[PATCH] powerpc/mm/cxl: Add barrier when setting mm cpumask

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

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 <andrew.donnellan@au1.ibm.com>
Reported-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
---
 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));
+		/*
+		 * 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;
-- 
2.13.5