linux-kernel - Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v19)

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Date:   Tue, 26 May 2020 14:41:14 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     libc-alpha <libc-alpha@...rceware.org>,
        Rich Felker <dalias@...c.org>,
        linux-api <linux-api@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ben Maurer <bmaurer@...com>, Dave Watson <davejwatson@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul <paulmck@...ux.vnet.ibm.com>, Paul Turner <pjt@...gle.com>,
        Joseph Myers <joseph@...esourcery.com>
Subject: Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v19)

* Mathieu Desnoyers:

> Something like this ?
>
> #ifdef __cplusplus
> # if  __cplusplus >= 201103L
> #  define rseq_static_assert (expr, diagnostic)         static_assert (expr, diagnostic)
> #  define rseq_alignof                                  alignof
> # endif
> #elif __STDC_VERSION__ >= 201112L
> # define rseq_static_assert (expr, diagnostic)          _Static_assert (expr, diagnostic)
> # define rseq_alignof                                   _Alignof
> #endif
>
> #ifndef rseq_static_assert
> # define rseq_static_assert (expr, diagnostic)          /* nothing */
> #endif

You can't have a space in #defines like that, no matter what GNU style
says. 8-)

> /* Ensure the compiler supports __attribute__ ((aligned)).  */
> rseq_static_assert ((rseq_alignof (struct rseq_cs) >= 32, "alignment"));
> rseq_static_assert ((rseq_alignof (struct rseq) >= 32, "alignment"));

You need to move the ; into rseq_static_assert.  And if you use explicit
arguments, you can't use double parentheses.

>> And something similar for _Alignas/attribute aligned,
>
> I don't see where _Alignas is needed here ?
>
> For attribute aligned, what would be the oldest supported C and C++
> standards ?

There are no standardized attributes for C, there is only _Alignas.
C++11 has an alignas specifier; it's not an attribute either.  I think
these are syntactically similar.

>> with an error for
>> older standards and !__GNUC__ compilers (because neither the type nor
>> __thread can be represented there).
>
> By "type" you mean "struct rseq" here ? What does it contain that requires
> a __GNUC__ compiler ?

__attribute__ and __thread support.

> About __thread, I recall other compilers have other means to declare it.
> In liburcu, I end up with the following:
>
> #if defined (__cplusplus) && (__cplusplus >= 201103L)
> # define URCU_TLS_STORAGE_CLASS thread_local
> #elif defined (__STDC_VERSION__) && (__STDC_VERSION__ >= 201112L)
> # define URCU_TLS_STORAGE_CLASS _Thread_local
> #elif defined (_MSC_VER)
> # define URCU_TLS_STORAGE_CLASS __declspec(thread)
> #else
> # define URCU_TLS_STORAGE_CLASS __thread
> #endif
>
> Would something along those lines be OK for libc ?

Yes, it would be okay (minus the Visual C++ part).  This part does not
have to go into UAPI headers first.  A fallback definition of __thread
should be okay.  Outside glibc, the TLS model declaration is optional, I
think.  The glibc *definition* ensures that the variable is
initial-exec.

Thanks,
Florian