mm/rmap: correct some obsolete comments of anon_vma

commit 2b575eb6 ("mm: convert anon_vma->lock to a mutex") changed
spinlock used to serialize access to vma list to mutex.  And further, the
commit 5a505085 ("mm/rmap: Convert the struct anon_vma::mutex to an
rwsem") converted the mutex to an rwsem for solving scalability problem.
So replace spinlock with rwsem to make comment uptodate.

Link: https://lkml.kernel.org/r/20210123072459.25903-1-linmiaohe@huawei.comSigned-off-by: NMiaohe Lin <linmiaohe@huawei.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

@@ -168,7 +168,7 @@ static void anon_vma_chain_link(struct vm_area_struct *vma,
  *
  * Anon-vma allocations are very subtle, because we may have
  * optimistically looked up an anon_vma in page_lock_anon_vma_read()
- * and that may actually touch the spinlock even in the newly
+ * and that may actually touch the rwsem even in the newly
  * allocated vma (it depends on RCU to make sure that the
  * anon_vma isn't actually destroyed).
  *
@@ -359,7 +359,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
 		goto out_error_free_anon_vma;
 
 	/*
-	 * The root anon_vma's spinlock is the lock actually used when we
+	 * The root anon_vma's rwsem is the lock actually used when we
 	 * lock any of the anon_vmas in this anon_vma tree.
 	 */
 	anon_vma->root = pvma->anon_vma->root;