[RFC PATCH 16/28] kernel/fork: assert no VMA readers during its destruction

[Suren Baghdasaryan <surenb@google.com>]

Assert there are no holders of VMA lock for reading when it is about to be
destroyed.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/mm.h | 8 ++++++++
 kernel/fork.c      | 2 ++
 2 files changed, 10 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dc72be923e5b..0d9c1563c354 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -676,6 +676,13 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos)
 	VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
 }
 
+static inline void vma_assert_no_reader(struct vm_area_struct *vma)
+{
+	VM_BUG_ON_VMA(rwsem_is_locked(&vma->lock) &&
+		      vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq),
+		      vma);
+}
+
 #else /* CONFIG_PER_VMA_LOCK */
 
 static inline void vma_init_lock(struct vm_area_struct *vma) {}
@@ -685,6 +692,7 @@ static inline bool vma_read_trylock(struct vm_area_struct *vma)
 static inline void vma_read_unlock(struct vm_area_struct *vma) {}
 static inline void vma_assert_locked(struct vm_area_struct *vma) {}
 static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos) {}
+static inline void vma_assert_no_reader(struct vm_area_struct *vma) {}
 
 #endif /* CONFIG_PER_VMA_LOCK */
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 1872ad549fed..b443ba3a247a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -487,6 +487,8 @@ static void __vm_area_free(struct rcu_head *head)
 {
 	struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
 						  vm_rcu);
+	/* The vma should either have no lock holders or be write-locked. */
+	vma_assert_no_reader(vma);
 	kmem_cache_free(vm_area_cachep, vma);
 }
 #endif
-- 
2.37.2.672.g94769d06f0-goog