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Subject: Re: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single
	ipi fallback for smp_call_function_many())
From: Suresh Siddha <suresh.b.siddha@intel.com>
Reply-To: Suresh Siddha <suresh.b.siddha@intel.com>
To: Nick Piggin <npiggin@suse.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, Oleg Nesterov <oleg@redhat.com>,
       Jens Axboe <jens.axboe@oracle.com>,
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Organization: Intel Corp
Date: Wed, 18 Feb 2009 10:43:40 -0800
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On Wed, 2009-02-18 at 05:59 -0800, Nick Piggin wrote:
> You're saying the problem is in generic_exec_single because I've
> removed the smp_mb that inadvertently also serialises memory with
> the x2apic on x86?

yes.

> 
> Indeed that could cause problems on some architectures which I
> had hoped to avoid. So the patch is probably better off to first
> add the smp_mb() to arch_send_call_function_xxx arch code, unless
> it is immediately obvious or confirmed by arch maintainer that
> such barrier is not required.

For x2apic specific operations we should add the smp_mb() sequence. But
we need to make sure that we don't end up doing it twice (once in
generic code and another in arch code) for all the ipi paths.

thanks,
suresh