[PATCH v3] slub: fix false-positive lockdep warning in free_partial()

From: Vladimir Davydov
Date: Wed Feb 05 2014 - 03:15:52 EST

Commit c65c1877bd68 ("slub: use lockdep_assert_held") requires
remove_partial() to be called with n->list_lock held, but free_partial()
called from kmem_cache_close() on cache destruction does not follow this
rule, leading to a warning:

WARNING: CPU: 0 PID: 2787 at mm/slub.c:1536 __kmem_cache_shutdown+0x1b2/0x1f0()
Modules linked in:
CPU: 0 PID: 2787 Comm: modprobe Tainted: G W 3.14.0-rc1-mm1+ #1
Hardware name:
0000000000000600 ffff88003ae1dde8 ffffffff816d9583 0000000000000600
0000000000000000 ffff88003ae1de28 ffffffff8107c107 0000000000000000
ffff880037ab2b00 ffff88007c240d30 ffffea0001ee5280 ffffea0001ee52a0
Call Trace:
[<ffffffff816d9583>] dump_stack+0x51/0x6e
[<ffffffff8107c107>] warn_slowpath_common+0x87/0xb0
[<ffffffff8107c145>] warn_slowpath_null+0x15/0x20
[<ffffffff811c7fe2>] __kmem_cache_shutdown+0x1b2/0x1f0
[<ffffffff811908d3>] kmem_cache_destroy+0x43/0xf0
[<ffffffffa013a123>] xfs_destroy_zones+0x103/0x110 [xfs]
[<ffffffffa0192b54>] exit_xfs_fs+0x38/0x4e4 [xfs]
[<ffffffff811036fa>] SyS_delete_module+0x19a/0x1f0
[<ffffffff816dfcd8>] ? retint_swapgs+0x13/0x1b
[<ffffffff810d2125>] ? trace_hardirqs_on_caller+0x105/0x1d0
[<ffffffff81359efe>] ? trace_hardirqs_on_thunk+0x3a/0x3f
[<ffffffff816e8539>] system_call_fastpath+0x16/0x1b

Although this cannot actually result in a race, because on cache
destruction there should not be any concurrent frees or allocations from
the cache, let's add spin_lock/unlock to free_partial() just to keep
lockdep happy.

Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
v3: fix the comment to free_partial()
v2: add a comment explaining why we need to take the lock

mm/slub.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index 0eeea85034c8..65a40d20ec80 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3185,12 +3185,13 @@ static void list_slab_objects(struct kmem_cache *s, struct page *page,
* Attempt to free all partial slabs on a node.
* This is called from kmem_cache_close(). We must be the last thread
- * using the cache and therefore we do not need to lock anymore.
+ * using the cache, but we still have to lock for lockdep's sake.
static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
struct page *page, *h;

+ spin_lock_irq(&n->list_lock);
list_for_each_entry_safe(page, h, &n->partial, lru) {
if (!page->inuse) {
remove_partial(n, page);
@@ -3200,6 +3201,7 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
"Objects remaining in %s on kmem_cache_close()");
+ spin_unlock_irq(&n->list_lock);


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