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