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

[Will Deacon <will.deacon@arm.com>]

[+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?

Will

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