Re: [PATCH] xen: Use wmb instead of rmb in xen_evtchn_do_upcall().

[Jeremy Fitzhardinge]

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@xxxxxxxxxxxxx>
>   
> > Signed-off-by: Isaku Yamahata <yamahata@xxxxxxxxxxxxx>
> > ---
> >  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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