[Lldb-commits] [PATCH] D154271: [lldb] Fix data race when interacting with python scripts

[Med Ismail Bennani via Phabricator via lldb-commits]

mib updated this revision to Diff 536540.
mib edited the summary of this revision.
mib added a comment.

Use `std::atomic<T>::fetch_{add, sub}` to address @JDevlieghere feedback.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D154271/new/

https://reviews.llvm.org/D154271

Files:
  lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPythonImpl.h
  lldb/source/Target/Process.cpp


Index: lldb/source/Target/Process.cpp
===================================================================
--- lldb/source/Target/Process.cpp
+++ lldb/source/Target/Process.cpp
@@ -2467,6 +2467,7 @@
 }
 
 void Process::LoadOperatingSystemPlugin(bool flush) {
+  std::lock_guard<std::recursive_mutex> guard(m_thread_mutex);
   if (flush)
     m_thread_list.Clear();
   m_os_up.reset(OperatingSystem::FindPlugin(this, nullptr));
Index: lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPythonImpl.h
===================================================================
--- lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPythonImpl.h
+++ lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPythonImpl.h
@@ -380,12 +380,22 @@
 
   uint32_t IsExecutingPython() const { return m_lock_count > 0; }
 
-  uint32_t IncrementLockCount() { return ++m_lock_count; }
+  uint32_t IncrementLockCount() {
+    // std::atomic::fetch_add is an atomic post-increment operation so we need
+    // to add 1 to its return value.
+    return m_lock_count.fetch_add(1, std::memory_order_relaxed) + 1;
+  }
 
   uint32_t DecrementLockCount() {
-    if (m_lock_count > 0)
-      --m_lock_count;
-    return m_lock_count;
+    // std::atomic::fetch_sub is an atomic post-increment operation so we need
+    // to subtract 1 to its return value.
+    uint32_t count = m_lock_count.fetch_sub(1, std::memory_order_relaxed) - 1;
+    if (static_cast<int32_t>(count) < 0) {
+      // Handle underflow here & reset count to zero.
+      count = 0;
+      m_lock_count.store(count, std::memory_order_relaxed);
+    }
+    return count;
   }
 
   enum ActiveIOHandler {
@@ -421,7 +431,7 @@
   bool m_session_is_active;
   bool m_pty_secondary_is_open;
   bool m_valid_session;
-  uint32_t m_lock_count;
+  std::atomic<uint32_t> m_lock_count;
   PyThreadState *m_command_thread_state;
 };
 


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