Re: [PATCH] slub: prevent validate_slab() error due to racecondition

[Christoph Lameter]

On Thu, 26 Apr 2012, Waiman Long wrote:

> The SLUB memory allocator was changed substantially from 3.0 to 3.1 by
> replacing some of page locking codes for updating the free object list
> of the slab with double-quadword atomic exchange (cmpxchg_double_slab)
> or a pseudo one using a page lock when debugging is turned on.  In the
> normal case, that should be enough to make sure that the slab is in a
> consistent state. However, when CONFIG_SLUB_DEBUG is turned on and the
> Redzone debugging flag is set, the Redzone bytes are also used to mark
> if an object is free or allocated. The extra state information in those
> Redzone bytes is not protected by the cmpxchg_double_slab(). As a
> result,
> validate_slab() may report a Redzone error if the validation is
> performed
> while racing with a free to a debugged slab.

Right. The problem is unique to validate_slab because no one else outside
the slab has access to that object that is to be freed.

> This patch fixes the BUG message by acquiring the node-level lock for
> slabs flagged for debugging to avoid this possible racing condition.
> The locking is done on the node-level lock instead of the more granular
> page lock because the new code may speculatively acquire the node-level
> lock later on. Acquiring the page lock and then the node lock may lead
> to potential deadlock.

Correct that would address the issue.

> As the increment of slab node count and insertion of the new slab into
> the partial or full slab list is not an atomic operation, there is a
> small time window where the two may not match. This patch temporarily
> works around this problem by allowing the node count to be one larger
> than the number of slab presents in the lists. This workaround may not
> work if more than one CPU is actively adding slab to the same node,
> but it should be good enough to workaround the problem in most cases.

Well yeah that is not a real fix. Its been racy for a long time.

> To really fix the issue, the overall synchronization between debug slub
> operations and slub validation needs a revisit.

True. The sync between validation and concurrent operations was not in
focus so far. Validation was used (at least by me) so far only to validate
that the slab structures are still okay after running some tests.

Lets just do this in steps. First patch here is a simple taking of the
node lock in free_debug_processing. This is similar to what you have done
but the changes are made to the debugging function instead. Then we can
look at how to address the slab counting issue in a separate patch.


Subject: slub: Take node lock during object free checks

This is needed for proper synchronization with validate_slab()
as pointed out by Waiman Long <Waiman.Long@xxxxxx>

Reported-by: Waiman Long <Waiman.Long@xxxxxx>
Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>


---
 mm/slub.c |   30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

Index: linux-2.6/mm/slub.c
===================================================================
--- linux-2.6.orig/mm/slub.c	2012-04-27 09:40:00.000000000 -0500
+++ linux-2.6/mm/slub.c	2012-04-27 09:50:15.000000000 -0500
@@ -1082,13 +1082,13 @@ bad:
 	return 0;
 }

-static noinline int free_debug_processing(struct kmem_cache *s,
-		 struct page *page, void *object, unsigned long addr)
+static noinline struct kmem_cache_node *free_debug_processing(
+	struct kmem_cache *s, struct page *page, void *object,
+	unsigned long addr, unsigned long *flags)
 {
-	unsigned long flags;
-	int rc = 0;
+	struct kmem_cache_node *n = get_node(s, page_to_nid(page));

-	local_irq_save(flags);
+	spin_lock_irqsave(&n->list_lock, *flags);
 	slab_lock(page);

 	if (!check_slab(s, page))
@@ -1126,15 +1126,19 @@ static noinline int free_debug_processin
 		set_track(s, object, TRACK_FREE, addr);
 	trace(s, page, object, 0);
 	init_object(s, object, SLUB_RED_INACTIVE);
-	rc = 1;
 out:
 	slab_unlock(page);
-	local_irq_restore(flags);
-	return rc;
+	/*
+	 * Keep node_lock to preserve integrity
+	 * until the object is actually freed
+	 */
+	return n;

 fail:
+	slab_unlock(page);
+	spin_unlock_irqrestore(&n->list_lock, *flags);
 	slab_fix(s, "Object at 0x%p not freed", object);
-	goto out;
+	return NULL;
 }

 static int __init setup_slub_debug(char *str)
@@ -1227,8 +1231,9 @@ static inline void setup_object_debug(st
 static inline int alloc_debug_processing(struct kmem_cache *s,
 	struct page *page, void *object, unsigned long addr) { return 0; }

-static inline int free_debug_processing(struct kmem_cache *s,
-	struct page *page, void *object, unsigned long addr) { return 0; }
+static inline struct kmem_cache_node *free_debug_processing(
+	struct kmem_cache *s, struct page *page, void *object,
+	unsigned long addr, unsigned long *flags) { return NULL; }

 static inline int slab_pad_check(struct kmem_cache *s, struct page *page)
 			{ return 1; }
@@ -2445,7 +2450,8 @@ static void __slab_free(struct kmem_cach

 	stat(s, FREE_SLOWPATH);

-	if (kmem_cache_debug(s) && !free_debug_processing(s, page, x, addr))
+	if (kmem_cache_debug(s) &&
+		!(n = free_debug_processing(s, page, x, addr, &flags)))
 		return;

 	do {
--
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