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

[Vlastimil Babka]

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
>>>    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?
> 
> 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.

> 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.

> 
>> 
>