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

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Message-ID: <20190910142120.GM9720@e119886-lin.cambridge.arm.com>
Date:   Tue, 10 Sep 2019 15:21:21 +0100
From:   Andrew Murray <andrew.murray@....com>
To:     Will Deacon <will@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] arm64: fix unreachable code issue with cmpxchg

On Tue, Sep 10, 2019 at 08:46:07AM +0100, Will Deacon 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.

I assume we're referring to stuff such as the following?

https://www.spinics.net/lists/arm-kernel/msg730329.html

Are these breakages limited to the out-of-line hacks made for LL/SC
atomics, or were there other breakages elsewhere?

Now that the out-of-line hacks have gone, I wonder if this is actually
still a problem anymore. In any case isn't the right thing to do there
to add the __always_inline to functions that use the register keyword
in a function currently annotated inline?

I'm happy to look into this if there is likely to be some benefit in
turning on CONFIG_OPTIMIZE_INLINING.

Thanks,

Andrew Murray

> 
> Will