linux-kernel - Re: [PATCH] x86/asm: use memory clobber in bitops that touch arbitrary memory

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Date:   Tue, 2 Apr 2019 10:59:05 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Paul McKenney <paulmck@...ux.ibm.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        James Y Knight <jyknight@...gle.com>, x86@...nel.org,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] x86/asm: use memory clobber in bitops that touch
 arbitrary memory

On Tue, Apr 2, 2019 at 9:27 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
>
>
> On Mon, Apr 01, 2019 at 06:24:08PM +0200, Alexander Potapenko wrote:
> > diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> > index d153d570bb04..20e4950827d9 100644
> > --- a/arch/x86/include/asm/bitops.h
> > +++ b/arch/x86/include/asm/bitops.h
> > @@ -111,7 +111,7 @@ clear_bit(long nr, volatile unsigned long *addr)
> >       } else {
> >               asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
> >                       : BITOP_ADDR(addr)
> > -                     : "Ir" (nr));
> > +                     : "Ir" (nr) : "memory");
> >       }
> >  }
>
> clear_bit() doesn't have a return value, so why are we now still using
> "+m" output ?
You're right, "m" should suffice. I'll update the patch.
By the way, now that we've added the memory barriers we can probably
remove the barrier() call from __clear_bit_unlock() and update the
comment accordingly.
For clear_bit() the memory barrier is missing on the IS_IMMEDIATE(nr)
path, so this one should be kept as is.

> AFAICT the only reason we did that was to clobber the variable, which
> you've (afaiu correctly) argued to be incorrect.
>
> So whould we not write this as:
>
>         asm volatile (LOCK_PREFIX __ASM_SIZE(btr) " %[nr], %[addr]"
>                 : : [addr] "m" (*addr), [nr] "Ir" (nr)
>                 : "memory");
>
> ?
>
> And the very same for _all_ other sites touched in this patch.



-- 
Alexander Potapenko
Software Engineer

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