Re: [PATCH v2 07/18] mm/ksw: add atomic watch on/off operations
From: Peter Zijlstra <peterz@infradead.org>
Date: 2025-09-04 06:47:17
Also in:
linux-mm, linux-perf-users, lkml
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()?