Re: [PATCH v2 07/18] mm/ksw: add atomic watch on/off operations

Peter Zijlstra
2025-09-04T06:47:08Z

On Thu, Sep 04, 2025 at 08:21:04AM +0800, Jinchao Wang wrote:

> +static DEFINE_PER_CPU(call_single_data_t,
> + watch_csd) = CSD_INIT(ksw_watch_on_local_cpu,
> + &watch_info);
> +

> +int ksw_watch_on(u64 watch_addr, u64 watch_len)
> +{
> + unsigned long flags;
> + int cpu;
> +
> + if (!watch_addr) {
> + pr_err("KSW: watch with invalid address\n");
> + return -EINVAL;
> + }
> +
> + spin_lock_irqsave(&watch_lock, flags);
> +
> + /*
> + * check if already watched
> + */
> + if (watch_info.addr != 0 && // not uninit
> + watch_info.addr != (unsigned long)&watch_holder && // installed
> + watch_addr != (unsigned long)&watch_holder) { //not restore
> + spin_unlock_irqrestore(&watch_lock, flags);
> + return -EBUSY;
> + }
> +
> + watch_info.addr = watch_addr;
> + watch_info.len = watch_len;
> +
> + spin_unlock_irqrestore(&watch_lock, flags);
> +
> + if (watch_addr == (unsigned long)&watch_holder)
> + pr_debug("KSW: watch off starting\n");
> + else
> + pr_debug("KSW: watch on starting\n");
> +
> + for_each_online_cpu(cpu) {
> + if (cpu == raw_smp_processor_id()) {
> + ksw_watch_on_local_cpu(&watch_info);
> + } else {
> + call_single_data_t *csd = &per_cpu(watch_csd, cpu);
> +
> + smp_call_function_single_async(cpu, csd);
> + }
> + }
> +
> + return 0;
> +}

What do you think happens when two ksw_watch_on() instances run
concurrently?

What happens if a CPU comes online/offline concurrently with
ksw_watch_on()?