Re: [PATCH] mm: slub: annotate kmem_cache_node->list_lock as raw_spinlock

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 2023/4/12 14:50, Vlastimil Babka wrote:
> 
> 
> On 4/12/23 08:44, Zhang, Qiang1 wrote:
>>>
>>>
>>> On 2023/4/11 22:19, Vlastimil Babka wrote:
>>>> On 4/11/23 16:08, Qi Zheng wrote:
>>>>>
>>>>>
>>>>> On 2023/4/11 21:40, Vlastimil Babka wrote:
>>>>>> On 4/11/23 15:08, Qi Zheng wrote:
>>>>>>> The list_lock can be held in the critical section of
>>>>>>> raw_spinlock, and then lockdep will complain about it
>>>>>>> like below:
>>>>>>>
>>>>>>>     =============================
>>>>>>>     [ BUG: Invalid wait context ]
>>>>>>>     6.3.0-rc6-next-20230411 #7 Not tainted
>>>>>>>     -----------------------------
>>>>>>>     swapper/0/1 is trying to lock:
>>>>>>>     ffff888100055418 (&n->list_lock){....}-{3:3}, at: ___slab_alloc+0x73d/0x1330
>>>>>>>     other info that might help us debug this:
>>>>>>>     context-{5:5}
>>>>>>>     2 locks held by swapper/0/1:
>>>>>>>      #0: ffffffff824e8160 (rcu_tasks.cbs_gbl_lock){....}-{2:2}, at: cblist_init_generic+0x22/0x2d0
>>>>>>>      #1: ffff888136bede50 (&ACCESS_PRIVATE(rtpcp, lock)){....}-{2:2}, at: cblist_init_generic+0x232/0x2d0
>>>>>>>     stack backtrace:
>>>>>>>     CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.3.0-rc6-next-20230411 #7
>>>>>>>     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
>>>>>>>     Call Trace:
>>>>>>>      <TASK>
>>>>>>>      dump_stack_lvl+0x77/0xc0
>>>>>>>      __lock_acquire+0xa65/0x2950
>>>>>>>      ? arch_stack_walk+0x65/0xf0
>>>>>>>      ? arch_stack_walk+0x65/0xf0
>>>>>>>      ? unwind_next_frame+0x602/0x8d0
>>>>>>>      lock_acquire+0xe0/0x300
>>>>>>>      ? ___slab_alloc+0x73d/0x1330
>>>>>>>      ? find_usage_forwards+0x39/0x50
>>>>>>>      ? check_irq_usage+0x162/0xa70
>>>>>>>      ? __bfs+0x10c/0x2c0
>>>>>>>      _raw_spin_lock_irqsave+0x4f/0x90
>>>>>>>      ? ___slab_alloc+0x73d/0x1330
>>>>>>>      ___slab_alloc+0x73d/0x1330
>>>>>>>      ? fill_pool+0x16b/0x2a0
>>>>>>>      ? look_up_lock_class+0x5d/0x160
>>>>>>>      ? register_lock_class+0x48/0x500
>>>>>>>      ? __lock_acquire+0xabc/0x2950
>>>>>>>      ? fill_pool+0x16b/0x2a0
>>>>>>>      kmem_cache_alloc+0x358/0x3b0
>>>>>>>      ? __lock_acquire+0xabc/0x2950
>>>>>>>      fill_pool+0x16b/0x2a0
>>>>>>>      ? __debug_object_init+0x292/0x560
>>>>>>>      ? lock_acquire+0xe0/0x300
>>>>>>>      ? cblist_init_generic+0x232/0x2d0
>>>>>>>      __debug_object_init+0x2c/0x560
>>>
>>> This "__debug_object_init" is because INIT_WORK() is called in
>>> cblist_init_generic(), so..
>>>
>>>>>>>      cblist_init_generic+0x147/0x2d0
>>>>>>>      rcu_init_tasks_generic+0x15/0x190
>>>>>>>      kernel_init_freeable+0x6e/0x3e0
>>>>>>>      ? rest_init+0x1e0/0x1e0
>>>>>>>      kernel_init+0x1b/0x1d0
>>>>>>>      ? rest_init+0x1e0/0x1e0
>>>>>>>      ret_from_fork+0x1f/0x30
>>>>>>>      </TASK>
>>>>>>>
>>>>>>> The fill_pool() can only be called in the !PREEMPT_RT kernel
>>>>>>> or in the preemptible context of the PREEMPT_RT kernel, so
>>>>>>> the above warning is not a real issue, but it's better to
>>>>>>> annotate kmem_cache_node->list_lock as raw_spinlock to get
>>>>>>> rid of such issue.
>>>>>>
>>>>>> + CC some RT and RCU people
>>>>>
>>>>> Thanks.
>>>>>
>>>>>>
>>>>>> AFAIK raw_spinlock is not just an annotation, but on RT it changes the
>>>>>> implementation from preemptible mutex to actual spin lock, so it would be
>>>>>
>>>>> Yeah.
>>>>>
>>>>>> rather unfortunate to do that for a spurious warning. Can it be somehow
>>>>>> fixed in a better way?
>>>
>>> ... probably a better fix is to drop locks and call INIT_WORK(), or make
>>> the cblist_init_generic() lockless (or part lockless), given it's just
>>> initializing the cblist, it's probably doable. But I haven't taken a
>>> careful look yet.
>>>
>>
>>
>> This is just one of the paths that triggers an invalid wait,  the following paths can also trigger:
>>
>> [  129.914547] [ BUG: Invalid wait context ]
>> [  129.914775] 6.3.0-rc1-yocto-standard+ #2 Not tainted
>> [  129.915044] -----------------------------
>> [  129.915272] kworker/2:0/28 is trying to lock:
>> [  129.915516] ffff88815660f570 (&c->lock){-.-.}-{3:3}, at: ___slab_alloc+0x68/0x12e0
>> [  129.915967] other info that might help us debug this:
>> [  129.916241] context-{5:5}
