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

On Thu, Sep 04, 2025 at 08:46:59AM +0200, Peter Zijlstra wrote:

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

quoted

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

quoted

+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()?

Thanks for pointing that out. I did not think much about it.
I will think more and address it in the next series.