linux-kernel - Re: [PATCH 00/22] add support for Clang LTO

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Message-ID: <20200703145151.GG9247@paulmck-ThinkPad-P72>
Date:   Fri, 3 Jul 2020 07:51:51 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marco Elver <elver@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>
Subject: Re: [PATCH 00/22] add support for Clang LTO

On Fri, Jul 03, 2020 at 03:25:23PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 03, 2020 at 03:13:30PM +0200, Peter Zijlstra wrote:
> > > The prototype for GCC is here: https://github.com/AKG001/gcc/
> > 
> > Thanks! Those test cases are somewhat over qualified though:
> > 
> >        static volatile _Atomic (TYPE) * _Dependent_ptr a;     		\
> 
> One question though; since its a qualifier, and we've recently spend a
> whole lot of effort to strip qualifiers in say READ_ONCE(), how does,
> and how do we want, this qualifier to behave.

Dereferencing a _Dependent_ptr pointer gives you something that is not
_Dependent_ptr, unless the declaration was like this:

	_Dependent_ptr _Atomic (TYPE) * _Dependent_ptr a;

And if I recall correctly, the current state is that assigning a
_Dependent_ptr variable to a non-_Dependent_ptr variable strips this
marking (though the thought was to be able to ask for a warning).

So, yes, it would be nice to be able to explicitly strip the
_Dependent_ptr, perhaps the kill_dependency() macro, which is already
in the C standard.

> C++ has very convenient means of manipulating qualifiers, so it's not
> much of a problem there, but for C it is, as we've found, really quite
> cumbersome. Even with _Generic() we can't manipulate individual
> qualifiers afaict.

Fair point, and in C++ this is a templated class, at least in the same
sense that std::atomic<> is a templated class.

But in this case, would kill_dependency do what you want?

							Thanx, Paul