Re: [PATCH] libstdc++: Fix wrong thread waking on notify [PR100334]

[Jonathan Wakely <jwakely@redhat.com>]

On 13/05/21 18:54 -0700, Thomas Rodgers wrote:
>From: Thomas Rodgers <rodgert@twrodgers.com>
>
>Please ignore the previous patch. This one removes the need to carry any
>extra state in the case of a 'laundered' atomic wait.
>
>libstdc++/ChangeLog:
>	* include/bits/atomic_wait.h (__waiter::_M_do_wait_v): loop
>	until value change observed.
>	(__waiter_base::_M_laundered): New member function.
>	(__watier_base::_M_notify): Check _M_laundered() to determine
>	whether to wake one or all.
>	(__detail::__atomic_compare): Return true if call to
>	__builtin_memcmp() == 0.
>	(__waiter_base::_S_do_spin_v): Adjust predicate.
>	* testsuite/29_atomics/atomic/wait_notify/100334.cc: New
>	test.
>---
> libstdc++-v3/include/bits/atomic_wait.h       | 28 ++++--
> .../29_atomics/atomic/wait_notify/100334.cc   | 94 +++++++++++++++++++
> 2 files changed, 114 insertions(+), 8 deletions(-)
> create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic/wait_notify/100334.cc
>
>diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
>index 984ed70f16c..07bb744d822 100644
>--- a/libstdc++-v3/include/bits/atomic_wait.h
>+++ b/libstdc++-v3/include/bits/atomic_wait.h
>@@ -181,11 +181,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> 	return false;
>       }
>
>+    // return true if equal
>     template<typename _Tp>
>       bool __atomic_compare(const _Tp& __a, const _Tp& __b)
>       {
> 	// TODO make this do the correct padding bit ignoring comparison
>-	return __builtin_memcmp(&__a, &__b, sizeof(_Tp)) != 0;
>+	return __builtin_memcmp(&__a, &__b, sizeof(_Tp)) == 0;
>       }
>
>     struct __waiter_pool_base
>@@ -300,14 +301,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> 	  explicit __waiter_base(const _Up* __addr) noexcept
> 	    : _M_w(_S_for(__addr))
> 	    , _M_addr(_S_wait_addr(__addr, &_M_w._M_ver))
>-	  {
>-	  }
>+	  { }
>+
>+	bool
>+	_M_laundered() const
>+	{ return _M_addr == &_M_w._M_ver; }
>
> 	void
> 	_M_notify(bool __all, bool __bare = false)
> 	{
>-	  if (_M_addr == &_M_w._M_ver)
>-	    __atomic_fetch_add(_M_addr, 1, __ATOMIC_ACQ_REL);
>+	  if (_M_laundered())
>+	    {
>+	      __atomic_fetch_add(_M_addr, 1, __ATOMIC_ACQ_REL);

Please mention this increment in the changelog.

OK for trunk and gcc-11 with that change, thanks.