mm, notifier: Catch sleeping/blocking for !blockable

We need to make sure implementations don't cheat and don't have a possible
schedule/blocking point deeply burried where review can't catch it.

I'm not sure whether this is the best way to make sure all the
might_sleep() callsites trigger, and it's a bit ugly in the code flow.
But it gets the job done.

Inspired by an i915 patch series which did exactly that, because the rules
haven't been entirely clear to us.

Link: https://lore.kernel.org/r/20190826201425.17547-5-daniel.vetter@ffwll.ch
Reviewed-by: Christian König <christian.koenig@amd.com> (v1)
Reviewed-by: Jérôme Glisse <jglisse@redhat.com> (v4)
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

mm/mmu_notifier.c

@@ -168,7 +168,13 @@ int __mmu_notifier_invalidate_range_start(struct mmu_notifier_range *range)
 	id = srcu_read_lock(&srcu);
 	hlist_for_each_entry_rcu(mn, &range->mm->mmu_notifier_mm->list, hlist) {
 		if (mn->ops->invalidate_range_start) {
-			int _ret = mn->ops->invalidate_range_start(mn, range);
+			int _ret;
+
+			if (!mmu_notifier_range_blockable(range))
+				non_block_start();
+			_ret = mn->ops->invalidate_range_start(mn, range);
+			if (!mmu_notifier_range_blockable(range))
+				non_block_end();
 			if (_ret) {
 				pr_info("%pS callback failed with %d in %sblockable context.\n",
 					mn->ops->invalidate_range_start, _ret,
@@ -210,8 +216,13 @@ void __mmu_notifier_invalidate_range_end(struct mmu_notifier_range *range,
 			mn->ops->invalidate_range(mn, range->mm,
 						  range->start,
 						  range->end);
-		if (mn->ops->invalidate_range_end)
+		if (mn->ops->invalidate_range_end) {
+			if (!mmu_notifier_range_blockable(range))
+				non_block_start();
 			mn->ops->invalidate_range_end(mn, range);
+			if (!mmu_notifier_range_blockable(range))
+				non_block_end();
+		}
 	}
 	srcu_read_unlock(&srcu, id);
 	lock_map_release(&__mmu_notifier_invalidate_range_start_map);