Re: perf: Add Raspberry Pi BCM2835 AXI PMU driver

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Race condition leaves PMU permanently bound to an offline CPU if a CPU goes offline during probe.
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--- Patch [1]: perf: Add Raspberry Pi BCM2835 AXI PMU driver ---
commit 0a57b13469fd53b31d259c68ce322f804972b736
Author: Ian Rogers <irogers@google.com>

perf: Add Raspberry Pi BCM2835 AXI PMU driver

This commit adds a new performance monitoring driver for the Raspberry Pi
AXI bus (BCM2835/2711), exposing system-level and VideoCore PMU hardware to
the Linux perf subsystem.

> diff --git a/drivers/perf/rpi_axi_pmu.c b/drivers/perf/rpi_axi_pmu.c
> new file mode 100644
> index 0000000000000..31fa94507f4ab
> --- /dev/null
> +++ b/drivers/perf/rpi_axi_pmu.c
[ ... ]
> +static int rpi_axi_pmu_offline_cpu(unsigned int cpu, struct hlist_node *node)
> +{
> +	struct rpi_axi_pmu *pmu = hlist_entry_safe(node, struct rpi_axi_pmu, cpuhp_node);
> +	unsigned long spinflags;
> +	unsigned int target;
> +
> +	if (!pmu->is_registered)
> +		return 0;

[Severity: Medium]
Does this early return leave the PMU permanently bound to an offline CPU if
a hotplug event occurs during probe?

Looking at rpi_axi_pmu__init(), there is a window where the hotplug state
is registered but the PMU is not yet marked as registered:

	ret = cpuhp_state_add_instance(rpi_axi_pmu_cpuhp_state, &pmu->cpuhp_node);
	...
	ret = perf_pmu_register(&pmu->pmu, PMU_NAME, /*type=*/-1);
	...
	pmu->is_registered = true;

If the designated CPU goes offline in this window, rpi_axi_pmu_offline_cpu()
will be called but immediately return 0. This bypasses the migration logic
that would normally update pmu->cpu to an online target.

Once the probe completes, the PMU would remain assigned to an offline CPU,
which could cause perf_event_open() to fail with -ENODEV until another
hotplug event forces a reassignment.

> +
> +	if (cpu != pmu->cpu)
> +		return 0;
> +
> +	target = cpumask_any_but(cpu_online_mask, cpu);
> +	if (target >= nr_cpu_ids)
> +		return 0;
> +
> +	perf_pmu_migrate_context(&pmu->pmu, cpu, target);
> +	pmu->cpu = target;
[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260813142608.2954099-1-irogers@google.com?part=1