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Diffstat (limited to 'kernel/trace/trace_clock.c')
-rw-r--r--kernel/trace/trace_clock.c44
1 files changed, 30 insertions, 14 deletions
diff --git a/kernel/trace/trace_clock.c b/kernel/trace/trace_clock.c
index aaf6793ededa..c1637f90c8a3 100644
--- a/kernel/trace/trace_clock.c
+++ b/kernel/trace/trace_clock.c
@@ -95,33 +95,49 @@ u64 notrace trace_clock_global(void)
{
unsigned long flags;
int this_cpu;
- u64 now;
+ u64 now, prev_time;
raw_local_irq_save(flags);
this_cpu = raw_smp_processor_id();
- now = sched_clock_cpu(this_cpu);
+
/*
- * If in an NMI context then dont risk lockups and return the
- * cpu_clock() time:
+ * The global clock "guarantees" that the events are ordered
+ * between CPUs. But if two events on two different CPUS call
+ * trace_clock_global at roughly the same time, it really does
+ * not matter which one gets the earlier time. Just make sure
+ * that the same CPU will always show a monotonic clock.
+ *
+ * Use a read memory barrier to get the latest written
+ * time that was recorded.
*/
- if (unlikely(in_nmi()))
- goto out;
+ smp_rmb();
+ prev_time = READ_ONCE(trace_clock_struct.prev_time);
+ now = sched_clock_cpu(this_cpu);
- arch_spin_lock(&trace_clock_struct.lock);
+ /* Make sure that now is always greater than prev_time */
+ if ((s64)(now - prev_time) < 0)
+ now = prev_time + 1;
/*
- * TODO: if this happens often then maybe we should reset
- * my_scd->clock to prev_time+1, to make sure
- * we start ticking with the local clock from now on?
+ * If in an NMI context then dont risk lockups and simply return
+ * the current time.
*/
- if ((s64)(now - trace_clock_struct.prev_time) < 0)
- now = trace_clock_struct.prev_time + 1;
+ if (unlikely(in_nmi()))
+ goto out;
- trace_clock_struct.prev_time = now;
+ /* Tracing can cause strange recursion, always use a try lock */
+ if (arch_spin_trylock(&trace_clock_struct.lock)) {
+ /* Reread prev_time in case it was already updated */
+ prev_time = READ_ONCE(trace_clock_struct.prev_time);
+ if ((s64)(now - prev_time) < 0)
+ now = prev_time + 1;
- arch_spin_unlock(&trace_clock_struct.lock);
+ trace_clock_struct.prev_time = now;
+ /* The unlock acts as the wmb for the above rmb */
+ arch_spin_unlock(&trace_clock_struct.lock);
+ }
out:
raw_local_irq_restore(flags);