We can use lockdep to avoid unnecessary work here, otherwise update the
code to logically evaluate all pertinent cases and share code with
vma_assert_write_locked().

Make it clear here that we treat the VMA being detached at this point as a
bug, this was only implicit before.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 include/linux/mmap_lock.h | 42 ++++++++++++++++++++++++++++++++++++---
 1 file changed, 39 insertions(+), 3 deletions(-)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index b00d34b5ad10..92ea07f0da4e 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -319,6 +319,10 @@ int vma_start_write_killable(struct vm_area_struct *vma)
 	return __vma_start_write(vma, mm_lock_seq, TASK_KILLABLE);
 }

+/**
+ * vma_assert_write_locked() - assert that @vma holds a VMA write lock.
+ * @vma: The VMA to assert.
+ */
 static inline void vma_assert_write_locked(struct vm_area_struct *vma)
 {
 	unsigned int mm_lock_seq;
@@ -326,16 +330,48 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma)
 	VM_BUG_ON_VMA(!__is_vma_write_locked(vma, &mm_lock_seq), vma);
 }

+/**
+ * vma_assert_locked() - assert that @vma holds either a VMA read or a VMA write
+ * lock and is not detached.
+ * @vma: The VMA to assert.
+ */
 static inline void vma_assert_locked(struct vm_area_struct *vma)
 {
-	unsigned int mm_lock_seq;
+	unsigned int refs;

 	/*
 	 * See the comment describing the vm_area_struct->vm_refcnt field for
 	 * details of possible refcnt values.
 	 */
-	VM_BUG_ON_VMA(refcount_read(&vma->vm_refcnt) <= 1 &&
-		      !__is_vma_write_locked(vma, &mm_lock_seq), vma);
+
+	/*
+	 * If read-locked or currently excluding readers, then the VMA is
+	 * locked.
+	 */
+#ifdef CONFIG_LOCKDEP
+	if (lock_is_held(&vma->vmlock_dep_map))
+		return;
+#endif
+
+	refs = refcount_read(&vma->vm_refcnt);
+
+	/*
+	 * In this case we're either read-locked, write-locked with temporary
+	 * readers, or in the midst of excluding readers, all of which means
+	 * we're locked.
+	 */
+	if (refs > 1)
+		return;
+
+	/* It is a bug for the VMA to be detached here. */
+	VM_BUG_ON_VMA(!refs, vma);
+
+	/*
+	 * OK, the VMA has a reference count of 1 which means it is either
+	 * unlocked and attached or write-locked, so assert that it is
+	 * write-locked.
+	 */
+	vma_assert_write_locked(vma);
 }

 static inline bool vma_is_attached(struct vm_area_struct *vma)
--
2.52.0