Re: [PATCH 2/2] ARM: Replace this_cpu_* with raw_cpu_* in panic_bad_stack()
From: Mark Rutland <mark.rutland@arm.com>
Date: 2022-08-25 13:34:28
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On Thu, Aug 25, 2022 at 02:31:54PM +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, function this_cpu_read() may trigger scheduling, see pseudocode below. Pseudocode of this_cpu_read(xx): preempt_disable_notrace(); raw_cpu_read(xx); if (unlikely(__preempt_count_dec_and_test())) __preempt_schedule_notrace(); 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 think if scheduling is a problem here, something should increment the preempt_count as is done on arm64, since any other operation in this function could end up causing preemption. Regardless, I also think it's sensible to use raw_cpu_*() here, but I don't think that actually fixes the problem the commit message describes. Thanks, Mark.
quoted hunk ↗ jump to hunk
Signed-off-by: Zhen Lei <redacted> --- KernelVersion: v6.0-rc2 arch/arm/kernel/traps.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c index 1518a1f443ff866..d5903d790cf3b7e 100644 --- a/arch/arm/kernel/traps.c +++ b/arch/arm/kernel/traps.c@@ -927,9 +927,9 @@ asmlinkage void handle_bad_stack(struct pt_regs *regs) { unsigned long tsk_stk = (unsigned long)current->stack; #ifdef CONFIG_IRQSTACKS - unsigned long irq_stk = (unsigned long)this_cpu_read(irq_stack_ptr); + unsigned long irq_stk = (unsigned long)raw_cpu_read(irq_stack_ptr); #endif - unsigned long ovf_stk = (unsigned long)this_cpu_read(overflow_stack_ptr); + unsigned long ovf_stk = (unsigned long)raw_cpu_read(overflow_stack_ptr); console_verbose(); pr_emerg("Insufficient stack space to handle exception!");-- 2.25.1
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