[libc-commits] [libc] [libc] Skip the RPC doorbell interrupt when the mailbox is uninitialized (PR #214037)

[Minseob Shin via libc-commits]

https://github.com/minseobshin11 updated https://github.com/llvm/llvm-project/pull/214037

>From 9e8a6a35589f84f8ecdea4763e48aaa72e580610 Mon Sep 17 00:00:00 2001
From: Minseob Shin <minseob.shin11 at gmail.com>
Date: Tue, 4 Aug 2026 13:39:03 -0500
Subject: [PATCH] [libc] Skip the RPC doorbell interrupt when the mailbox is
 uninitialized

---
 libc/shared/rpc.h                             | 15 +++--
 libc/test/src/__support/RPC/CMakeLists.txt    | 10 ++++
 .../src/__support/RPC/rpc_doorbell_test.cpp   | 57 +++++++++++++++++++
 3 files changed, 77 insertions(+), 5 deletions(-)
 create mode 100644 libc/test/src/__support/RPC/rpc_doorbell_test.cpp

diff --git a/libc/shared/rpc.h b/libc/shared/rpc.h
index 5bc518ab97af7..eb6051f329e4a 100644
--- a/libc/shared/rpc.h
+++ b/libc/shared/rpc.h
@@ -158,11 +158,16 @@ template <bool Invert> struct Process {
     if (rpc::is_first_lane(lane_mask)) {
       if (!__scoped_atomic_fetch_add(doorbell->value, 1UL, __ATOMIC_RELAXED,
                                      __MEMORY_SCOPE_SYSTEM)) {
-        __scoped_atomic_store_n(doorbell->mailbox,
-                                static_cast<uint64_t>(doorbell->event_id),
-                                __ATOMIC_RELAXED, __MEMORY_SCOPE_SYSTEM);
-        __scoped_atomic_thread_fence(__ATOMIC_RELEASE, __MEMORY_SCOPE_SYSTEM);
-        signal_interrupt(event_id);
+        // A doorbell that is not interrupt-backed has no mailbox. The counter
+        // incremented above already publishes the work, so skipping the
+        // interrupt is safe.
+        if (doorbell->mailbox) {
+          __scoped_atomic_store_n(doorbell->mailbox,
+                                  static_cast<uint64_t>(doorbell->event_id),
+                                  __ATOMIC_RELAXED, __MEMORY_SCOPE_SYSTEM);
+          __scoped_atomic_thread_fence(__ATOMIC_RELEASE, __MEMORY_SCOPE_SYSTEM);
+          signal_interrupt(event_id);
+        }
       }
     }
 #endif
diff --git a/libc/test/src/__support/RPC/CMakeLists.txt b/libc/test/src/__support/RPC/CMakeLists.txt
index 8f79a49cec40c..11d09226077c8 100644
--- a/libc/test/src/__support/RPC/CMakeLists.txt
+++ b/libc/test/src/__support/RPC/CMakeLists.txt
@@ -9,3 +9,13 @@ add_libc_test(
   DEPENDS
    libc.src.__support.RPC.rpc
 )
+
+add_libc_test(
+  rpc_doorbell_test
+  SUITE
+    libc-rpc-tests
+  SRCS
+    rpc_doorbell_test.cpp
+  DEPENDS
+   libc.src.__support.RPC.rpc
+)
diff --git a/libc/test/src/__support/RPC/rpc_doorbell_test.cpp b/libc/test/src/__support/RPC/rpc_doorbell_test.cpp
new file mode 100644
index 0000000000000..f836b29f6bcce
--- /dev/null
+++ b/libc/test/src/__support/RPC/rpc_doorbell_test.cpp
@@ -0,0 +1,57 @@
+//===-- tests for the RPC doorbell ----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/RPC/rpc.h"
+
+#include "test/UnitTest/Test.h"
+
+namespace {
+enum { port_count = 4 };
+
+using ProcType = LIBC_NAMESPACE::rpc::Process<false>;
+
+enum { alloc_size = ProcType::allocation_size(port_count, 1) };
+
+alignas(64) char buffer[alloc_size] = {0};
+
+// At namespace scope so the doorbell never refers to a dead local.
+uint64_t pending = 0;
+} // namespace
+
+// A doorbell can be only partially initialized. The AMDGPU offload plugin sets
+// 'value' to the address of a field inside its HSA signal, so it is never null,
+// while 'mailbox' is that signal's event mailbox pointer, which is null unless
+// the signal is backed by an interrupt. Ringing such a doorbell must not
+// dereference the null mailbox, and must still publish the pending work so a
+// polling server makes progress.
+TEST(LlvmLibcRPCDoorbell, NotifyWithoutMailbox) {
+  ProcType Proc(port_count, buffer);
+
+  pending = 0;
+  Proc.doorbell->value = &pending;
+  Proc.doorbell->mailbox = nullptr;
+  Proc.doorbell->event_id = 0;
+
+  Proc.notify(/*lane_mask=*/1);
+
+  // notify() has no effect when built with MSVC.
+#ifndef _MSC_VER
+  EXPECT_EQ(pending, static_cast<uint64_t>(1));
+#endif
+}
+
+// A doorbell that was never configured at all is ignored entirely.
+TEST(LlvmLibcRPCDoorbell, NotifyWithoutDoorbell) {
+  ProcType Proc(port_count, buffer);
+
+  Proc.doorbell->value = nullptr;
+  Proc.doorbell->mailbox = nullptr;
+  Proc.doorbell->event_id = 0;
+
+  Proc.notify(/*lane_mask=*/1);
+}