Messages in this thread | | | Date | Thu, 25 Aug 2022 14:29:51 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH 1/2] arm64/traps: Replace this_cpu_* with raw_cpu_* in panic_bad_stack() |
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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.
> 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.
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 >
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