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

On 2022/8/25 21:29, Mark Rutland wrote:

On Thu, Aug 25, 2022 at 02:31:53PM +0800, Zhen Lei wrote:

quoted

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.

quoted

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.

quoted

Signed-off-by: Zhen Lei <redacted>
---
 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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