[PATCH] [libstdc++] Fix "bare" notifications dropped by waiters check

Richard Biener richard.guenther@gmail.com
Thu Apr 22 08:31:46 GMT 2021


On Thu, Apr 22, 2021 at 4:29 AM Thomas Rodgers
<rodgert@appliantology.com> wrote:
>
> From: Thomas Rodgers <rodgert@twrodgers.com>
>
> NOTE - This patch also needs to be backported to gcc-11 in order for
> semaphore release() to work correctly on non-futex platforms.
>
> Tested sparc-sun-solaris2.11
>
> For types that track whether or not there extant waiters (e.g.
> semaphore) internally, the __atomic_notify_address_bare() call was
> introduced to avoid the overhead of loading the atomic count of
> waiters. For platforms that don't have Futex, however, there was
> still a check for waiters, and seeing that there are none (because
> in the bare case, the count is not incremented), the notification
> is dropped. This commit addresses that case.

This is OK for the GCC 11 branch if approved for trunk.

Richard.

> libstdc++-v3/ChangeLog:
>         * include/bits/atomic_wait.h: Always notify waiters in the
>         in the case of 'bare' address notification.
> ---
>  libstdc++-v3/include/bits/atomic_wait.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
> index 0ac5575190c..984ed70f16c 100644
> --- a/libstdc++-v3/include/bits/atomic_wait.h
> +++ b/libstdc++-v3/include/bits/atomic_wait.h
> @@ -226,9 +226,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>
>        void
> -      _M_notify(const __platform_wait_t* __addr, bool __all) noexcept
> +      _M_notify(const __platform_wait_t* __addr, bool __all, bool __bare) noexcept
>        {
> -       if (!_M_waiting())
> +       if (!(__bare || _M_waiting()))
>           return;
>
>  #ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
> @@ -304,11 +304,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           }
>
>         void
> -       _M_notify(bool __all)
> +       _M_notify(bool __all, bool __bare = false)
>         {
>           if (_M_addr == &_M_w._M_ver)
>             __atomic_fetch_add(_M_addr, 1, __ATOMIC_ACQ_REL);
> -         _M_w._M_notify(_M_addr, __all);
> +         _M_w._M_notify(_M_addr, __all, __bare);
>         }
>
>         template<typename _Up, typename _ValFn,
> @@ -452,7 +452,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      __atomic_notify_address(const _Tp* __addr, bool __all) noexcept
>      {
>        __detail::__bare_wait __w(__addr);
> -      __w._M_notify(__all);
> +      __w._M_notify(__all, true);
>      }
>
>    // This call is to be used by atomic types which track contention externally
> @@ -464,7 +464,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      __detail::__platform_notify(__addr, __all);
>  #else
>      __detail::__bare_wait __w(__addr);
> -    __w._M_notify(__all);
> +    __w._M_notify(__all, true);
>  #endif
>    }
>  _GLIBCXX_END_NAMESPACE_VERSION
> --
> 2.30.2
>


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