linux-kernel - Re: [PATCH] riscv/barrier: Define __smp_{store_release,load

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Date:   Wed, 28 Feb 2018 01:15:24 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     Daniel Lustig <dlustig@...dia.com>
Cc:     Palmer Dabbelt <palmer@...ive.com>, albert@...ive.com,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv/barrier: Define __smp_{store_release,load_acquire}

On Tue, Feb 27, 2018 at 02:20:37PM -0800, Daniel Lustig wrote:
> On 2/27/2018 10:21 AM, Palmer Dabbelt wrote:
> > On Mon, 26 Feb 2018 18:24:11 PST (-0800), parri.andrea@...il.com wrote:
> >> Introduce __smp_{store_release,load_acquire}, and rely on the generic
> >> definitions for smp_{store_release,load_acquire}. This avoids the use
> >> of full ("rw,rw") fences on SMP.
> >>
> >> Signed-off-by: Andrea Parri <parri.andrea@...il.com>
> >> ---
> >>  arch/riscv/include/asm/barrier.h | 15 +++++++++++++++
> >>  1 file changed, 15 insertions(+)
> >>
> >> diff --git a/arch/riscv/include/asm/barrier.h b/arch/riscv/include/asm/barrier.h
> >> index 5510366d169ae..d4628e4b3a5ea 100644
> >> --- a/arch/riscv/include/asm/barrier.h
> >> +++ b/arch/riscv/include/asm/barrier.h
> >> @@ -38,6 +38,21 @@
> >>  #define __smp_rmb()    RISCV_FENCE(r,r)
> >>  #define __smp_wmb()    RISCV_FENCE(w,w)
> >>
> >> +#define __smp_store_release(p, v)                    \
> >> +do {                                    \
> >> +    compiletime_assert_atomic_type(*p);                \
> >> +    RISCV_FENCE(rw,w);                        \
> >> +    WRITE_ONCE(*p, v);                        \
> >> +} while (0)
> >> +
> >> +#define __smp_load_acquire(p)                        \
> >> +({                                    \
> >> +    typeof(*p) ___p1 = READ_ONCE(*p);                \
> >> +    compiletime_assert_atomic_type(*p);                \
> >> +    RISCV_FENCE(r,rw);                        \
> >> +    ___p1;                                \
> >> +})
> >> +
> >>  /*
> >>   * This is a very specific barrier: it's currently only used in two places in
> >>   * the kernel, both in the scheduler.  See include/linux/spinlock.h for the two
> > 
> > I'm adding Daniel just in case I misunderstood what's going on here,
> > but these look good to me.  As this is a non-trivial memory model
> > change I'm going to let it bake in linux-next for a bit just so it
> > gets some visibility.
> 
> Looks good to me too.  In particular, it also covers the
> Write->release(p)->acquire(p)->Write ordering that we were debating
> in the broader LKMM thread, which is good.

Yeah, I think that other changes would be required to completely cover
the issues debated in that thread: I plan to prepare and to post a new
series/RFC to address those (unless someone precedes me of course ;-).

  Andrea


> 
> Dan
> 
> > 
> > Thanks