[Lldb-commits] [lldb] Fix single thread stepping timeout race condition (PR #104195)

[via lldb-commits]

https://github.com/jeffreytan81 created https://github.com/llvm/llvm-project/pull/104195

This PR fixes a potential race condition in https://github.com/llvm/llvm-project/pull/90930. 

This race can happen because the original code set `m_info->m_isAlive = true` **after** the timer thread is created. So if there is a context switch happens and timer thread checks `m_info->m_isAlive` before main thread got a chance to run `m_info->m_isAlive = true`, the timer thread may treat `ThreadPlanSingleThreadTimeout` as not alive and simply exit.



>From 5d413e737dd522603ab22105946671d95dc2d038 Mon Sep 17 00:00:00 2001
From: jeffreytan81 <jeffreytan at fb.com>
Date: Wed, 14 Aug 2024 12:17:22 -0700
Subject: [PATCH] Fix single thread stepping timeout race condition

---
 lldb/source/Target/ThreadPlanSingleThreadTimeout.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lldb/source/Target/ThreadPlanSingleThreadTimeout.cpp b/lldb/source/Target/ThreadPlanSingleThreadTimeout.cpp
index 0a939d55f4ce49..806ba95c508b7c 100644
--- a/lldb/source/Target/ThreadPlanSingleThreadTimeout.cpp
+++ b/lldb/source/Target/ThreadPlanSingleThreadTimeout.cpp
@@ -29,10 +29,10 @@ ThreadPlanSingleThreadTimeout::ThreadPlanSingleThreadTimeout(
     : ThreadPlan(ThreadPlan::eKindSingleThreadTimeout, "Single thread timeout",
                  thread, eVoteNo, eVoteNoOpinion),
       m_info(info), m_state(State::WaitTimeout) {
-  // TODO: reuse m_timer_thread without recreation.
-  m_timer_thread = std::thread(TimeoutThreadFunc, this);
   m_info->m_isAlive = true;
   m_state = m_info->m_last_state;
+  // TODO: reuse m_timer_thread without recreation.
+  m_timer_thread = std::thread(TimeoutThreadFunc, this);
 }
 
 ThreadPlanSingleThreadTimeout::~ThreadPlanSingleThreadTimeout() {