linux-kernel - Re: [PATCH 1/3] glibc: Perform rseq registration at C startup and thread creation (v21)

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Date:   Wed, 24 Jun 2020 16:20:47 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Carlos O'Donell <carlos@...hat.com>,
        Joseph Myers <joseph@...esourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        libc-alpha@...rceware.org, Thomas Gleixner <tglx@...utronix.de>,
        Ben Maurer <bmaurer@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <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@...r.kernel.org,
        linux-api@...r.kernel.org
Subject: Re: [PATCH 1/3] glibc: Perform rseq registration at C startup and thread creation (v21)

* Mathieu Desnoyers:

> diff --git a/manual/threads.texi b/manual/threads.texi
> index bb7a42c655..d5069d5581 100644
> --- a/manual/threads.texi
> +++ b/manual/threads.texi

> +@...typevar {struct rseq} __rseq_abi
> +@...ndards{Linux, sys/rseq.h}
> +@...glibc{} implements a @code{__rseq_abi} TLS symbol to interact with
> +the Restartable Sequences system call.  The layout of this structure is
> +defined by the @file{sys/rseq.h} header.  Registration of each thread's
> +@...e{__rseq_abi} is performed by @theglibc{} at library initialization
> +and thread creation. The manual for the rseq system call can be found
> +at @uref{https://git.kernel.org/pub/scm/libs/librseq/librseq.git/tree/doc/man/rseq.2}.

Should be “creation.  The” (two spaces after a sentence-ending period).

> diff --git a/sysdeps/unix/sysv/linux/sys/rseq.h b/sysdeps/unix/sysv/linux/sys/rseq.h
> new file mode 100644
> index 0000000000..5e118c1781
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/sys/rseq.h

> +#ifdef __cplusplus
> +# if  __cplusplus >= 201103L
> +#  define __rseq_static_assert(expr, diagnostic) static_assert (expr, diagnostic)
> +#  define __rseq_alignof(type)                   alignof (type)
> +#  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_tls_storage_class                _Thread_local
> +#endif
> +
> +#ifndef __rseq_static_assert
> +/* Try to use _Static_assert macro from sys/cdefs.h.  */
> +# ifdef _Static_assert
> +#  define __rseq_static_assert(expr, diagnostic) _Static_assert (expr, diagnostic)
> +# else
> +#  define __rseq_static_assert(expr, diagnostic) /* Nothing.  */
> +# endif
> +#endif
> +
> +/* Rely on GNU extensions for older standards and tls model.  */
> +#ifdef __GNUC__
> +# ifndef __rseq_alignof
> +#  define __rseq_alignof(x) __alignof__ (x)
> +# endif
> +# define __rseq_tls_model_ie __attribute__ ((__tls_model__ ("initial-exec")))
> +#else
> +/* Specifying the TLS model on the declaration is optional.  */
> +# define __rseq_tls_model_ie /* Nothing.  */
> +#endif

I'm still worried that __rseq_static_assert and __rseq_alignof will show
up in the UAPI with textually different definitions.  (This does not
apply to __rseq_tls_model_ie.)

Is my worry unfounded?

Thanks,
Florian