From cffd6e116711640d070d9690480b95c5a6de7154 Mon Sep 17 00:00:00 2001
From: George Melikov <mail@gmelikov.ru>
Date: Mon, 23 Jan 2017 21:35:58 +0300
Subject: [PATCH] OpenZFS 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>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/3746
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/260af64
Closes #5625
---
 module/zfs/zrlock.c | 50 +++++++++++++++++++++------------------------
 1 file changed, 23 insertions(+), 27 deletions(-)

diff --git a/module/zfs/zrlock.c b/module/zfs/zrlock.c
index 137875fe0..02795112f 100644
--- a/module/zfs/zrlock.c
+++ b/module/zfs/zrlock.c
@@ -20,7 +20,8 @@
  */
 /*
  * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2014 by Delphix. All rights reserved.
+ * Copyright (c) 2014, 2015 by Delphix. All rights reserved.
+ * Copyright 2016 The MathWorks, Inc. All rights reserved.
  */
 
 /*
@@ -72,37 +73,32 @@ 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);
+				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;
 			}
-			zrl->zr_owner = curthread;
-			zrl->zr_caller = zc;
-#endif
-			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