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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