From: Ondřej Surý <ondrej@sury.org>
Date: Thu, 22 Nov 2018 10:42:12 +0000 (+0100)
Subject: Implement isc_rwlock_downgrade using pthreads and single atomic_bool
X-Git-Tag: v9.15.1~25^2~1
X-Git-Url: http://git.ipfire.org/gitweb/index.cgi?a=commitdiff_plain;h=4501f646eecd49bda19862187d4f550218bdaf8f;p=thirdparty%2Fbind9.git

Implement isc_rwlock_downgrade using pthreads and single atomic_bool
---

diff --git a/lib/isc/include/isc/rwlock.h b/lib/isc/include/isc/rwlock.h
index 50d06d72d00..5551c1b6239 100644
--- a/lib/isc/include/isc/rwlock.h
+++ b/lib/isc/include/isc/rwlock.h
@@ -36,6 +36,7 @@ typedef enum {
 
 struct isc_rwlock {
 	pthread_rwlock_t	rwlock;
+	atomic_bool             downgrade;
 };
 
 #else /* HAVE_PTHREAD_RWLOCK_RDLOCK */
diff --git a/lib/isc/rwlock.c b/lib/isc/rwlock.c
index 2619619d1b4..7d5e14ea4eb 100644
--- a/lib/isc/rwlock.c
+++ b/lib/isc/rwlock.c
@@ -39,15 +39,32 @@ isc_rwlock_init(isc_rwlock_t *rwl, unsigned int read_quota,
 	UNUSED(read_quota);
 	UNUSED(write_quota);
 	REQUIRE(pthread_rwlock_init(&rwl->rwlock, NULL) == 0);
+	atomic_init(&rwl->downgrade, false);
 	return (ISC_R_SUCCESS);
 }
 
 isc_result_t
 isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 	switch (type) {
-	case isc_rwlocktype_read: REQUIRE(pthread_rwlock_rdlock(&rwl->rwlock) == 0); break;
-	case isc_rwlocktype_write: REQUIRE(pthread_rwlock_wrlock(&rwl->rwlock) == 0); break;
-	default: INSIST(0);
+	case isc_rwlocktype_read:
+		REQUIRE(pthread_rwlock_rdlock(&rwl->rwlock) == 0);
+		break;
+	case isc_rwlocktype_write:
+		while (true) {
+			REQUIRE(pthread_rwlock_wrlock(&rwl->rwlock) == 0);
+			/* Unlock if in middle of downgrade operation */
+			if (atomic_load_release(&rwl->downgrade)) {
+				REQUIRE(pthread_rwlock_unlock(&rwl->rwlock)
+					== 0);
+				while (atomic_load(&rwl->downgrade));
+				continue;
+			}
+			break;
+		}
+		break;
+	default:
+		INSIST(0);
+		ISC_UNREACHABLE();
 	}
 	return (ISC_R_SUCCESS);
 }
@@ -61,6 +78,10 @@ isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 		break;
 	case isc_rwlocktype_write:
 		ret = pthread_rwlock_trywrlock(&rwl->rwlock);
+		if ((ret == 0) && atomic_load(&rwl->downgrade)) {
+			isc_rwlock_unlock(rwl, type);
+			return (ISC_R_LOCKBUSY);
+		}
 		break;
 	default: INSIST(0);
 	}
@@ -88,8 +109,10 @@ isc_rwlock_tryupgrade(isc_rwlock_t *rwl) {
 
 void
 isc_rwlock_downgrade(isc_rwlock_t *rwl) {
+	atomic_store_acquire(&rwl->downgrade, true);
 	isc_rwlock_unlock(rwl, isc_rwlocktype_write);
 	isc_rwlock_lock(rwl, isc_rwlocktype_read);
+	atomic_store_acquire(&rwl->downgrade, false);
 }
 
 void