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.
>
>>
>
