linux-kernel - Re: [PATCH tip/core/rcu 48/55] powerpc: strengthen value-returning-atomics memory barriers

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Date:	Fri, 9 Sep 2011 11:43:38 -0700
From:	Olof Johansson <olof@...om.net>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, patches@...aro.org,
	anton@...ba.org, benh@...nel.crashing.org, paulus@...ba.org,
	linuxppc-dev@...abs.org
Subject: Re: [PATCH tip/core/rcu 48/55] powerpc: strengthen
 value-returning-atomics memory barriers

On Fri, Sep 9, 2011 at 10:34 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Fri, Sep 09, 2011 at 10:23:33AM -0700, Olof Johansson wrote:
>> [+linuxppc-dev]
>>
>> On Tue, Sep 6, 2011 at 11:00 AM, Paul E. McKenney
>> <paulmck@...ux.vnet.ibm.com> wrote:
>> > The trailing isync/lwsync in PowerPC value-returning atomics needs
>> > to be a sync in order to provide the required ordering properties.
>> > The leading lwsync/eieio can remain, as the remainder of the required
>> > ordering guarantees are provided by the atomic instructions: Any
>> > reordering will cause the stwcx to fail, which will result in a retry.
>>
>> Admittedly, my powerpc barrier memory is starting to fade, but isn't
>> isync sufficient here? It will make sure all instructions before it
>> have retired, and will restart any speculative/issued instructions
>> beyond it.
>>
>> lwsync not being sufficient makes sense since a load can overtake it.
>
> As I understand it, although isync waits for the prior stwcx to execute,
> it does not guarantee that the corresponding store is visible to all
> processors before any following loads.

Ah yes, combined with brushing up on the semantics in
memory-barriers.txt, this sounds reasonable to me.

>> > diff --git a/arch/powerpc/include/asm/synch.h b/arch/powerpc/include/asm/synch.h
>> > index d7cab44..4d97fbe 100644
>> > --- a/arch/powerpc/include/asm/synch.h
>> > +++ b/arch/powerpc/include/asm/synch.h
>> > @@ -37,11 +37,7 @@ static inline void isync(void)
>> >  #endif
>> >
>> >  #ifdef CONFIG_SMP
>> > -#define __PPC_ACQUIRE_BARRIER                          \
>> > -       START_LWSYNC_SECTION(97);                       \
>> > -       isync;                                          \
>> > -       MAKE_LWSYNC_SECTION_ENTRY(97, __lwsync_fixup);
>> > -#define PPC_ACQUIRE_BARRIER    "\n" stringify_in_c(__PPC_ACQUIRE_BARRIER)
>> > +#define PPC_ACQUIRE_BARRIER    "\n" stringify_in_c(sync;)
>>
>> This can just be done as "\n\tsync\n" instead of the stringify stuff.
>
> That does sound a bit more straightforward, now that you mention it.  ;-)

With that change, I'm:

Acked-by: Olof Johansson <olof@...om.net>

But at least Ben or Anton should sign off on it too.


-Olof
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