Subject: [PATCH v6 4/4] ARM: Add KGDB/KDB FIQ debugger generic code

From: [email-masked] (Daniel Thompson)
Date: Thu, 26 Jun 2014 13:48:35 +0100

On 24/06/14 17:22, Nicolas Pitre wrote:
> On Tue, 24 Jun 2014, Daniel Thompson wrote:
> 
>> From: Anton Vorontsov <[email-masked]>
>>
>> The FIQ debugger may be used to debug situations when the kernel stuck
>> in uninterruptable sections, e.g. the kernel infinitely loops or
>> deadlocked in an interrupt or with interrupts disabled.
>>
>> By default KGDB FIQ is disabled in runtime, but can be enabled with
>> kgdb_fiq.enable=1 kernel command line option.
>>
>> Signed-off-by: Anton Vorontsov <[email-masked]>
>> Signed-off-by: John Stultz <[email-masked]>
>> Signed-off-by: Daniel Thompson <[email-masked]>
>> Cc: Russell King <[email-masked]>
>> Cc: Ben Dooks <[email-masked]>
>> Cc: Dave Martin <[email-masked]>
>> ---
>>  arch/arm/Kconfig                 |   2 +
>>  arch/arm/Kconfig.debug           |  18 ++++++
>>  arch/arm/include/asm/kgdb.h      |   7 +++
>>  arch/arm/kernel/Makefile         |   1 +
>>  arch/arm/kernel/kgdb_fiq.c       | 124 +++++++++++++++++++++++++++++++++++++++
>>  arch/arm/kernel/kgdb_fiq_entry.S |  87 +++++++++++++++++++++++++++
>>  6 files changed, 239 insertions(+)
>>  create mode 100644 arch/arm/kernel/kgdb_fiq.c
>>  create mode 100644 arch/arm/kernel/kgdb_fiq_entry.S
> 
> [...]
> 
>> +static long kgdb_fiq_setup_stack(void *info)
>> +{
>> +	struct pt_regs regs;
>> +
>> +	regs.ARM_sp = __get_free_pages(GFP_KERNEL, THREAD_SIZE_ORDER) +
>> +			THREAD_START_SP;
>> +	WARN_ON(!regs.ARM_sp);
> 
> Isn't this rather fatal if you can't allocate any stack? Why not using 
> BUG_ON(), or better yet propagate a proper error code back?

Thanks for raising this.

I think we can get rid of the allocation altogether. This stack is *way*
oversized (it only needs to be 12 bytes).

>> +
>> +	set_fiq_regs(&regs);
>> +	return 0;
>> +}
>> +
>> +/**
>> + * kgdb_fiq_enable_nmi - Manage NMI-triggered entry to KGDB
>> + * @on: Flag to either enable or disable an NMI
>> + *
>> + * This function manages NMIs that usually cause KGDB to enter. That is, not
>> + * all NMIs should be enabled or disabled, but only those that issue
>> + * kgdb_handle_exception().
>> + *
>> + * The call counts disable requests, and thus allows to nest disables. But
>> + * trying to enable already enabled NMI is an error.
>> + */
>> +static void kgdb_fiq_enable_nmi(bool on)
>> +{
>> +	static atomic_t cnt;
>> +	int ret;
>> +
>> +	ret = atomic_add_return(on ? 1 : -1, &cnt);
>> +	if (ret > 1 && on) {
>> +		/*
>> +		 * There should be only one instance that calls this function
>> +		 * in "enable, disable" order. All other users must call
>> +		 * disable first, then enable. If not, something is wrong.
>> +		 */
>> +		WARN_ON(1);
>> +		return;
>> +	}
> 
> Minor style suggestion:
> 
> 	/*
> 	 * There should be only one instance that calls this function
> 	 * in "enable, disable" order. All other users must call
> 	 * disable first, then enable. If not, something is wrong.
> 	 */
> 	if (WARN_ON(ret > 1 && on))
> 		return;

Will adopt this style.

> Other than that...
> 
> Acked-by: Nicolas Pitre <[email-masked]>

Thanks for review.