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Date:   Tue, 23 Apr 2019 19:39:12 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Randy Dunlap <rdunlap@infradead.org>, akpm@linux-foundation.org,
        broonie@kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-next@vger.kernel.org, mhocko@suse.cz,
        mm-commits@vger.kernel.org, sfr@canb.auug.org.au,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>
Subject: Re: mmotm 2019-04-19-14-53 uploaded (objtool)
Message-ID: <20190423173912.GJ12232@hirez.programming.kicks-ass.net>
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On Tue, Apr 23, 2019 at 09:07:01AM -0700, Andy Lutomirski wrote:
> > diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> > index 22ba683afdc2..c82abd6e4ca3 100644
> > --- a/arch/x86/include/asm/uaccess.h
> > +++ b/arch/x86/include/asm/uaccess.h
> > @@ -427,10 +427,11 @@ do {                                    \
> > ({                                \
> >    __label__ __pu_label;                    \
> >    int __pu_err = -EFAULT;                    \
> > -    __typeof__(*(ptr)) __pu_val;                \
> > -    __pu_val = x;                        \
> > +    __typeof__(*(ptr)) __pu_val = (x);            \
> > +    __typeof__(ptr) __pu_ptr = (ptr);            \
> 
> Hmm.  I wonder if this forces the address calculation to be done
> before STAC, which means that gcc can’t use mov ..., %gs:(fancy
> stuff).  It probably depends on how clever the optimizer is. Have you
> looked at the generated code?

I have not; will do before posting the real patch.