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

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Message-ID: <1940294182.34562.1590506525684.JavaMail.zimbra@efficios.com>
Date:   Tue, 26 May 2020 11:22:05 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Florian Weimer <fweimer@...hat.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)

----- On May 26, 2020, at 10:57 AM, Florian Weimer fweimer@...hat.com wrote:

> * Mathieu Desnoyers:
> 
>>> Like the attribute, it needs to come right after the struct keyword, I
>>> think.  (Trailing attributes can be ambiguous, but not in this case.)
>>
>> Nope. _Alignas really _is_ special :-(
>>
>> struct _Alignas (16) blah {
>>         int a;
>> };
>>
>> p.c:1:8: error: expected ‘{’ before ‘_Alignas’
>>  struct _Alignas (16) blah {
> 
> Meh, yet another unnecessary C++ incompatibility.  C does not support
> empty structs, so I assume they didn't see the field requirement as a
> burden.

Indeed, it's weird.

> 
>> One last thing I'm planning to add in sys/rseq.h to cover acessing the
>> rseq_cs pointers with both the UAPI headers and the glibc struct rseq
>> declarations:
>>
>> /* The rseq_cs_ptr macro can be used to access the pointer to the current
>>    rseq critical section descriptor.  */
>> #ifdef __LP64__
>> # define rseq_cs_ptr(rseq) \
>>            ((const struct rseq_cs *) (rseq)->rseq_cs.ptr)
>> #else /* __LP64__ */
>> # define rseq_cs_ptr(rseq) \
>>            ((const struct rseq_cs *) (rseq)->rseq_cs.ptr.ptr32)
>> #endif /* __LP64__ */
>>
>> Does it make sense ?
> 
> Written this way, it's an aliasing violation.  I don't think it's very
> useful.

OK, I'll just remove it.

Thanks,

Mathieu


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