Re: [PATCH v2 05/11] arm64: entry: Add entry and exit functions for debug exceptions

[Will Deacon <will@kernel.org>]

On Mon, May 12, 2025 at 06:43:20PM +0100, Ada Couprie Diaz wrote:
> Duplicate the `debug_exception_entry()` and `debug_exception_exit()`

nit: it's debug_exception_enter().

> functions from mm/fault.c, as they are needed to split
> the debug exceptions entry paths from the current unified one.
> The original will be removed in a cleanup patch.
> 
> Signed-off-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
> ---
>  arch/arm64/kernel/entry-common.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index b260ddc4d3e9..92d78b329e67 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -418,6 +418,28 @@ static inline void fp_user_discard(void)
>  	}
>  }
>  
> +/*
> + * In debug exception context, we explicitly disable preemption despite
> + * having interrupts disabled.
> + * This serves two purposes: it makes it much less likely that we would
> + * 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)
> +{
> +	preempt_disable();
> +
> +	/* This code is a bit fragile.  Test it. */
> +	RCU_LOCKDEP_WARN(!rcu_is_watching(), "exception_enter didn't work");
> +}
> +NOKPROBE_SYMBOL(debug_exception_enter);
> +
> +static void debug_exception_exit(struct pt_regs *regs)
> +{
> +	preempt_enable_no_resched();
> +}
> +NOKPROBE_SYMBOL(debug_exception_exit);

Could you make these externally visible (i.e. drop the 'static') so that
you can remove the definitions from mm/fault.c at the same time? Then
you could add the 'static' back at the end when there are no more
external callers.

Will