On Mon, 2017-08-28 at 11:55 +0530, Aneesh Kumar K.V wrote:
> We need to add memory barrier so that the page table walk doesn't happen
> before the cpumask is set and made visible to the other cpus. We need
> to use a sync here instead of lwsync because lwsync is not sufficient for
> store/load ordering.
>
> We also need to add an if (mm) check so that we do the right thing when called
> with a kernel context. For kernel context, we have mm = NULL. W.r.t kernel
> address we can skip setting the mm cpumask.
>
> Fixes: 0f4bc0932e ("powerpc/mm/cxl: Add the fault handling cpu to mm cpumask")
> Cc: Andrew Donnellan <[email protected]>
> Reported-by: Benjamin Herrenschmidt <[email protected]>
> Reported-by: Dan Carpenter <[email protected]>
> Signed-off-by: Aneesh Kumar K.V <[email protected]>
> ---
> drivers/misc/cxl/fault.c | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/misc/cxl/fault.c b/drivers/misc/cxl/fault.c
> index ab507e4ed69b..caed2a523bee 100644
> --- a/drivers/misc/cxl/fault.c
> +++ b/drivers/misc/cxl/fault.c
> @@ -141,9 +141,19 @@ int cxl_handle_mm_fault(struct mm_struct *mm, u64 dsisr,
> u64 dar)
> /*
> * Add the fault handling cpu to task mm cpumask so that we
> * can do a safe lockless page table walk when inserting the
> - * hash page table entry.
> + * hash page table entry. This function get called with a
> + * valid mm for all user space applications. Hence using
> + * if (mm) check is sufficient here.
> */
> - cpumask_set_cpu(smp_processor_id(), mm_cpumask(mm));
> + if (mm) {
> + cpumask_set_cpu(smp_processor_id(), mm_cpumask(mm));
First test if it's already set as this should be quite common and the
cost of setting is a full atomic.
> + /*
> + * We need to make sure we walk the table only after
> + * we update the cpumask. The other side of the barrier is
> + * explained * in serialize_against_pte_lookup()
> + */
> + smp_mb();
> + }
> if ((result = copro_handle_mm_fault(mm, dar, dsisr, &flt))) {
> pr_devel("copro_handle_mm_fault failed: %#x\n", result);
> return result;
