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SubjectRe: [PATCH v2] slub: fix false-positive lockdep warning in free_partial()
On 02/05/2014 12:01 PM, David Rientjes wrote:
> On Wed, 5 Feb 2014, Vladimir Davydov wrote:
>
>> 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@parallels.com>
>> ---
>> v2: add a comment explaining why we need to take the lock
>>
>> mm/slub.c | 6 ++++++
>> 1 file changed, 6 insertions(+)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 0eeea85034c8..24bf05e962ff 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -3191,6 +3191,11 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
>> {
>> struct page *page, *h;
>>
>> + /*
>> + * the lock is for lockdep's sake, not for any actual
>> + * race protection
>> + */
> I think Christoph was referring to altering the comment for this function
> which still says "We must be the last thread using the cache and therefore
> we do not need to lock anymore."

Oh, sorry, I didn't notice that. Will amend.

Thanks.

>
>> + 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 +3205,7 @@ 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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