>> [  129.916392] 3 locks held by kworker/2:0/28:
>> [  129.916642]  #0: ffff888100084d48 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work+0x515/0xba0
>> [  129.917145]  #1: ffff888100c17dd0 ((work_completion)(&(&krcp->monitor_work)->work)){+.+.}-{0:0}, at: process_on0
>> [  129.917758]  #2: ffff8881565f8508 (krc.lock){....}-{2:2}, at: kfree_rcu_monitor+0x29f/0x810
>> [  129.918207] stack backtrace:
>> [  129.918374] CPU: 2 PID: 28 Comm: kworker/2:0 Not tainted 6.3.0-rc1-yocto-standard+ #2
>> [  129.918784] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.1-0-g3208b098f51a-prebuilt.qemu.o4
>> [  129.919397] Workqueue: events kfree_rcu_monitor
>> [  129.919662] Call Trace:
>> [  129.919812]  <TASK>
>> [  129.919941]  dump_stack_lvl+0x64/0xb0
>> [  129.920171]  dump_stack+0x10/0x20
>> [  129.920372]  __lock_acquire+0xeb8/0x3a80
>> [  129.920603]  ? ret_from_fork+0x2c/0x50
>> [  129.920824]  ? __pfx___lock_acquire+0x10/0x10
>> [  129.921068]  ? unwind_next_frame.part.0+0x1ba/0x3c0
>> [  129.921343]  ? ret_from_fork+0x2c/0x50
>> [  129.921573]  ? __this_cpu_preempt_check+0x13/0x20
>> [  129.921847]  lock_acquire+0x194/0x480
>> [  129.922060]  ? ___slab_alloc+0x68/0x12e0
>> [  129.922293]  ? __pfx_lock_acquire+0x10/0x10
>> [  129.922529]  ? __pfx_mark_lock.part.0+0x10/0x10
>> [  129.922778]  ? __kasan_check_read+0x11/0x20
>> [  129.922998]  ___slab_alloc+0x9a/0x12e0
>> [  129.923222]  ? ___slab_alloc+0x68/0x12e0
>> [  129.923452]  ? __pfx_mark_lock.part.0+0x10/0x10
>> [  129.923706]  ? __kasan_check_read+0x11/0x20
>> [  129.923937]  ? fill_pool+0x22a/0x370
>> [  129.924161]  ? __lock_acquire+0xf5b/0x3a80
>> [  129.924387]  ? fill_pool+0x22a/0x370
>> [  129.924590]  __slab_alloc.constprop.0+0x5b/0x90
>> [  129.924832]  kmem_cache_alloc+0x296/0x3d0
>> [  129.925073]  ? fill_pool+0x22a/0x370
>> [  129.925291]  fill_pool+0x22a/0x370
>> [  129.925495]  ? __pfx_fill_pool+0x10/0x10
>> [  129.925718]  ? __pfx___lock_acquire+0x10/0x10
>> [  129.926034]  ? __kasan_check_read+0x11/0x20
>> [  129.926269]  ? check_chain_key+0x200/0x2b0
>> [  129.926503]  __debug_object_init+0x82/0x8c0
>> [  129.926734]  ? __pfx_lock_release+0x10/0x10
>> [  129.926984]  ? __pfx___debug_object_init+0x10/0x10
>> [  129.927249]  ? __kasan_check_read+0x11/0x20
>> [  129.927498]  ? do_raw_spin_unlock+0x9c/0x100
>> [  129.927758]  debug_object_activate+0x2d1/0x2f0
>> [  129.928022]  ? __pfx_debug_object_activate+0x10/0x10
>> [  129.928300]  ? __this_cpu_preempt_check+0x13/0x20
>> [  129.928583]  __call_rcu_common.constprop.0+0x94/0xeb0
>> [  129.928897]  ? __this_cpu_preempt_check+0x13/0x20
>> [  129.929186]  ? __pfx_rcu_work_rcufn+0x10/0x10
>> [  129.929459]  ? __pfx___call_rcu_common.constprop.0+0x10/0x10
>> [  129.929803]  ? __pfx_lock_acquired+0x10/0x10
>> [  129.930067]  ? __pfx_do_raw_spin_trylock+0x10/0x10
>> [  129.930363]  ? kfree_rcu_monitor+0x29f/0x810
>> [  129.930627]  call_rcu+0xe/0x20
>> [  129.930821]  queue_rcu_work+0x4f/0x60
>> [  129.931050]  kfree_rcu_monitor+0x5d3/0x810
>> [  129.931302]  ? __pfx_kfree_rcu_monitor+0x10/0x10
>> [  129.931587]  ? __this_cpu_preempt_check+0x13/0x20
>> [  129.931878]  process_one_work+0x607/0xba0
>> [  129.932129]  ? __pfx_process_one_work+0x10/0x10
>> [  129.932408]  ? worker_thread+0xd6/0x710
>> [  129.932653]  worker_thread+0x2d4/0x710
>> [  129.932888]  ? __pfx_worker_thread+0x10/0x10
>> [  129.933154]  kthread+0x18b/0x1c0
>> [  129.933363]  ? __pfx_kthread+0x10/0x10
>> [  129.933598]  ret_from_fork+0x2c/0x50
>> [  129.933825]  </TASK>
>>
>> Maybe no need to convert ->list_lock to raw_spinlock.
>>
>> --- a/lib/debugobjects.c
>> +++ b/lib/debugobjects.c
>> @@ -562,10 +562,10 @@ __debug_object_init(void *addr, const struct debug_obj_descr *descr, int onstack
>>          unsigned long flags;
>>
>>          /*
>> -        * On RT enabled kernels the pool refill must happen in preemptible
>> +        * The pool refill must happen in preemptible
>>           * context:
>>           */
>> -       if (!IS_ENABLED(CONFIG_PREEMPT_RT) || preemptible())
>> +       if (preemptible())
>>                  fill_pool();
> 
> +CC Peterz
> 
> Aha so this is in fact another case where the code is written with
> actual differences between PREEMPT_RT and !PREEMPT_RT in mind, but
> CONFIG_PROVE_RAW_LOCK_NESTING always assumes PREEMPT_RT?

