[PATCH] libstdc++: Add C++2a synchronization support

[Jonathan Wakely]

On 25/11/20 10:35 +0000, Jonathan Wakely wrote:
>On 25/11/20 01:07 +0000, Jonathan Wakely wrote:
>>On 24/11/20 23:45 +0000, Jonathan Wakely wrote:
>>>On 21/11/20 16:36 -0800, H.J. Lu wrote:
>>>>On Sat, Nov 21, 2020 at 9:40 AM Jonathan Wakely via Gcc-patches
>>>><gcc-patches@gcc.gnu.org> wrote:
>>>>>
>>>>>On 21/11/20 17:04 +0000, Jonathan Wakely wrote:
>>>>>>On 21/11/20 16:16 +0100, Andreas Schwab wrote:
>>>>>>>In file included from /daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/bits/shared_ptr_atomic.h:33,
>>>>>>>              from /daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/memory:78,
>>>>>>>              from /daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/m68k-linux/bits/stdc++.h:82,
>>>>>>>              from /daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/m68k-linux/bits/extc++.h:32,
>>>>>>>              from /daten/aranym/gcc/gcc-20201121/libstdc++-v3/testsuite/17_intro/headers/c++2020/all_attributes.cc:37:
>>>>>>>/daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/bits/atomic_base.h: In member function 'void std::atomic_flag::wait(bool, std::memory_order) const':
>>>>>>>/daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/bits/atomic_base.h:239: error: no matching function for call to '__atomic_wait(const __atomic_flag_data_type*, bool&, std::atomic_flag::wait(bool, std::memory_order) const::<lambda()>)'
>>>>>>>In file included from /daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/bits/atomic_base.h:41,
>>>>>>>              from /daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/bits/shared_ptr_atomic.h:33,
>>>>>>>              from /daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/memory:78,
>>>>>>>              from /daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/m68k-linux/bits/stdc++.h:82,
>>>>>>>              from /daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/m68k-linux/bits/extc++.h:32,
>>>>>>>              from /daten/aranym/gcc/gcc-20201121/libstdc++-v3/testsuite/17_intro/headers/c++2020/all_attributes.cc:37:
>>>>>>>/daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/bits/atomic_wait.h:265: note: candidate: 'template<class _Tp, class _Pred> void std::__atomic_wait(const _Tp*, _Tp, _Pred)'
>>>>>>>/daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/bits/atomic_wait.h:265: note:   template argument deduction/substitution failed:
>>>>>>>In file included from /daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/bits/shared_ptr_atomic.h:33,
>>>>>>>              from /daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/memory:78,
>>>>>>>              from /daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/m68k-linux/bits/stdc++.h:82,
>>>>>>>              from /daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/m68k-linux/bits/extc++.h:32,
>>>>>>>              from /daten/aranym/gcc/gcc-20201121/libstdc++-v3/testsuite/17_intro/headers/c++2020/all_attributes.cc:37:
>>>>>>>/daten/aranym/gcc/gcc-20201121/Build/m68k-linux/libstdc++-v3/include/bits/atomic_base.h:239: note:   deduced conflicting types for parameter '_Tp' ('unsigned char' and 'bool')
>>>>>>
>>>>>>I'm testing this.
>>>>>
>>>>>I'm committing this instead, it's the same but also disables
>>>>>29_atomics/atomic/wait_notify/generic.cc on non-linux targets.
>>>>>
>>>>>Tested sparc-solaris2.11 and powerpc64le-linux.
>>>>>
>>>>>There are still some timeouts on linux:
>>>>>
>>>>>FAIL: 30_threads/latch/3.cc execution test
>>>>>FAIL: 30_threads/semaphore/try_acquire_for.cc execution test
>>>>>
>>>>
>>>>I opened:
>>>>
>>>>https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97936
>>>
>>>I've disabled the failing tests for now. They can be re-enabled after
>>>the problem is found and fixed.
>>
>>I was finally able to reproduce the hangs, and I think this is the
>>fix:
>>
>>--- a/libstdc++-v3/include/bits/atomic_wait.h
>>+++ b/libstdc++-v3/include/bits/atomic_wait.h
>>@@ -100,9 +100,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>           auto __e = syscall (SYS_futex, static_cast<const void*>(__addr),
>>                                 static_cast<int>(__futex_wait_flags::__wait_private),
>>                                   __val, nullptr);
>>-           if (!__e)
>>+           if (!__e || errno == EAGAIN)
>>             break;
>>-           else if (!(errno == EINTR || errno == EAGAIN))
>>+           else if (errno != EINTR)
>>             __throw_system_error(__e);
>>         }
>>      }
>>
>>The problem is that we're going into a busy loop when SYS_futex
>>returns EAGAIN, but that means the current value doesn't match the
>>expected value, so we should return not keep waiting for the value to
>>change.
>
>I've pushed that as ad9cbcee543ecccd79fa49dafcd925532d2ce210 but there
>are still other FAILs to be fixed.

Except that what I pushed was not what I wrote above, as noticed by
Jakub. I need to get more sleep.

Fixed with this patch, pushed to trunk.


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