Re: [PATCH -fixes] drivers: perf: Fix smp_processor_id() use in preemptible code

[Will Deacon <will@kernel.org>]

On Mon, Aug 26, 2024 at 06:52:10PM +0200, Alexandre Ghiti wrote:
> As reported in [1], the use of smp_processor_id() in
> pmu_sbi_device_probe() must be protected by disabling the preemption, so
> simple use get_cpu()/put_cpu() instead.
> 
> Reported-by: Nam Cao <namcao@linutronix.de>
> Closes: https://lore.kernel.org/linux-riscv/20240820074925.ReMKUPP3@linutronix.de/ [1]
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
>  drivers/perf/riscv_pmu_sbi.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/perf/riscv_pmu_sbi.c b/drivers/perf/riscv_pmu_sbi.c
> index 31a17a56eb3b..25b1b699b3e2 100644
> --- a/drivers/perf/riscv_pmu_sbi.c
> +++ b/drivers/perf/riscv_pmu_sbi.c
> @@ -1373,11 +1373,15 @@ static int pmu_sbi_device_probe(struct platform_device *pdev)
>  
>  	/* SBI PMU Snapsphot is only available in SBI v2.0 */
>  	if (sbi_v2_available) {
> +		int cpu;
> +
>  		ret = pmu_sbi_snapshot_alloc(pmu);
>  		if (ret)
>  			goto out_unregister;
>  
> -		ret = pmu_sbi_snapshot_setup(pmu, smp_processor_id());
> +		cpu = get_cpu();
> +
> +		ret = pmu_sbi_snapshot_setup(pmu, cpu);
>  		if (ret) {
>  			/* Snapshot is an optional feature. Continue if not available */
>  			pmu_sbi_snapshot_free(pmu);
> @@ -1391,6 +1395,7 @@ static int pmu_sbi_device_probe(struct platform_device *pdev)
>  			 */
>  			static_branch_enable(&sbi_pmu_snapshot_available);
>  		}
> +		put_cpu();

Are you sure it's safe to enable the static key with preemption disabled?
I thought that could block on a mutex.

Will