Re: [RFC PATCH v2 1/4] arm64/entry: Make debug_exception_enter/exit() noinstr

On Thu, 13 Aug 2026 06:49:27 +0000
Hongyan Xia <[email protected]> wrote:

> From: Hongyan Xia <[email protected]>
> 
> Commit 879a6754d3d11e30af24b7dc486f561510d62641 ran into a crash because
> debug_exception_enter/exit() triggered page faults caused by perf dwarf
> call graph tracing. That patch was a band-aid on top.
> 
> Instead of trying to band-aid all possible paths that can happen during
> instrumentation or perf tracing, simply force-inline these functions and
> use the notrace variant of preempt handling to avoid all the complexity.
> 
> Drop the RCU_LOCKDEP_WARN(): arm64_enter_el1_dbg() runs first and
> enters NMI context via ct_nmi_enter(), so RCU is always watching by the
> time debug_exception_enter() runs.
> 
> Also mark __preempt_count_add/sub() __always_inline so the preempt
> count updates fold into the noinstr callers instead of becoming
> outlined calls to instrumentable text.

Looks good to me as a kprobe maintainer.

Reviewed-by: Masami Hiramatsu (Google) <[email protected]>

Thanks,

> 
> Signed-off-by: Hongyan Xia <[email protected]>
> ---
>  arch/arm64/include/asm/preempt.h |  4 ++--
>  arch/arm64/kernel/entry-common.c | 13 ++++---------
>  2 files changed, 6 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/preempt.h b/arch/arm64/include/asm/preempt.h
> index 932ea4b62042..326f221c3f56 100644
> --- a/arch/arm64/include/asm/preempt.h
> +++ b/arch/arm64/include/asm/preempt.h
> @@ -41,14 +41,14 @@ static inline bool test_preempt_need_resched(void)
>  	return !current_thread_info()->preempt.need_resched;
>  }
>  
> -static inline void __preempt_count_add(int val)
> +static __always_inline void __preempt_count_add(int val)
>  {
>  	u32 pc = READ_ONCE(current_thread_info()->preempt.count);
>  	pc += val;
>  	WRITE_ONCE(current_thread_info()->preempt.count, pc);
>  }
>  
> -static inline void __preempt_count_sub(int val)
> +static __always_inline void __preempt_count_sub(int val)
>  {
>  	u32 pc = READ_ONCE(current_thread_info()->preempt.count);
>  	pc -= val;
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index ceb4eb11232a..d5c1d6defe29 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -292,20 +292,15 @@ static __always_inline void fpsimd_syscall_exit(void)
>   * accidentally schedule in exception context and it will force a warning
>   * if we somehow manage to schedule by accident.
>   */
> -static void debug_exception_enter(struct pt_regs *regs)
> +static __always_inline void debug_exception_enter(struct pt_regs *regs)
>  {
> -	preempt_disable();
> -
> -	/* This code is a bit fragile.  Test it. */
> -	RCU_LOCKDEP_WARN(!rcu_is_watching(), "exception_enter didn't work");
> +	preempt_disable_notrace();
>  }
> -NOKPROBE_SYMBOL(debug_exception_enter);
>  
> -static void debug_exception_exit(struct pt_regs *regs)
> +static __always_inline void debug_exception_exit(struct pt_regs *regs)
>  {
> -	preempt_enable_no_resched();
> +	preempt_enable_no_resched_notrace();
>  }
> -NOKPROBE_SYMBOL(debug_exception_exit);
>  
>  UNHANDLED(el1t, 64, sync)
>  UNHANDLED(el1t, 64, irq)
> -- 
> 2.47.3
> 


-- 
Masami Hiramatsu (Google) <[email protected]>