linux-kernel - Re: [PATCH] arm64: fix unreachable code issue with cmpxchg

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Date:   Tue, 10 Sep 2019 14:24:15 +0100
From:   Will Deacon <will@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Andrew Murray <andrew.murray@....com>,
        Mark Rutland <mark.rutland@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Borislav Petkov <bp@...e.de>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] arm64: fix unreachable code issue with cmpxchg

On Tue, Sep 10, 2019 at 10:04:24AM +0200, Arnd Bergmann wrote:
> On Tue, Sep 10, 2019 at 9:46 AM Will Deacon <will@...nel.org> wrote:
> >
> > On Mon, Sep 09, 2019 at 10:21:35PM +0200, Arnd Bergmann wrote:
> > > On arm64 build with clang, sometimes the __cmpxchg_mb is not inlined
> > > when CONFIG_OPTIMIZE_INLINING is set.
> >
> > Hmm. Given that CONFIG_OPTIMIZE_INLINING has also been shown to break
> > assignment of local 'register' variables on GCC, perhaps we should just
> > disable that option for arm64 (at least) since we don't have any toolchains
> > that seem to like it very much! I'd certainly prefer that over playing
> > whack-a-mole with __always_inline.
> 
> Right, but I can also see good reasons to keep going:
> 
> - In theory, CONFIG_OPTIMIZE_INLINING is the right thing to do -- the compilers
>   also make some particularly bad decisions around inlining when each inline
>   turns into an __always_inline, as has been the case in Linux for a long time.
>   I think in most cases, we get better object code with CONFIG_OPTIMIZE_INLINING
>   and in the cases where this is worse, it may be better to fix the compiler.
>   The new "asm_inline"  macro should also help with that.

Sure, in theory, but it looks like there isn't a single arm64 compiler out
there which gets it right.

> - The x86 folks have apparently whacked most of the moles already, see this
>   commit from 2008
> 
>    commit 3f9b5cc018566ad9562df0648395649aebdbc5e0
>    Author: Ingo Molnar <mingo@...e.hu>
>    Date:   Fri Jul 18 16:30:05 2008 +0200
> 
>     x86: re-enable OPTIMIZE_INLINING
> 
>     re-enable OPTIMIZE_INLINING more widely. Jeff Dike fixed the remaining
>     outstanding issue in this commit:
> 
>     | commit 4f81c5350b44bcc501ab6f8a089b16d064b4d2f6
>     | Author: Jeff Dike <jdike@...toit.com>
>     | Date:   Mon Jul 7 13:36:56 2008 -0400
>     |
>     |     [UML] fix gcc ICEs and unresolved externs
>     [...]
>     |    This patch reintroduces unit-at-a-time for gcc >= 4.0,
> bringing back the
>     |    possibility of Uli's crash.  If that happens, we'll debug it.
> 
>     it's still default-off and thus opt-in.

This appears to be fixing an ICE, whereas the issue reported recently for
arm64 gcc was silent miscompilation of atomics in some cases. Unfortunately,
I can't seem to find the thread :/ Mark, you were on that one too, right?

> - The inlining decisions of gcc and clang are already very different, and
>    the bugs we are finding around that are much more common than
>    the difference between CONFIG_OPTIMIZE_INLINING=y/n on a
>    given compiler.

Sorry, not sure that you're getting at here.

Anyway, the second version of your patch looks fine, but I would still
prefer to go the extra mile and disable CONFIG_OPTIMIZE_INLINING altogether
given that I don't think it's a safe option to enable for us.

Will