[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 <oleg@tv-sign.ru>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Christoph Lameter <clameter@engr.sgi.com>
Acked-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

@@ -183,7 +183,7 @@ void __init anon_vma_init(void)
  */
 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();
@@ -195,9 +195,16 @@ static struct anon_vma *page_lock_anon_vma(struct page *page)
 
 	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();
 }
 
 /*
@@ -333,7 +340,8 @@ static int page_referenced_anon(struct page *page)
 		if (!mapcount)
 			break;
 	}
-	spin_unlock(&anon_vma->lock);
+
+	page_unlock_anon_vma(anon_vma);
 	return referenced;
 }
 
@@ -802,7 +810,8 @@ static int try_to_unmap_anon(struct page *page, int migration)
 		if (ret == SWAP_FAIL || !page_mapped(page))
 			break;
 	}
-	spin_unlock(&anon_vma->lock);
+
+	page_unlock_anon_vma(anon_vma);
 	return ret;
 }