linux-kernel - Re: [PATCH v4] locking/csd_lock: change csdlock_debug from early_param to _

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Message-ID: <fd69f464-4cc9-859e-d38d-bda85e6b33a6@huawei.com>
Date:   Fri, 27 May 2022 14:49:03 +0800
From:   Chen Zhongjin <chenzhongjin@...wei.com>
To:     <paulmck@...nel.org>
CC:     <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-arch@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
        <stable@...r.kernel.org>, <peterz@...radead.org>,
        <tglx@...utronix.de>, <namit@...are.com>, <gor@...ux.ibm.com>,
        <rdunlap@...radead.org>, <mingo@...nel.org>, <jgross@...e.com>,
        <gregkh@...uxfoundation.org>, <mpe@...erman.id.au>
Subject: Re: [PATCH v4] locking/csd_lock: change csdlock_debug from
 early_param to __setup

Hi,

On 2022/5/18 9:11, Paul E. McKenney wrote:
> On Tue, May 17, 2022 at 11:22:04AM +0800, Chen Zhongjin wrote:
>> On 2022/5/10 17:46, Chen Zhongjin wrote:
>>> csdlock_debug uses early_param and static_branch_enable() to enable
>>> csd_lock_wait feature, which triggers a panic on arm64 with config:
>>> CONFIG_SPARSEMEM=y
>>> CONFIG_SPARSEMEM_VMEMMAP=n
>>>
>>> With CONFIG_SPARSEMEM_VMEMMAP=n, __nr_to_section is called in
>>> static_key_enable() and returns NULL which makes NULL dereference
>>> because mem_section is initialized in sparse_init() which is later
>>> than parse_early_param() stage.
>>>
>>> For powerpc this is also broken, because early_param stage is
>>> earlier than jump_label_init() so static_key_enable won't work.
>>> powerpc throws an warning: "static key 'xxx' used before call
>>> to jump_label_init()".
>>>
>>> Thus, early_param is too early for csd_lock_wait to run
>>> static_branch_enable(), so changes it to __setup to fix these.
>>>
>>> Fixes: 8d0968cc6b8f ("locking/csd_lock: Add boot parameter for controlling CSD lock debugging")
>>> Cc: stable@...r.kernel.org
>>> Reported-by: Chen jingwen <chenjingwen6@...wei.com>
>>> Signed-off-by: Chen Zhongjin <chenzhongjin@...wei.com>
>>> ---
>>> Change v3 -> v4:
>>> Fix title and description because this fix is also applied
>>> to powerpc.
>>> For more detailed arm64 bug report see:
>>> https://lore.kernel.org/linux-arm-kernel/e8715911-f835-059d-27f8-cc5f5ad30a07@huawei.com/t/
>>>
>>> Change v2 -> v3:
>>> Add module name in title
>>>
>>> Change v1 -> v2:
>>> Fix return 1 for __setup
>>> ---
>>>  kernel/smp.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/kernel/smp.c b/kernel/smp.c
>>> index 65a630f62363..381eb15cd28f 100644
>>> --- a/kernel/smp.c
>>> +++ b/kernel/smp.c
>>> @@ -174,9 +174,9 @@ static int __init csdlock_debug(char *str)
>>>  	if (val)
>>>  		static_branch_enable(&csdlock_debug_enabled);
>>>  
>>> -	return 0;
>>> +	return 1;
>>>  }
>>> -early_param("csdlock_debug", csdlock_debug);
>>> +__setup("csdlock_debug=", csdlock_debug);
>>>  
>>>  static DEFINE_PER_CPU(call_single_data_t *, cur_csd);
>>>  static DEFINE_PER_CPU(smp_call_func_t, cur_csd_func);
>>
>> Ping for review. Thanks！
> 
> I have pulled it into -rcu for testing and further review.  It might
> well need to go through some other path, though.
>> 								Thanx, Paul
> .

So did it have any result? Do we have any idea to fix that except delaying the
set timing? I guess that maybe not using static_branch can work for this, but it
still needs to be evaluated for performance influence of not enabled situation.

Best,
Chen