Deadlock during rundown using interpreter.

Identified deadlock between finalizer thread and rundown enumerating
all interpreter method. Since rundown will query for method name
in callback when iterating interpreter methods, interp_jit_info_foreach,
that might lead to additional loader activity. The hash map in
interpreter keeping the methods is locked with default JIT memory manager,
but since the callback might end up in mono_class_create_from_typedef
that will take loader lock, we get the following lock order on that
code path, memory manager->loader lock. Finalizer thread invokes
OnThreadExiting using interpreter and that might end up with a reverse
lock order, loader lock->memory manager on that code path, so these
two have a potential to deadlock. This is not a problem under JIT
or AOT since the JIT hash table is lock free therefore not causing
any deadlocks due to lock order between memory manager and loader lock.

Could be fixed by changing into a lock free hash table in interpreters
for interp_code_hash might be to risky at this point. A more safe fix
is to take a copy of the pointers while holding lock and then iterate
using local copy (simple array of pointers). Since this method is
only called during rundown, only when using interpreter, and only
include the pointers (InterpMethod *) we use with the callback,
it will have some temporary memory impact (allocating an array
of pointers), but will mitigate the deadlock since we can safely
call iterator callback without holding the lock. It will also
improve interpreter performance in situations where we run session
rundown, since lock will be held a much shorter amount of time.

src/mono/mono/mini/interp/interp.c

@@ -7435,28 +7435,44 @@ interp_invalidate_transformed (void)
 }
 
 typedef struct {
-	InterpJitInfoFunc func;
-	gpointer user_data;
-} InterpJitInfoFuncUserData;
+	MonoJitInfo **jit_info_array;
+	gint size;
+	gint next;
+} InterpCopyJitInfoFuncUserData;
 
 static void
-interp_call_jit_info_func (gpointer imethod, gpointer user_data)
+interp_copy_jit_info_func (gpointer imethod, gpointer user_data)
 {
-	InterpJitInfoFuncUserData *data = (InterpJitInfoFuncUserData *)user_data;
-	data->func (((InterpMethod *)imethod)->jinfo, data->user_data);
+	InterpCopyJitInfoFuncUserData *data = (InterpCopyJitInfoFuncUserData*)user_data;
+	if (data->next < data->size)
+		data->jit_info_array [data->next++] = ((InterpMethod *)imethod)->jinfo;
 }
 
 static void
 interp_jit_info_foreach (InterpJitInfoFunc func, gpointer user_data)
 {
-	InterpJitInfoFuncUserData data = {func, user_data};
+	InterpCopyJitInfoFuncUserData copy_jit_info_data;
 
 	// FIXME: Enumerate all memory managers
 	MonoJitMemoryManager *jit_mm = get_default_jit_mm ();
 
-	jit_mm_lock (jit_mm);
-	mono_internal_hash_table_apply (&jit_mm->interp_code_hash, interp_call_jit_info_func, &data);
-	jit_mm_unlock (jit_mm);
+	// Can't keep memory manager lock while iterating and calling callback since it might take other locks
+	// causing poential deadlock situations. Instead, create copy of interpreter imethod jinfo pointers into
+	// plain array and use pointers from array when when running callbacks.
+	copy_jit_info_data.size = mono_atomic_load_i32 (&(jit_mm->interp_code_hash.num_entries));
+	copy_jit_info_data.next = 0;
+	copy_jit_info_data.jit_info_array = (MonoJitInfo**) g_new (MonoJitInfo*, copy_jit_info_data.size);
+	if (copy_jit_info_data.jit_info_array) {
+		jit_mm_lock (jit_mm);
+		mono_internal_hash_table_apply (&jit_mm->interp_code_hash, interp_copy_jit_info_func, &copy_jit_info_data);
+		jit_mm_unlock (jit_mm);
+	}
+
+	if (copy_jit_info_data.jit_info_array) {
+		for (size_t i = 0; i < copy_jit_info_data.next; ++i)
+			func (copy_jit_info_data.jit_info_array [i], user_data);
+		g_free (copy_jit_info_data.jit_info_array);
+	}
 }
 
 static void