From: Willy Tarreau <w@1wt.eu>
Date: Sat, 17 Oct 2020 16:55:18 +0000 (+0200)
Subject: MINOR: lb/map: use seek lock and read locks where appropriate
X-Git-Tag: v2.3-dev8~50
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ae99aeb135ac18b1442ddf9eeccb16cb35d2d04d;p=thirdparty%2Fhaproxy.git

MINOR: lb/map: use seek lock and read locks where appropriate

- map_get_server_hash() doesn't need a write lock since it only
  reads the array, let's only use a read lock here.

- map_get_server_rr() only needs exclusivity to adjust the rr_idx
  while looking for its entry. Since this one is not used by
  map_get_server_hash(), let's turn this lock to a seek lock that
  doesn't block reads.

With 8 threads, no significant performance difference was noticed
given that lookups are usually instant with this LB algo so the
lock contention is rare.
---

diff --git a/src/lb_map.c b/src/lb_map.c
index 1432913316..b735678a8c 100644
--- a/src/lb_map.c
+++ b/src/lb_map.c
@@ -216,7 +216,7 @@ struct server *map_get_server_rr(struct proxy *px, struct server *srvtoavoid)
 	int newidx, avoididx;
 	struct server *srv, *avoided;
 
-	HA_RWLOCK_WRLOCK(LBPRM_LOCK, &px->lbprm.lock);
+	HA_RWLOCK_SKLOCK(LBPRM_LOCK, &px->lbprm.lock);
 	if (px->lbprm.tot_weight == 0) {
 		avoided = NULL;
 		goto out;
@@ -248,7 +248,7 @@ struct server *map_get_server_rr(struct proxy *px, struct server *srvtoavoid)
 		px->lbprm.map.rr_idx = avoididx;
 
   out:
-	HA_RWLOCK_WRUNLOCK(LBPRM_LOCK, &px->lbprm.lock);
+	HA_RWLOCK_SKUNLOCK(LBPRM_LOCK, &px->lbprm.lock);
 	/* return NULL or srvtoavoid if found */
 	return avoided;
 }
@@ -265,10 +265,10 @@ struct server *map_get_server_hash(struct proxy *px, unsigned int hash)
 {
 	struct server *srv = NULL;
 
-	HA_RWLOCK_WRLOCK(LBPRM_LOCK, &px->lbprm.lock);
+	HA_RWLOCK_RDLOCK(LBPRM_LOCK, &px->lbprm.lock);
 	if (px->lbprm.tot_weight)
 		srv = px->lbprm.map.srv[hash % px->lbprm.tot_weight];
-	HA_RWLOCK_WRUNLOCK(LBPRM_LOCK, &px->lbprm.lock);
+	HA_RWLOCK_RDUNLOCK(LBPRM_LOCK, &px->lbprm.lock);
 	return srv;
 }