Re: [PATCH] arm64: smp: fix smp_send_stop() behaviour

[Cristian Marussi <cristian.marussi@arm.com>]

Hi Will

Thanks for the review.

On 17/06/2019 19:09, Will Deacon wrote:
> [+James M]
> 
> On Thu, Jun 13, 2019 at 01:21:46PM +0100, Cristian Marussi wrote:
>> On a 2-CPUs system, when one CPU is already online if the other
>> panics while starting-up, smp_send_stop() will fail to send any
>> STOP message to the other already online core, resulting in a
>> system still responsive and alive at the end of the panic procedure.
>> This patch makes smp_send_stop() account also for the online status
>> of the calling CPU while evaluating how many CPUs are effectively
>> online: this way, an adequate number of STOPs is sent, so enforcing
>> a proper freeze of the system at the end of panic even under the
>> above conditions.
>>
>> Reported-by: Dave Martin <Dave.Martin@arm.com>
>> Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
>> ---
>>
>> This peculiar panic-procedure behaviour was exposed hitting a BUG()
>> while running a KSFT cpu-hotplug test on a 2-core ARMv8 model.
>> Such trigger-BUG() was fixed by a distinct commit already included
>> in Linux 5.2-rc4 [0]
>>
>> [0] https://lore.kernel.org/linux-arm-kernel/1559576102-12156-1-git-send-email-Dave.Martin@arm.com/
>> ---
>>  arch/arm64/kernel/smp.c | 12 +++++++++---
>>  1 file changed, 9 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
>> index bb4b3f07761a..c7d604427883 100644
>> --- a/arch/arm64/kernel/smp.c
>> +++ b/arch/arm64/kernel/smp.c
>> @@ -971,8 +971,14 @@ void tick_broadcast(const struct cpumask *mask)
>>  void smp_send_stop(void)
>>  {
>>  	unsigned long timeout;
>> +	unsigned int this_cpu_online = cpu_online(smp_processor_id());
>>  
>> -	if (num_online_cpus() > 1) {
>> +	/*
>> +	 * If this CPU isn't fully online, it will not be counted in
>> +	 * num_online_cpus(): on a 2-CPU system this situation will
>> +	 * result in no message being sent to the other already online CPU.
>> +	 */
>> +	if (num_online_cpus() > this_cpu_online) {
>>  		cpumask_t mask;
>>  
>>  		cpumask_copy(&mask, cpu_online_mask);
>> @@ -985,10 +991,10 @@ void smp_send_stop(void)
>>  
>>  	/* Wait up to one second for other CPUs to stop */
>>  	timeout = USEC_PER_SEC;
>> -	while (num_online_cpus() > 1 && timeout--)
>> +	while (num_online_cpus() > this_cpu_online && timeout--)
>>  		udelay(1);
>>  
>> -	if (num_online_cpus() > 1)
>> +	if (num_online_cpus() > this_cpu_online)
>>  		pr_warning("SMP: failed to stop secondary CPUs %*pbl\n",
>>  			   cpumask_pr_args(cpu_online_mask));
> 
> Whilst this looks ok to me, I'm worried about whether or not we have this
> sort of logic elsewhere. For example, do we need to fix
> crash_smp_send_stop() (and possibly machine_kexec()) too?

I think we certainly have such logic in crash_smp_send_stop() too at least,
maybe it is just less easily exposed given the different use case of the function.

This wanted to be just a fix only against the observed troubled panic, but I
could extend it to cover similar issues spotted by code analysis, if deemed worth.

Thanks

Cristian


> 
> Will
> 


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