On Tue, 2013-09-24 at 09:32 +0200, Ingo Molnar wrote:
> (netdev Cc:-ed)
>
> * Christoph Lameter <c...@linux.com> wrote:
>
> > These location triggered during testing with KVM.
> >
> > These are fetches without preemption off where we judged that
> > to be more performance efficient or where other means of
> > providing synchronization (BH handling) are available.
>
> > Index: linux/include/net/snmp.h
> > ===================================================================
> > --- linux.orig/include/net/snmp.h 2013-09-12 13:26:29.216103951 -0500
> > +++ linux/include/net/snmp.h 2013-09-12 13:26:29.208104037 -0500
> > @@ -126,7 +126,7 @@ struct linux_xfrm_mib {
> > extern __typeof__(type) __percpu *name[SNMP_ARRAY_SZ]
> >
> > #define SNMP_INC_STATS_BH(mib, field) \
> > - __this_cpu_inc(mib[0]->mibs[field])
> > + raw_cpu_inc(mib[0]->mibs[field])
> >
> > #define SNMP_INC_STATS_USER(mib, field) \
> > this_cpu_inc(mib[0]->mibs[field])
> > @@ -141,7 +141,7 @@ struct linux_xfrm_mib {
> > this_cpu_dec(mib[0]->mibs[field])
> >
> > #define SNMP_ADD_STATS_BH(mib, field, addend) \
> > - __this_cpu_add(mib[0]->mibs[field], addend)
> > + raw_cpu_add(mib[0]->mibs[field], addend)
>
> Are the networking folks fine with allowing unafe operations of SNMP stats
> in preemptible sections, or should the kernel produce an optional warning
> message if CONFIG_PREEMPT_DEBUG=y and these ops are used in preemptible
> (non-bh, non-irq-handler, non-irqs-off, etc.) sections?
>
> RAW_SNMP_*_STATS() ops could be used to annotate those places where that
> kind of usage is safe.
I would rather not use RAW_ prefix in the macro, but add debugging
check to make sure we use _BH() variant in the right context.
BUG_ON(!in_softirq())
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