Merge tag 'csd-lock.2024.11.16a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull CSD-lock update from Paul McKenney:
 "This switches from sched_clock() to ktime_get_mono_fast_ns(), which on
  x86 switches from the rdtsc instruction to the rdtscp instruction,
  thus avoiding instruction reorderings that cause false-positive
  reports of CSD-lock stalls of almost 2^46 nanoseconds. These false
  positives are rare, but really are seen in the wild"

* tag 'csd-lock.2024.11.16a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
  locking/csd-lock: Switch from sched_clock() to ktime_get_mono_fast_ns()

Author: torvalds

kernel/smp.c

@@ -246,7 +246,7 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in
 		return true;
 	}
 
-	ts2 = sched_clock();
+	ts2 = ktime_get_mono_fast_ns();
 	/* How long since we last checked for a stuck CSD lock.*/
 	ts_delta = ts2 - *ts1;
 	if (likely(ts_delta <= csd_lock_timeout_ns * (*nmessages + 1) *
@@ -321,7 +321,7 @@ static void __csd_lock_wait(call_single_data_t *csd)
 	int bug_id = 0;
 	u64 ts0, ts1;
 
-	ts1 = ts0 = sched_clock();
+	ts1 = ts0 = ktime_get_mono_fast_ns();
 	for (;;) {
 		if (csd_lock_wait_toolong(csd, ts0, &ts1, &bug_id, &nmessages))
 			break;