Change the control flow in atomic_compare_exchange_strong to avoid a potential deadlock.

When you assign a shared_ptr, the deleter gets called and assigned. In this routine, the assignment happens inside a critical section, which could (potentially) lead to a deadlock, if the deleter did something wonky. Now we swap the old value with an (empty) temporary shared_ptr, and then let the temporary delete the old value when it goes out of scope (after the lock has been released).  This should fix PR#27724. Thanks to Hans Boehm for the bug report and the suggested fix.



git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@269965 91177308-0d34-0410-b5e6-96231b3b80d8

include/memory

@@ -5541,14 +5541,17 @@ template <class _Tp>
 bool
 atomic_compare_exchange_strong(shared_ptr<_Tp>* __p, shared_ptr<_Tp>* __v, shared_ptr<_Tp> __w)
 {
+    shared_ptr<_Tp> __temp;
     __sp_mut& __m = __get_sp_mut(__p);
     __m.lock();
     if (__p->__owner_equivalent(*__v))
     {
+        _VSTD::swap(__temp, *__p);
         *__p = __w;
         __m.unlock();
         return true;
     }
+    _VSTD::swap(__temp, *__v);
     *__v = *__p;
     __m.unlock();
     return false;