[PATCH] [libstdc++] Fix test timeout in stop_calback/destroy.cc

[Thomas Rodgers]

From: Thomas Rodgers <rodg...@twrodgers.com>

A change was made to __atomic_semaphore::_S_do_try_acquire() to
(ideally) let the compare_exchange reload the value of __old rather than
always reloading it twice. This causes _M_acquire to spin indefinitely
if the value of __old is already 0.

libstdc++/ChangeLog:
        * include/bits/semaphore_base.h: Always reload __old in
        __atomic_semaphore::_S_do_try_acquire().
        * testsuite/30_threads/stop_token/stop_callback/destroy.cc
        re-enable testcase.
---
 libstdc++-v3/include/bits/semaphore_base.h       | 16 ++++++----------
 .../stop_token/stop_callback/destroy.cc          |  2 --
 2 files changed, 6 insertions(+), 12 deletions(-)

diff --git a/libstdc++-v3/include/bits/semaphore_base.h 
b/libstdc++-v3/include/bits/semaphore_base.h
index 35469e443b0..84b33423fff 100644
--- a/libstdc++-v3/include/bits/semaphore_base.h
+++ b/libstdc++-v3/include/bits/semaphore_base.h
@@ -196,9 +196,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     __atomic_semaphore& operator=(const __atomic_semaphore&) = delete;
 
     static _GLIBCXX_ALWAYS_INLINE bool
-    _S_do_try_acquire(__detail::__platform_wait_t* __counter,
-                     __detail::__platform_wait_t& __old) noexcept
+    _S_do_try_acquire(__detail::__platform_wait_t* __counter) noexcept
     {
+      auto __old = __atomic_impl::load(__counter, memory_order::acquire);
       if (__old == 0)
        return false;
 
@@ -211,18 +211,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _GLIBCXX_ALWAYS_INLINE void
     _M_acquire() noexcept
     {
-      auto __old = __atomic_impl::load(&_M_counter, memory_order::acquire);
       auto const __pred =
-       [this, &__old] { return _S_do_try_acquire(&this->_M_counter, __old); };
+       [this] { return _S_do_try_acquire(&this->_M_counter); };
       std::__atomic_wait_address_bare(&_M_counter, __pred);
     }
 
     bool
     _M_try_acquire() noexcept
     {
-      auto __old = __atomic_impl::load(&_M_counter, memory_order::acquire);
       auto const __pred =
-       [this, &__old] { return _S_do_try_acquire(&this->_M_counter, __old); };
+       [this] { return _S_do_try_acquire(&this->_M_counter); };
       return std::__detail::__atomic_spin(__pred);
     }
 
@@ -231,9 +229,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _M_try_acquire_until(const chrono::time_point<_Clock,
                           _Duration>& __atime) noexcept
       {
-       auto __old = __atomic_impl::load(&_M_counter, memory_order_relaxed);
        auto const __pred =
-         [this, &__old] { return _S_do_try_acquire(&this->_M_counter, __old); 
};
+         [this] { return _S_do_try_acquire(&this->_M_counter); };
 
        return __atomic_wait_address_until_bare(&_M_counter, __pred, __atime);
       }
@@ -243,9 +240,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _M_try_acquire_for(const chrono::duration<_Rep, _Period>& __rtime)
        noexcept
       {
-       auto __old = __atomic_impl::load(&_M_counter, memory_order_relaxed);
        auto const __pred =
-         [this, &__old] { return _S_do_try_acquire(&this->_M_counter, __old); 
};
+         [this] { return _S_do_try_acquire(&this->_M_counter); };
 
        return __atomic_wait_address_for_bare(&_M_counter, __pred, __rtime);
       }
diff --git 
a/libstdc++-v3/testsuite/30_threads/stop_token/stop_callback/destroy.cc 
b/libstdc++-v3/testsuite/30_threads/stop_token/stop_callback/destroy.cc
index c2cfba027cb..061ed448c33 100644
--- a/libstdc++-v3/testsuite/30_threads/stop_token/stop_callback/destroy.cc
+++ b/libstdc++-v3/testsuite/30_threads/stop_token/stop_callback/destroy.cc
@@ -21,8 +21,6 @@
 // { dg-require-effective-target pthread }
 // { dg-require-gthreads "" }
 
-// { dg-skip-if "FIXME: times out" { *-*-* } }
-
 #include <stop_token>
 #include <atomic>
 #include <thread>
-- 
2.30.2