[PATCH 3/6] powerpc/mm: Ensure cpumask update is ordered

[Michael Ellerman]

Nicholas Piggin <npiggin at gmail.com> writes:

> On Mon, 24 Jul 2017 21:20:07 +1000
> Nicholas Piggin <npiggin at gmail.com> wrote:
>
>> On Mon, 24 Jul 2017 14:28:00 +1000
>> Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote:
>> 
>> > There is no guarantee that the various isync's involved with
>> > the context switch will order the update of the CPU mask with
>> > the first TLB entry for the new context being loaded by the HW.
>> > 
>> > Be safe here and add a memory barrier to order any subsequent
>> > load/store which may bring entries into the TLB.
>> > 
>> > The corresponding barrier on the other side already exists as
>> > pte updates use pte_xchg() which uses __cmpxchg_u64 which has
>> > a sync after the atomic operation.
>> > 
>> > Signed-off-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>
>> > ---
>> >  arch/powerpc/include/asm/mmu_context.h | 1 +
>> >  1 file changed, 1 insertion(+)
>> > 
>> > diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
>> > index ed9a36ee3107..ff1aeb2cd19f 100644
>> > --- a/arch/powerpc/include/asm/mmu_context.h
>> > +++ b/arch/powerpc/include/asm/mmu_context.h
>> > @@ -110,6 +110,7 @@ static inline void switch_mm_irqs_off(struct mm_struct *prev,
>> >  	/* Mark this context has been used on the new CPU */
>> >  	if (!cpumask_test_cpu(smp_processor_id(), mm_cpumask(next))) {
>> >  		cpumask_set_cpu(smp_processor_id(), mm_cpumask(next));
>> > +		smp_mb();
>> >  		new_on_cpu = true;
>> >  	}
>> >    
>> 
>> I think this is the right thing to do, but it should be commented.
>> Is hwsync the right barrier? (i.e., it will order the page table walk)
>
> After some offline discussion, I think we have an agreement that
> this is the right barrier, as it orders with the subsequent load
> of next->context.id that the mtpid depends on (or slbmte for HPT).
>
> So we should have a comment here to that effect, and including
> the pte_xchg comments from your changelog. Some comment (at least
> refer back to here) added at pte_xchg too please.
>
> Other than that your series seems good to me if you repost it you
> can add
>
> Reviewed-by: Nicholas Piggin <npiggin at gmail.com>
>
> This one out of the series is the bugfix so it should go to stable
> as well, right?

So I'm waiting on a v2?

cheers