Messages in this thread | | | Date | Tue, 10 Jun 2008 08:57:37 +0100 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH] xen: Use wmb instead of rmb in xen_evtchn_do_upcall(). |
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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@eu.citrix.com> > >> Signed-off-by: Isaku Yamahata <yamahata@valinux.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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