[libcxx-commits] [PATCH] D105758: Hold mutex lock while notify_all is called at notify_all_at_thread_exit

[Vinícius dos Santos Oliveira via Phabricator via libcxx-commits]

vinipsmaker created this revision.
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Releasing the mutex before the call to notify_all is an optimization.
This optimization cannot be used here. The thread waiting on the
condition might destroy the associated resources -- mutex + condition
variable -- and the notifier thread will access an destroyed variable

- the condition variable. In fact, notify_all_at_thread_exit is meant

exactly to join on detached threads, and the waiting thread doesn't
expect for the notifier thread to access any further shared resources,
making this scenario very likely to happen. The waiting thread might
awake spuriously on the release of the mutex lock. The reorder is
necessary to prevent this race.

Further details can be found at
https://cplusplus.github.io/LWG/issue3343.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D105758

Files:
  libcxx/src/thread.cpp


Index: libcxx/src/thread.cpp
===================================================================
--- libcxx/src/thread.cpp
+++ libcxx/src/thread.cpp
@@ -158,8 +158,8 @@
     for (_Notify::iterator i = notify_.begin(), e = notify_.end();
             i != e; ++i)
     {
-        i->second->unlock();
         i->first->notify_all();
+        i->second->unlock();
     }
     for (_AsyncStates::iterator i = async_states_.begin(), e = async_states_.end();
             i != e; ++i)


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