linux-kernel - Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v20)

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Message-ID: <1953500643.51064.1591186963416.JavaMail.zimbra@efficios.com>
Date:   Wed, 3 Jun 2020 08:22:43 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     carlos <carlos@...hat.com>, Joseph Myers <joseph@...esourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        libc-alpha <libc-alpha@...rceware.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ben Maurer <bmaurer@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Rich Felker <dalias@...c.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup
 and thread creation (v20)

----- On Jun 3, 2020, at 8:05 AM, Florian Weimer fweimer@...hat.com wrote:

> * Mathieu Desnoyers:
> 
>> +#ifdef __cplusplus
>> +# if  __cplusplus >= 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) >= 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
> 
> This does not seem to work.  I get this with GCC 9:
> 
> In file included from /tmp/cih_test_gsrKbC.cc:8:0:
> ../sysdeps/unix/sysv/linux/sys/rseq.h:42:50: error: attribute ignored
> [-Werror=attributes]
> #  define __rseq_alignas(x)                      alignas (x)
>                                                  ^
> ../sysdeps/unix/sysv/linux/sys/rseq.h:122:14: note: in expansion of macro
> ‘__rseq_alignas’
>     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 ?

> 
> 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 patchset,
or should I re-spin another round of the series ?
> 
> The resut looks okay to me.
> 
> I'm still waiting for feedback from other maintainers whether the level
> of documentation and testing is appropriate.

OK.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com