Re-enable spin wait on ethreads rwlocks

Spin wait on most ethread rwlocks used by the runtime system was
unintentionally disabled during development. Spin wait has now been enabled
again. This bug appeared in commit 59ee2a593090e7d53c97ceba63cbd300d1b9657e,
i.e., it has not been seen in any released versions.

1 files changed, 19 insertions, 12 deletions

diff --git a/erts/lib_src/common/ethr_mutex.c b/erts/lib_src/common/ethr_mutex.c
index aac0d44a32..ccceb61714 100644
--- a/erts/lib_src/common/ethr_mutex.c
+++ b/erts/lib_src/common/ethr_mutex.c
@@ -35,7 +35,7 @@
 
 #define ETHR_SPIN_WITH_WAITERS 1
 
-#define ETHR_MTX_MAX_FLGS_SPIN 1000
+#define ETHR_MTX_MAX_FLGS_SPIN 10
 
 #ifdef ETHR_USE_OWN_RWMTX_IMPL__
 static int default_rwmtx_main_spincount;
@@ -593,24 +593,31 @@ initial_spincount(struct ethr_mutex_base_ *mtxb)
 static ETHR_INLINE int
 update_spincount(struct ethr_mutex_base_ *mtxb,
 		 ethr_ts_event *tse,
-		 int *start_scnt,
+		 int *scnt_state,
 		 int *scnt)
 {
-    int sscnt = *start_scnt;
-    if (sscnt < 0) {
-	*scnt = ((tse->iflgs & ETHR_TS_EV_MAIN_THR)
-		 ? mtxb->main_scnt
-		 : mtxb->aux_scnt);
-	*scnt -= ETHR_MTX_MAX_FLGS_SPIN;
+    int state = *scnt_state;
+    if (state <= 0) {
+	/* Here state is max spincount to do on event negated */
+	*scnt = -state;
     }
     else {
+	/* Here state is initial spincount made on flags */
 	*scnt = ((tse->iflgs & ETHR_TS_EV_MAIN_THR)
 		 ? mtxb->main_scnt
 		 : mtxb->aux_scnt);
-	*scnt -= sscnt;
-	if (*scnt > 0 && sscnt < ETHR_MTX_MAX_FLGS_SPIN) {
-	    *scnt = ETHR_MTX_MAX_FLGS_SPIN - sscnt;
-	    *start_scnt = -1;
+	if (*scnt <= state)
+	    *scnt = 0;
+	else {
+	    if (*scnt <= ETHR_MTX_MAX_FLGS_SPIN)
+		*scnt_state = 0; /* No spin on event */
+	    else {
+		 /* Spin on event after... */
+		*scnt_state = -1*(*scnt - ETHR_MTX_MAX_FLGS_SPIN);
+		/* ... we have spun on flags */
+		*scnt = ETHR_MTX_MAX_FLGS_SPIN;
+	    }
+	    *scnt -= state;
 	    return 0;
 	}
     }