5981 Deadlock in dmu_objset_find_dp

@@ -157,12 +157,12 @@
         ASSERT(rrl->rr_writer == NULL);
         refcount_destroy(&rrl->rr_anon_rcount);
         refcount_destroy(&rrl->rr_linked_rcount);
 }
 
-void
-rrw_enter_read(rrwlock_t *rrl, void *tag)
+static void
+rrw_enter_read_impl(rrwlock_t *rrl, boolean_t prio, void *tag)
 {
         mutex_enter(&rrl->rr_lock);
 #if !defined(DEBUG) && defined(_KERNEL)
         if (rrl->rr_writer == NULL && !rrl->rr_writer_wanted &&
             !rrl->rr_track_all) {

@@ -174,11 +174,11 @@
 #endif
         ASSERT(rrl->rr_writer != curthread);
         ASSERT(refcount_count(&rrl->rr_anon_rcount) >= 0);
 
         while (rrl->rr_writer != NULL || (rrl->rr_writer_wanted &&
-            refcount_is_zero(&rrl->rr_anon_rcount) &&
+            refcount_is_zero(&rrl->rr_anon_rcount) && !prio &&
             rrn_find(rrl) == NULL))
                 cv_wait(&rrl->rr_cv, &rrl->rr_lock);
 
         if (rrl->rr_writer_wanted || rrl->rr_track_all) {
                 /* may or may not be a re-entrant enter */

@@ -190,10 +190,29 @@
         ASSERT(rrl->rr_writer == NULL);
         mutex_exit(&rrl->rr_lock);
 }
 
 void
+rrw_enter_read(rrwlock_t *rrl, void *tag)
+{
+        rrw_enter_read_impl(rrl, B_FALSE, tag);
+}
+
+/*
+ * take a read lock even if there are pending write lock requests. if we want
+ * to take a lock reentrantly, but from different threads (that have a
+ * relationship to each other), the normal detection mechanism to overrule
+ * the pending writer does not work, so we have to give an explicit hint here.
+ */
+void
+rrw_enter_read_prio(rrwlock_t *rrl, void *tag)
+{
+        rrw_enter_read_impl(rrl, B_TRUE, tag);
+}
+
+
+void
 rrw_enter_write(rrwlock_t *rrl)
 {
         mutex_enter(&rrl->rr_lock);
         ASSERT(rrl->rr_writer != curthread);