mmap locking API: add mmap_assert_locked() and mmap_assert_write_locked()

Add new APIs to assert that mmap_sem is held.

Using this instead of rwsem_is_locked and lockdep_assert_held[_write]
makes the assertions more tolerant of future changes to the lock type.

Signed-off-by: Michel Lespinasse <walken@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Laurent Dufour <ldufour@linux.ibm.com>
Cc: Liam Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ying Han <yinghan@google.com>
Link: http://lkml.kernel.org/r/20200520052908.204642-10-walken@google.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmu_notifier.c

@@ -609,7 +609,7 @@ int __mmu_notifier_register(struct mmu_notifier *subscription,
 	struct mmu_notifier_subscriptions *subscriptions = NULL;
 	int ret;
 
-	lockdep_assert_held_write(&mm->mmap_sem);
+	mmap_assert_write_locked(mm);
 	BUG_ON(atomic_read(&mm->mm_users) <= 0);
 
 	if (IS_ENABLED(CONFIG_LOCKDEP)) {
@@ -761,7 +761,7 @@ struct mmu_notifier *mmu_notifier_get_locked(const struct mmu_notifier_ops *ops,
 	struct mmu_notifier *subscription;
 	int ret;
 
-	lockdep_assert_held_write(&mm->mmap_sem);
+	mmap_assert_write_locked(mm);
 
 	if (mm->notifier_subscriptions) {
 		subscription = find_get_mmu_notifier(mm, ops);
@@ -1006,7 +1006,7 @@ int mmu_interval_notifier_insert_locked(
 		mm->notifier_subscriptions;
 	int ret;
 
-	lockdep_assert_held_write(&mm->mmap_sem);
+	mmap_assert_write_locked(mm);
 
 	if (!subscriptions || !subscriptions->has_itree) {
 		ret = __mmu_notifier_register(NULL, mm);