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

[Ada Couprie Diaz <ada.coupriediaz@arm.com>]

On 20/05/2025 16:36, Will Deacon wrote:

> 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().
Thanks,fixed for v3.
>> 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
If you are happy with some header noise, adding `debug_exception_enter()`
and `debug_exception_exit()` to include/asm/exception.h` so that they can
be visible in `mm/fault.c` during the clean up and removing them when
done, I would much prefer doing it that way !

It felt strange to just have some duplicate code, but I didn't know
what would be acceptable.
I will make the change for v3, unless the header noise mentioned above is
an issue.

Thanks,
Ada