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From:   Florian Weimer <fweimer@redhat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     carlos <carlos@redhat.com>, Joseph Myers <joseph@codesourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        libc-alpha <libc-alpha@sourceware.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ben Maurer <bmaurer@fb.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Paul <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
        Rich Felker <dalias@libc.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>
Subject: Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v20)
References: <20200527185130.5604-1-mathieu.desnoyers@efficios.com>
        <20200527185130.5604-2-mathieu.desnoyers@efficios.com>
        <87d06gxsla.fsf@oldenburg2.str.redhat.com>
        <1953500643.51064.1591186963416.JavaMail.zimbra@efficios.com>
Date:   Wed, 03 Jun 2020 14:31:54 +0200
In-Reply-To: <1953500643.51064.1591186963416.JavaMail.zimbra@efficios.com>
        (Mathieu Desnoyers's message of "Wed, 3 Jun 2020 08:22:43 -0400
        (EDT)")
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* Mathieu Desnoyers:

> ----- On Jun 3, 2020, at 8:05 AM, Florian Weimer fweimer@redhat.com wrote:
>
>> * Mathieu Desnoyers:
>>=20
>>> +#ifdef __cplusplus
>>> +# if  __cplusplus >=3D 201103L
>>> +#  define __rseq_static_assert(expr, diagnostic) static_assert (expr,
>>> diagnostic)
>>> +#  define __rseq_alignof(type)                   alignof (type)
>>> +#  define __rseq_alignas(x)                      alignas (x)
>>> +#  define __rseq_tls_storage_class               thread_local
>>> +# endif
>>> +#elif (defined __STDC_VERSION__ ? __STDC_VERSION__ : 0) >=3D 201112L
>>> +# define __rseq_static_assert(expr, diagnostic)  _Static_assert (expr,
>>> diagnostic)
>>> +# define __rseq_alignof(type)                    _Alignof (type)
>>> +# define __rseq_alignas(x)                       _Alignas (x)
>>> +# define __rseq_tls_storage_class                _Thread_local
>>> +#endif
>>=20
>> This does not seem to work.  I get this with GCC 9:
>>=20
>> In file included from /tmp/cih_test_gsrKbC.cc:8:0:
>> ../sysdeps/unix/sysv/linux/sys/rseq.h:42:50: error: attribute ignored
>> [-Werror=3Dattributes]
>> #  define __rseq_alignas(x)                      alignas (x)
>>                                                  ^
>> ../sysdeps/unix/sysv/linux/sys/rseq.h:122:14: note: in expansion of macro
>> =E2=80=98__rseq_alignas=E2=80=99
>>     uint32_t __rseq_alignas (32) version;
>>              ^
>
> Is that when compiling C or C++ code ? If it's C code, I would expect
> "_Alignas" to be used, not "alignas".
>
> Which exact version of gcc do you use ?

C++ code.  CXX was set to this compiler at configure time:

gcc version 9.3.1 20200408 (Red Hat 9.3.1-2) (GCC)=20

>> In any case, these changes really have to go into the UAPI header first.
>> Only the __thread handling should remain.  Otherwise, we'll have a tough
>> situation on our hands changing the UAPI header, without introducing
>> macro definition conflicts.  I'd suggest to stick to the aligned
>> attribute for the time being, like the current UAPI headers.
>
> OK. Should I do that in a separate patch, or you do it on top of my patch=
set,
> or should I re-spin another round of the series ?

I think the initial commit should mirror the current UAPI header
contents.

Keep the macros for the UAPI patch though. 8-)  We can pick up these
changes once they have been merged into Linux.

Thanks,
Florian