# Re: \[PATCH\] arm64: smp: fix smp\_send\_stop() behaviour

- From: Cristian Marussi \<cristian.marussi@arm.com\>
- Date: 2019-06-18 09:56:50 
- List: linux-arm-kernel
- Page: https://ratatoskr.run/linux-arm-kernel/2019/06/8782873
- Thread: https://ratatoskr.run/linux-arm-kernel/2019/06/8782873/t.md

```text
Hi Itaru

thanks for the review.

On 17/06/2019 20:58, Itaru Kitayama wrote:
> Could you avoid using the magic number like in udelay()?
> 

If you mean udelay(1) it is just that I avoided modifying anything which was not
strictly related to the fix addressed by this patch.

Thanks

Cristian

> On Thu, Jun 13, 2019 at 21:22 Cristian Marussi [ref]
> 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));
>>
>> --
>> 2.17.1
>>
>>
>> _______________________________________________
>> linux-arm-kernel mailing list
>> linux-arm-kernel@lists.infradead.org
>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>>
> 


_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
```