linux-kernel - Re: [PATCH 4.19 079/271] x86/atomic: Fix smp_mb__{before,after}

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Message-ID: <20190726110135.GO31381@hirez.programming.kicks-ass.net>
Date:   Fri, 26 Jul 2019 13:01:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jari Ruusu <jari.ruusu@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 079/271] x86/atomic: Fix
 smp_mb__{before,after}_atomic()

On Fri, Jul 26, 2019 at 01:18:06PM +0300, Jari Ruusu wrote:
> Greg Kroah-Hartman wrote:
> > [ Upstream commit 69d927bba39517d0980462efc051875b7f4db185 ]
> > 
> > Recent probing at the Linux Kernel Memory Model uncovered a
> > 'surprise'. Strongly ordered architectures where the atomic RmW
> > primitive implies full memory ordering and
> > smp_mb__{before,after}_atomic() are a simple barrier() (such as x86)
> > fail for:
> > 
> >         *x = 1;
> >         atomic_inc(u);
> >         smp_mb__after_atomic();
> >         r0 = *y;
> 
> [snip]
> 
> > --- a/arch/x86/include/asm/atomic.h
> > +++ b/arch/x86/include/asm/atomic.h
> > @@ -54,7 +54,7 @@ static __always_inline void arch_atomic_add(int i, atomic_t *v)
> >  {
> >         asm volatile(LOCK_PREFIX "addl %1,%0"
> >                      : "+m" (v->counter)
> > -                    : "ir" (i));
> > +                    : "ir" (i) : "memory");
> >  }
> > 
> >  /**
> 
> Shouldn't those clobber contraints actually be:  "memory","cc"
> That is because addl subl (and other) machine instructions
> actually modify the flags register too.
> 
> gcc docs say: The "cc" clobber indicates that the assembler
> code modifies the flags register.

GCC x86 assumes any asm() will clobber "cc".