linux-kernel - Re: [PATCH] xen: Use wmb instead of rmb in xen_evtchn_do

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Message-ID: <484F76BB.20507@goop.org>
Date:	Wed, 11 Jun 2008 07:54:51 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Isaku Yamahata <yamahata@...inux.co.jp>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-ia64-devel@...ts.xensource.com,
	Samuel Thibault <samuel.thibault@...citrix.com>,
	Keir Fraser <Keir.Fraser@...citrix.com>
Subject: Re: [PATCH] xen: Use wmb instead of rmb in xen_evtchn_do_upcall().

Nick Piggin wrote:
> On Tuesday 10 June 2008 17:57, Jeremy Fitzhardinge wrote:
>   
>> Nick Piggin wrote:
>>     
>>> On Tuesday 10 June 2008 17:35, Isaku Yamahata wrote:
>>>       
>>>> This patch is ported one from 534:77db69c38249 of linux-2.6.18-xen.hg.
>>>> Use wmb instead of rmb to enforce ordering between
>>>> evtchn_upcall_pending and evtchn_pending_sel stores
>>>> in xen_evtchn_do_upcall().
>>>>         
>>> There are a whole load of places in the kernel that should be using
>>> smp_ variants of memory barriers. This seemed to me like one of them,
>>> but I could be wrong.
>>>       
>> No, it needs to be an unconditional barrier.  This is synchronizing with
>> the hypervisor - even if the kernel is compiled UP, the SMP hypervisor
>> may be testing/setting the events pending bits from another (physical) cpu.
>>     
>
> OK. What you *really* want is smp_*mb_even_if_compiled_for_UP() ;)
> That is, a small set of primitives that are compiled with CONFIG_SMP
> (and given some xxx_ prefix to distinguish).
>   

We already have a set of sync_* for atomic ops which are always locked.

> IO barriers are probably the best thing you can use for the moment.
>   

It is conceptually similar, I suppose.

       J
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