linux-kernel - Re: [PATCH v4 4/4] arm64: lto: Strengthen READ_ONCE() to acquire when CONFIG

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Date:   Tue, 3 Nov 2020 13:13:29 +0000
From:   Will Deacon <will@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/4] arm64: lto: Strengthen READ_ONCE() to acquire
 when CONFIG_LTO=y

On Tue, Nov 03, 2020 at 12:58:45PM +0000, Mark Rutland wrote:
> On Tue, Nov 03, 2020 at 12:17:21PM +0000, Will Deacon wrote:
> > When building with LTO, there is an increased risk of the compiler
> > converting an address dependency headed by a READ_ONCE() invocation
> > into a control dependency and consequently allowing for harmful
> > reordering by the CPU.
> > 
> > Ensure that such transformations are harmless by overriding the generic
> > READ_ONCE() definition with one that provides acquire semantics when
> > building with LTO.
> > 
> > Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Signed-off-by: Will Deacon <will@...nel.org>
> 
> [...]
> 
> Could we add a note above __READ_ONCE() along the lines of the commit
> message, e.g.
> 
> /*
>  * With LTO a compiler might convert an address dependency headed by a
>  * READ_ONCE() into a control dependency, allowing for harmful
>  * reordering by the CPU.
>  *
>  * To prevent this, upgrade READ_OONCE() to provide acquire semantics
>  * when building with LTO.

It's not halloween any moooore :)

But yes, I'll add something to that effect, cheers.

Will