Maybe we should make CONFIG_PROVE_RAW_LOCK_NESTING depend on
CONFIG_PREEMPT_RT:

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index f0d5b82e478d..257b170aacb6 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1262,6 +1262,7 @@ config PROVE_LOCKING
  config PROVE_RAW_LOCK_NESTING
         bool "Enable raw_spinlock - spinlock nesting checks"
         depends on PROVE_LOCKING
+       depends on PREEMPT_RT
         default n
         help
          Enable the raw_spinlock vs. spinlock nesting checks which ensure

> 
>>          db = get_bucket((unsigned long) addr);
>>
>>
>>
>> Thanks
>> Zqiang
>>
>>>
>>>
>>> Regards,
>>> Boqun
>>>
>>>>>
>>>>> It's indeed unfortunate for the warning in the commit message. But
>>>>> functions like kmem_cache_alloc(GFP_ATOMIC) may indeed be called
>>>>> in the critical section of raw_spinlock or in the hardirq context, which
>>>>
>>>> Hmm, I thought they may not, actually.
>>>>
>>>>> will cause problem in the PREEMPT_RT kernel. So I still think it is
>>>>> reasonable to convert kmem_cache_node->list_lock to raw_spinlock type.
>>>>
>>>> It wouldn't be the complete solution anyway. Once we allow even a GFP_ATOMIC
>>>> slab allocation for such context, it means also page allocation can happen
>>>> to refill the slabs, so lockdep will eventually complain about zone->lock,
>>>> and who knows what else.
>>>
>>> Oh, indeed. :(
>>>
>>>>
>>>>> In addition, there are many fix patches for this kind of warning in the
>>>>> git log, so I also think there should be a general and better solution. :)
>>>>
>>>> Maybe, but given above, I doubt it's this one.
>>>>
>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>> -- 
>>> Thanks,
>>> Qi

-- 
Thanks,
Qi