Re: [PATCH] Adjust wait logic to limit spurious evalution of wait predicate.

[Thomas Rodgers <rodgert@appliantology.com>]

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And this time, with patch.


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commit fe8959f61a0945662c2521b3ea47fbc2b751e881
Author: Thomas Rodgers <rodgert@appliantology.com>
Date:   Sun May 24 15:00:13 2020 -0700

    Adjust wait logic to limit spurious evalution of wait predicate.
    
            * include/bits/atomic_wait.h (__waiters::_M_do_wait): adjust wakeup
              logic.

diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
index 92c1e2526ed..cce11ae1cf5 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -138,24 +138,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 
       void
-      _M_do_wait(__platform_wait_t __version) noexcept
+      _M_do_wait(__platform_wait_t __old) noexcept
       {
-#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
-	__platform_wait(&_M_ver, __version);
-#else
-	__platform_wait_t __cur = 0;
-	while (__cur <= __version)
+	__platform_wait_t __cur;
+	__atomic_load(&_M_ver, &__cur, __ATOMIC_ACQUIRE);
+	while (__cur == __old)
 	  {
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
+	    __platform_wait(&_M_ver, __cur);
+#else
 	    __waiters::__lock_t __l(_M_mtx);
 	    auto __e = __gthread_cond_wait(&_M_cv, __l.mutex()->native_handle());
 	    if (__e)
 	      std::terminate();
-	    __platform_wait_t __last = __cur;
-	    __atomic_load(&_M_ver, &__cur, __ATOMIC_ACQUIRE);
-	    if (__cur < __last)
-	      break; // break the loop if version overflows
-	  }
 #endif
+	    __atomic_load(&_M_ver, &__cur, __ATOMIC_ACQUIRE);
+	  }
       }
 
       __platform_wait_t

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> On May 24, 2020, at 3:06 PM, Thomas Rodgers <rodgert@appliantology.com> wrote:
> 
> 	* include/bits/atomic_wait.h (__waiters::_M_do_wait): adjust wakeup logic.