3746 ZRLs are racy

Authored by: Will Andrews <will@freebsd.org>
Reviewed by: Boris Protopopov <bprotopopov@hotmail.com>
Reviewed by: Pavel Zakharov <pavel.zakha@gmail.com>
Reviewed by: Yuri Pankov <yuri.pankov@gmail.com>
Reviewed by: Justin T. Gibbs <gibbs@scsiguy.com>
Approved by: Matt Ahrens <mahrens@delphix.com>

usr/src/uts/common/fs/zfs/zrlock.c

@@ -21,6 +21,7 @@
 /*
  * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2014, 2015 by Delphix. All rights reserved.
+ * Copyright 2016 The MathWorks, Inc. All rights reserved.
  */
 
 /*
@@ -71,37 +72,32 @@ zrl_destroy(zrlock_t *zrl)
 void
 zrl_add_impl(zrlock_t *zrl, const char *zc)
 {
-	uint32_t n = (uint32_t)zrl->zr_refcount;
-
-	while (n != ZRL_LOCKED) {
-		uint32_t cas = atomic_cas_32(
-		    (uint32_t *)&zrl->zr_refcount, n, n + 1);
-		if (cas == n) {
-			ASSERT3S((int32_t)n, >=, 0);
+	for (;;) {
+		uint32_t n = (uint32_t)zrl->zr_refcount;
+		while (n != ZRL_LOCKED) {
+			uint32_t cas = atomic_cas_32(
+			    (uint32_t *)&zrl->zr_refcount, n, n + 1);
+			if (cas == n) {
+				ASSERT3S((int32_t)n, >=, 0);
 #ifdef	ZFS_DEBUG
-			if (zrl->zr_owner == curthread) {
-				DTRACE_PROBE2(zrlock__reentry,
-				    zrlock_t *, zrl, uint32_t, n);
-			}
-			zrl->zr_owner = curthread;
-			zrl->zr_caller = zc;
+				if (zrl->zr_owner == curthread) {
+					DTRACE_PROBE2(zrlock__reentry,
+					    zrlock_t *, zrl, uint32_t, n);
+				}
+				zrl->zr_owner = curthread;
+				zrl->zr_caller = zc;
 #endif
-			return;
+				return;
+			}
+			n = cas;
 		}
-		n = cas;
-	}
 
-	mutex_enter(&zrl->zr_mtx);
-	while (zrl->zr_refcount == ZRL_LOCKED) {
-		cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
+		mutex_enter(&zrl->zr_mtx);
+		while (zrl->zr_refcount == ZRL_LOCKED) {
+			cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
+		}
+		mutex_exit(&zrl->zr_mtx);
 	}
-	ASSERT3S(zrl->zr_refcount, >=, 0);
-	zrl->zr_refcount++;
-#ifdef	ZFS_DEBUG
-	zrl->zr_owner = curthread;
-	zrl->zr_caller = zc;
-#endif
-	mutex_exit(&zrl->zr_mtx);
 }
 
 void