queue(struct umtx_q *uq, int q)
 		uh = uq->uq_spare_queue;
 		uh->key = uq->uq_key;
 		LIST_INSERT_HEAD(&uc->uc_queue[q], uh, link);
+#ifdef UMTX_PROFILING
+		uc->length++;
+		if (uc->length > uc->max_length) {
+			uc->max_length = uc->length;
+			if (uc->max_length > max_length)
+				max_length = uc->max_length;	
+		}
+#endif
 	}
 	uq->uq_spare_queue = NULL;
 
 	TAILQ_INSERT_TAIL(&uh->head, uq, uq_link);
 	uh->length++;
-#ifdef UMTX_PROFILING
-	uc->length++;
-	if (uc->length > uc->max_length) {
-		uc->max_length = uc->length;
-		if (uc->max_length > max_length)
-			max_length = uc->max_length;	
-	}
-#endif
 	uq->uq_flags |= UQF_UMTXQ;
 	uq->uq_cur_queue = uh;
 	return;
@@ -572,13 +572,13 @@ umtxq_remove_queue(struct umtx_q *uq, int q)
 		uh = uq->uq_cur_queue;
 		TAILQ_REMOVE(&uh->head, uq, uq_link);
 		uh->length--;
-#ifdef UMTX_PROFILING
-		uc->length--;
-#endif
 		uq->uq_flags &= ~UQF_UMTXQ;
 		if (TAILQ_EMPTY(&uh->head)) {
 			KASSERT(uh->length == 0,
 			    ("inconsistent umtxq_queue length"));
+#ifdef UMTX_PROFILING
+			uc->length--;
+#endif
 			LIST_REMOVE(uh, link);
 		} else {
 			uh = LIST_FIRST(&uc->uc_spare_queue);