linux-kernel - Re: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function

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Date:	Fri, 20 Feb 2009 11:56:40 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Nick Piggin <npiggin@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Oleg Nesterov <oleg@...hat.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single
	ipi fallback for smp_call_function_many())

* Suresh Siddha <suresh.b.siddha@...el.com> wrote:

> On Thu, 2009-02-19 at 04:20 -0800, Ingo Molnar wrote:
> > Could you please refresh this patch to latest tip:master? The 
> > APIC drivers moved to arch/x86/kernel/apic/.
> 
> Appended the refreshed patch. Thanks.

thanks. Two details i noticed:

Firstly:

> +++ b/arch/x86/kernel/apic/x2apic_cluster.c
> +++ b/arch/x86/kernel/apic/x2apic_phys.c

how about x2apic_uv.c? It uses uv_write_global_mmr64() in its 
IPI sending method, which uses:

static inline void uv_write_global_mmr64(int pnode, unsigned long offset,
                                unsigned long val)
{
        *uv_global_mmr64_address(pnode, offset) = val;
}

which uses ->mmr_base, which is mapped via:

        init_extra_mapping_uc(UV_LOCAL_MMR_BASE, UV_LOCAL_MMR_SIZE);

so it should be fine because uncached - but at minimum we should 
put a comment into x2apic_uv.c that the generic IPI code relies 
on the lowlevel code serializing - i.e. relies on the UC PAT 
attribute.

Secondly, you added smp_mb(), which will translate to an MFENCE. 

But in theory it should be enough to have a wmb() here. [Note, 
not an smp_wmb() that i suggested before.] That will translate 
to an SFENCE - which will serialize writes but still allows 
reads/prefetches to pass.

So the question is, is an SFENCE there enough to serialize the 
WRMSR with previous memory-writes? It's not specified in the 
x2apic docs as far as i could see.

	Ingo
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