Messages in this thread | | | Subject | Re: [PATCH 1/2] arm64/traps: Replace this_cpu_* with raw_cpu_* in panic_bad_stack() | From | "Leizhen (ThunderTown)" <> | Date | Fri, 26 Aug 2022 11:25:39 +0800 |
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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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