3 files changed, 39 insertions, 10 deletions

diff --git a/test/ruby/test_thread.rb b/test/ruby/test_thread.rb
index c08d41cb86..e4d6e1ad62 100644
--- a/test/ruby/test_thread.rb
+++ b/test/ruby/test_thread.rb
@@ -1649,4 +1649,32 @@ q.pop
       end
     end;
   end
+
+  # [Bug #21926]
+  def test_thread_join_during_finalizers
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}", timeout: 30)
+    begin;
+      require 'open3'
+
+      class ProcessWrapper
+        def initialize
+          @stdin, @stdout, @stderr, @wait_thread = Open3.popen3("cat") # hangs until we close our stdin side
+          ObjectSpace.define_finalizer(self, self.class.make_finalizer(@stdin, @stdout, @stderr, @wait_thread))
+        end
+
+        def self.make_finalizer(stdin, stdout, stderr, wait_thread)
+          proc do
+            stdin.close rescue nil
+            stdout.close rescue nil
+            stderr.close rescue nil
+            wait_thread.value
+          end
+        end
+      end
+
+      50.times { ProcessWrapper.new }
+      GC.stress = true
+      1000.times { Object.new }
+    end;
+  end
 end
diff --git a/thread_pthread.c b/thread_pthread.c
index 9c7754067b..df2940d5cf 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -1099,23 +1099,21 @@ thread_sched_to_waiting_until_wakeup(struct rb_thread_sched *sched, rb_thread_t
 
     RB_VM_SAVE_MACHINE_CONTEXT(th);
 
-    if (ubf_set(th, ubf_waiting, (void *)th)) {
-        return;
-    }
 
     RB_INTERNAL_THREAD_HOOK(RUBY_INTERNAL_THREAD_EVENT_SUSPENDED, th);
 
     thread_sched_lock(sched, th);
     {
-        if (!RUBY_VM_INTERRUPTED(th->ec)) {
+        // NOTE: there's a lock ordering inversion here with the ubf call, but it's benign.
+        if (ubf_set(th, ubf_waiting, (void *)th)) {
+            RUBY_DEBUG_LOG("th:%u interrupted", rb_th_serial(th));
+        }
+        else {
             bool can_direct_transfer = !th_has_dedicated_nt(th);
             // NOTE: th->status is set before and after this sleep outside of this function in `sleep_forever`
             thread_sched_wakeup_next_thread(sched, th, can_direct_transfer);
             thread_sched_wait_running_turn(sched, th, can_direct_transfer);
         }
-        else {
-            RUBY_DEBUG_LOG("th:%u interrupted", rb_th_serial(th));
-        }
     }
     thread_sched_unlock(sched, th);
 
diff --git a/thread_pthread_mn.c b/thread_pthread_mn.c
index 9651baf51f..0d774c67b5 100644
--- a/thread_pthread_mn.c
+++ b/thread_pthread_mn.c
@@ -67,12 +67,15 @@ thread_sched_wait_events(struct rb_thread_sched *sched, rb_thread_t *th, int fd,
 
     uint32_t event_serial = ++th->sched.event_serial; // overflow is okay
 
-    if (ubf_set(th, ubf_event_waiting, (void *)th)) {
-        return false;
-    }
 
     thread_sched_lock(sched, th);
     {
+        // NOTE: there's a lock ordering inversion here with the ubf call, but it's benign.
+        if (ubf_set(th, ubf_event_waiting, (void *)th)) {
+            thread_sched_unlock(sched, th);
+            return false;
+        }
+
         if (timer_thread_register_waiting(th, fd, events, rel, event_serial)) {
             RUBY_DEBUG_LOG("wait fd:%d", fd);