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

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Message-ID: <484E33F1.5000503@goop.org>
Date:	Tue, 10 Jun 2008 08:57:37 +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 17:35, 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.

> Also, if you do that can you get rid of the ifdef? If it really *really*
> mattered, we could introduce smp_mb before/after xchg... but if you
> use smp_wmb anyway then it definitely does not matter because that is a
> noop on x86.
>   

Yes, I'd like to lose the #ifdef.  Unfortunately I think putting a 
"lock; addl $0,0(%%esp)" style barrier had a measurable negative 
performance impact, but I may be thinking of something else.  I don't 
know how expensive sfence is.

The alternative is to make ia64's xchg a barrier (or to add a barrier 
variant of it).  It seems like a wart to have a cross-architecture 
function like xchg(), but then have different architectures differ in 
important details like barrier-ness.

    J

> Thanks,
> Nick
>
>   
>  Cc: Samuel Thibault <samuel.thibault@...citrix.com>
>   
>> Signed-off-by: Isaku Yamahata <yamahata@...inux.co.jp>
>> ---
>>  drivers/xen/events.c |    2 +-
>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/drivers/xen/events.c b/drivers/xen/events.c
>> index 73d78dc..332dd63 100644
>> --- a/drivers/xen/events.c
>> +++ b/drivers/xen/events.c
>> @@ -529,7 +529,7 @@ void xen_evtchn_do_upcall(struct pt_regs *regs)
>>
>>  #ifndef CONFIG_X86 /* No need for a barrier -- XCHG is a barrier on x86.
>> */ /* Clear master flag /before/ clearing selector flag. */
>> -		rmb();
>> +		wmb();
>>  #endif
>>  		pending_words = xchg(&vcpu_info->evtchn_pending_sel, 0);
>>  		while (pending_words != 0) {
>>     

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