Re: [PATCH 1/4] hwtracing: hisi_ptt: Make cpumask only present online CPUs

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Wed, 15 Mar 2023 17:43:13 +0800
Yicong Yang <yangyicong@huawei.com> wrote:

> From: Yicong Yang <yangyicong@hisilicon.com>
> 
> perf will try to start PTT trace on every CPU presented in cpumask sysfs
> attribute and it will fail to start on offline CPUs(see the comments in
> perf_event_open()). But the driver is using cpumask_of_node() to export
> the available cpumask which may include offline CPUs and may fail the
> perf unintendedly. Fix this by only export the online CPUs of the node.

There isn't clear documentation that I can find for cpumask_of_node()
and chasing through on arm64 (which is what we care about for this driver)
it's maintained via numa_add_cpu() numa_remove_cpu()
Those are called in arch/arm64/kernel/smp.c in locations that are closely coupled
with set_cpu_online(cpu, XXX);
https://elixir.bootlin.com/linux/v6.3-rc4/source/arch/arm64/kernel/smp.c#L246
https://elixir.bootlin.com/linux/v6.3-rc4/source/arch/arm64/kernel/smp.c#L303

Now there are races when the two might not be in sync but in this case
we are just exposing the result to userspace, so chances of a race
after this sysfs attribute has been read seems much higher to me and
I don't think we can do anything about that.

Is there another path that I'm missing where online and node masks are out
of sync?

Jonathan


> 
> Fixes: ff0de066b463 ("hwtracing: hisi_ptt: Add trace function support for HiSilicon PCIe Tune and Trace device")
> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>

> ---
>  drivers/hwtracing/ptt/hisi_ptt.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hwtracing/ptt/hisi_ptt.c b/drivers/hwtracing/ptt/hisi_ptt.c
> index 30f1525639b5..0a10c7ec46ad 100644
> --- a/drivers/hwtracing/ptt/hisi_ptt.c
> +++ b/drivers/hwtracing/ptt/hisi_ptt.c
> @@ -487,9 +487,18 @@ static ssize_t cpumask_show(struct device *dev, struct device_attribute *attr,
>  			    char *buf)
>  {
>  	struct hisi_ptt *hisi_ptt = to_hisi_ptt(dev_get_drvdata(dev));
> -	const cpumask_t *cpumask = cpumask_of_node(dev_to_node(&hisi_ptt->pdev->dev));
> +	cpumask_var_t mask;
> +	ssize_t n;
>  
> -	return cpumap_print_to_pagebuf(true, buf, cpumask);
> +	if (!alloc_cpumask_var(&mask, GFP_KERNEL))
> +		return 0;
> +
> +	cpumask_and(mask, cpumask_of_node(dev_to_node(&hisi_ptt->pdev->dev)),
> +		    cpu_online_mask);
> +	n = cpumap_print_to_pagebuf(true, buf, mask);
> +	free_cpumask_var(mask);
> +
> +	return n;
>  }
>  static DEVICE_ATTR_RO(cpumask);
>