[PATCH 1/2] arm64: smp: Add function to determine if cpus are stuck in the kernel

[Suzuki K Poulose]

On 17/06/16 11:27, Mark Rutland wrote:
> On Fri, Jun 17, 2016 at 10:34:56AM +0100, James Morse wrote:
>> kernel/smp.c has a fancy counter that keeps track of the number of CPUs
>> it marked as not-present and left in cpu_park_loop(). If there are any
>> CPUs spinning in here, features like kexec or hibernate may release them
>> by overwriting this memory.
>>
>> This problem also occurs on machines using spin-tables to release
>> secondary cores.
>> After commit 44dbcc93ab67 ("arm64: Fix behavior of maxcpus=N")
>> we bring all known cpus into the secondary holding pen, but may not bring
>> them up depending on 'maxcpus'. This memory can't be re-used by kexec
>> or hibernate.
>>
>> Add a function cpus_are_stuck_in_kernel() to determine if either of these
>> cases have occurred.
>>
>> Signed-off-by: James Morse <james.morse at arm.com>
>> Cc: Suzuki K Poulose <suzuki.poulose at arm.com>
>> ---
>>   arch/arm64/include/asm/smp.h | 20 ++++++++++++++++++++
>>   arch/arm64/kernel/smp.c      | 13 +++++++++++++
>>   2 files changed, 33 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h
>> index 433e50405274..4be755bcc07a 100644
>> --- a/arch/arm64/include/asm/smp.h
>> +++ b/arch/arm64/include/asm/smp.h
>> @@ -124,6 +124,26 @@ static inline void cpu_panic_kernel(void)
>>   	cpu_park_loop();
>>   }
>>
>> +/*
>> + * Kernel features such as hibernate and kexec depend on cpu hotplug to know
>> + * they can replace any kernel memory they are not using themselves.
>> + *
>> + * There are two corner cases:
>> + * If a secondary CPU fails to come online, (e.g. due to mismatched features),
>> + * it will try to call cpu_die(). If this fails, it increases the counter
>> + * cpus_stuck_in_kernel and sits in cpu_park_loop(). The memory containing
>> + * this function must not be re-used for anything else as the 'stuck' core
>> + * is executing it.
>
> It might also be stuck in __no_granule_support, if it never made it to C
> code. In that case, the CPU in charge of bringing up that new CPU will
> increment the counter in __cpu_up.

Just to clarify, *in all the cases*, the CPU in charge of bringing up updates
the cpus_stuck_in_kernel.

>
> There might be other reasons we do something like that in future, so it
> might be better to be a little less specific and say something like:
>
> 	If a secondary CPU enters the kernel but fails to come online,
> 	(e.g. due to mismatched features), and cannot exit the kernel,
> 	we increment cpus_stuck_in_kernel and leave the CPU in a
> 	quiesecent loop within the kernel text. The memory containing
> 	this loop must not be re-used for anything else as the 'stuck'
> 	core is executing it.

Agree.

>> +bool cpus_are_stuck_in_kernel(void);
>> +
>>   #endif /* ifndef __ASSEMBLY__ */
>>
>>   #endif /* ifndef __ASM_SMP_H */
>> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
>> index 678e0842cb3b..e197502f94fd 100644
>> --- a/arch/arm64/kernel/smp.c
>> +++ b/arch/arm64/kernel/smp.c
>> @@ -909,3 +909,16 @@ int setup_profiling_timer(unsigned int multiplier)
>>   {
>>   	return -EINVAL;
>>   }
>> +
>> +bool cpus_are_stuck_in_kernel(void)
>> +{
>> +	bool ret = !!cpus_stuck_in_kernel;
>> +#ifdef CONFIG_HOTPLUG_CPU
>> +	int any_cpu = raw_smp_processor_id();
>> +
>> +	if (num_possible_cpus() > 1 && !cpu_ops[any_cpu]->cpu_die)
>> +		ret = true;
>> +#endif

Minor nit: Moving the cpu_die check to a static inline function with
an obvious name might make the code look better.

	return !!cpus_stuck_in_kernel || !have_cpu_die() ?

Eitherway,

Reviewed-by: Suzuki K Poulose <suzuki.poulose at arm.com>

Cheers
Suzuki

>> +
>> +	return ret;
>> +}
>> --
>> 2.8.0.rc3
>>
>