Free rb_native_thread memory at fork

peterzhu2118 · peterzhu2118 · commit 0ba2a485336e · 2025-06-11T16:24:20.000-04:00
We never freed any resources of rb_native_thread at fork because it would
cause it to hang. This is because it called rb_native_cond_destroy for
condition variables.  We can't call rb_native_cond_destroy here because
according to the specs of pthread_cond_destroy:

    Attempting to destroy a condition variable upon which other threads
    are currently blocked results in undefined behavior.

Specifically, glibc's pthread_cond_destroy waits on all the other listeners.
Since after forking all the threads are dead, the condition variable's
listeners will never wake up, so it will hang forever.

This commit changes it to only free the memory and none of the condition
variables.
diff --git a/thread.c b/thread.c
@@ -519,12 +519,8 @@ thread_cleanup_func(void *th_ptr, int atfork)
     th->locking_mutex = Qfalse;
     thread_cleanup_func_before_exec(th_ptr);
 
-    /*
-     * Unfortunately, we can't release native threading resource at fork
-     * because libc may have unstable locking state therefore touching
-     * a threading resource may cause a deadlock.
-     */
     if (atfork) {
+        native_thread_destroy_atfork(th->nt);
         th->nt = NULL;
         return;
     }
diff --git a/thread_pthread.c b/thread_pthread.c
@@ -1816,6 +1816,27 @@ native_thread_assign(struct rb_native_thread *nt, rb_thread_t *th)
     th->nt = nt;
 }
 
+static void
+native_thread_destroy_atfork(struct rb_native_thread *nt)
+{
+    if (nt) {
+        /* We can't call rb_native_cond_destroy here because according to the
+         * specs of pthread_cond_destroy:
+         *
+         *   Attempting to destroy a condition variable upon which other threads
+         *   are currently blocked results in undefined behavior.
+         *
+         * Specifically, glibc's pthread_cond_destroy waits on all the other
+         * listeners. Since after forking all the threads are dead, the condition
+         * variable's listeners will never wake up, so it will hang forever.
+         */
+
+        RB_ALTSTACK_FREE(nt->altstack);
+        ruby_xfree(nt->nt_context);
+        ruby_xfree(nt);
+    }
+}
+
 static void
 native_thread_destroy(struct rb_native_thread *nt)
 {
@@ -1826,9 +1847,7 @@ native_thread_destroy(struct rb_native_thread *nt)
             rb_native_cond_destroy(&nt->cond.intr);
         }
 
-        RB_ALTSTACK_FREE(nt->altstack);
-        ruby_xfree(nt->nt_context);
-        ruby_xfree(nt);
+        native_thread_destroy_atfork(nt);
     }
 }
 
