[PATCH] adapt page_lock_anon_vma() to PREEMPT_RCU

page_lock_anon_vma() uses spin_lock() to block RCU. This doesn't work with
PREEMPT_RCU, we have to do rcu_read_lock() explicitely. Otherwise, it is
theoretically possible that slab returns anon_vma's memory to the system
before we do spin_unlock(&anon_vma->lock).

[ Hugh points out that this only matters for PREEMPT_RCU, which isn't merged
yet, and may never be. Regardless, this patch is conceptually the
right thing to do, even if it doesn't matter at this point. - Linus ]

Signed-off-by: Oleg Nesterov <[e-mail]>
Cc: Paul McKenney <[e-mail]>
Cc: Nick Piggin <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Acked-by: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/rmap.c

···
  */
 static struct anon_vma *page_lock_anon_vma(struct page *page)
 {
-	struct anon_vma *anon_vma = NULL;
+	struct anon_vma *anon_vma;
 	unsigned long anon_mapping;
 
 	rcu_read_lock();
···
 
 	anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
 	spin_lock(&anon_vma->lock);
+	return anon_vma;
 out:
 	rcu_read_unlock();
-	return anon_vma;
+	return NULL;
+}
+
+static void page_unlock_anon_vma(struct anon_vma *anon_vma)
+{
+	spin_unlock(&anon_vma->lock);
+	rcu_read_unlock();
 }
 
 /*
···
 		if (!mapcount)
 			break;
 	}
-	spin_unlock(&anon_vma->lock);
+
+	page_unlock_anon_vma(anon_vma);
 	return referenced;
 }
 
···
 		if (ret == SWAP_FAIL || !page_mapped(page))
 			break;
 	}
-	spin_unlock(&anon_vma->lock);
+
+	page_unlock_anon_vma(anon_vma);
 	return ret;
 }