Re: [PATCH 1/2] arm64/traps: Replace this_cpu_* with raw_cpu_* in panic_bad_stack()

From: "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Russell King <linux@armlinux.org.uk>,
	<linux-arm-kernel@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>, <patches@armlinux.org.uk>
Subject: Re: [PATCH 1/2] arm64/traps: Replace this_cpu_* with raw_cpu_* in panic_bad_stack()
Date: Fri, 26 Aug 2022 11:25:39 +0800	[thread overview]
Message-ID: <569fd51d-aca2-f1c0-d90e-2ae34e4f6f99@huawei.com> (raw)
In-Reply-To: <Ywd5T0+APXqP5PXp@FVFF77S0Q05N>

On 2022/8/25 21:29, Mark Rutland wrote:
> On Thu, Aug 25, 2022 at 02:31:53PM +0800, Zhen Lei wrote:
>> The hardware automatically disable the IRQ interrupt before jumping to the
>> interrupt or exception vector. Therefore, the preempt_disable() operation
>> in this_cpu_read() after macro expansion is unnecessary. In fact, before
>> commit 8168f098867f ("arm64: entry: split bad stack entry"), the operation
>> this_cpu_read() precedes arm64_enter_nmi(). If set_preempt_need_resched()
>> is called before stack overflow, this_cpu_read() may trigger scheduling,
>> see pseudocode below.
>>
>> Pseudocode of this_cpu_read(xx) when CONFIG_PREEMPTION=y:
>> preempt_disable_notrace();
>> raw_cpu_read(xx);
>> if (unlikely(__preempt_count_dec_and_test()))
>> 	__preempt_schedule_notrace();
> 
> Ok, but in mainline we have commit 8168f098867f; so we cannot reach here
> without having fiddled with the preempt count.
> 
> Are you saying that some stable kernel is broken because it lacks commit
> 8168f098867f? Is so, I think the right fix is to backport commit 8168f098867f,
> and that is then irrelevant to this change.

Yes, after backport commit 8168f098867f, the risk is gone.

> 
>> Therefore, use raw_cpu_* instead of this_cpu_* to eliminate potential
>> hazards. At the very least, it reduces a few lines of assembly code.
> 
> I'm happy to use raw_cpu_*() here, to minimize the work we have to do, any any
> risks with e.g. instrumentation, but as above I don't think the case mentioned
> in the commit message is relevant.

OK, I will delete the description about risk.

> 
> Thanks,
> Mark.
> 
>>
>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>> ---
>>  arch/arm64/kernel/traps.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
>> index b7fed33981f7b76..e6b6f4650e3d895 100644
>> --- a/arch/arm64/kernel/traps.c
>> +++ b/arch/arm64/kernel/traps.c
>> @@ -871,8 +871,8 @@ DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)], overflow_stack)
>>  void panic_bad_stack(struct pt_regs *regs, unsigned long esr, unsigned long far)
>>  {
>>  	unsigned long tsk_stk = (unsigned long)current->stack;
>> -	unsigned long irq_stk = (unsigned long)this_cpu_read(irq_stack_ptr);
>> -	unsigned long ovf_stk = (unsigned long)this_cpu_ptr(overflow_stack);
>> +	unsigned long irq_stk = (unsigned long)raw_cpu_read(irq_stack_ptr);
>> +	unsigned long ovf_stk = (unsigned long)raw_cpu_ptr(overflow_stack);
>>  
>>  	console_verbose();
>>  	pr_emerg("Insufficient stack space to handle exception!");
>> -- 
>> 2.25.1
>>
> .
> 

-- 
Regards,
  Zhen Lei

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