add debug bit to count contended locks

Thank you to phk for the improvement over the initial suggestion

bin/varnishd/VSC_lck.vsc

@@ -30,5 +30,27 @@
 	:oneliner:	Lock Operations
 
 
+.. varnish_vsc:: dbg_busy
+	:type:	counter
+	:level:	debug
+	:oneliner:	Contended lock operations
+
+	If the ``lck`` debug bit is set: Lock operations which
+	returned EBUSY on the first locking attempt.
+
+	If the ``lck`` debug bit is unset, this counter will never be
+	incremented even if lock operations are contended.
+
+.. varnish_vsc:: dbg_try_fail
+	:type:	counter
+	:level:	debug
+	:oneliner:	Contended trylock operations
+
+	If the ``lck`` debug bit is set: Trylock operations which
+	returned EBUSY.
+
+	If the ``lck`` debug bit is unset, this counter will never be
+	incremented even if lock operations are contended.
+
 .. varnish_vsc_end::	lck
 

bin/varnishd/cache/cache_lck.c

@@ -107,11 +107,20 @@ void v_matchproto_()
 Lck__Lock(struct lock *lck, const char *p, int l)
 {
 	struct ilck *ilck;
+	int r = EINVAL;
 
 	CAST_OBJ_NOTNULL(ilck, lck->priv, ILCK_MAGIC);
 	if (DO_DEBUG(DBG_WITNESS))
 		Lck_Witness_Lock(ilck, p, l, "");
-	AZ(pthread_mutex_lock(&ilck->mtx));
+	else if (DO_DEBUG(DBG_LCK)) {
+		r = pthread_mutex_trylock(&ilck->mtx);
+		if (r == EBUSY)
+			ilck->stat->dbg_busy++;
+		else
+			AZ(r);
+	}
+	if (r)
+		AZ(pthread_mutex_lock(&ilck->mtx));
 	AZ(ilck->held);
 	ilck->stat->locks++;
 	ilck->owner = pthread_self();
@@ -162,7 +171,8 @@ Lck__Trylock(struct lock *lck, const char *p, int l)
 		ilck->held = 1;
 		ilck->stat->locks++;
 		ilck->owner = pthread_self();
-	}
+	} else if (DO_DEBUG(DBG_LCK))
+		ilck->stat->dbg_try_fail++;
 	return (r);
 }
 

include/tbl/debug_bits.h

@@ -51,6 +51,7 @@ DEBUG_BIT(VMOD_SO_KEEP,		vmod_so_keep,	"Keep copied VMOD libraries")
 DEBUG_BIT(PROCESSORS,		processors,	"Fetch/Deliver processors")
 DEBUG_BIT(PROTOCOL,		protocol,	"Protocol debugging")
 DEBUG_BIT(VCL_KEEP,		vcl_keep,	"Keep VCL C and so files")
+DEBUG_BIT(LCK,			lck,		"Additional lock statistics")
 #undef DEBUG_BIT
 
 /*lint -restore */