Similar to how decrementing rb->next too early can cause data_head to
(temporarily) be observed to go backward, so too can this happen when
we increment too late.
This barrier() ensures the rb->head load happens after the increment,
both the one in the 'goto again' path, as the one from
perf_output_get_handle() -- albeit very unlikely to matter for the
latter.
Fixes: ef60777c9abd ("perf: Optimize the perf_output() path by removing
IRQ-disables")
Suggested-by: Yabin Cui <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
---
kernel/events/ring_buffer.c | 9 +++++++++
1 file changed, 9 insertions(+)
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -48,6 +48,15 @@ static void perf_output_put_handle(struc
unsigned long head;
again:
+ /*
+ * In order to avoid publishing a head value that goes backwards,
+ * we must ensure the load of @rb->head happens after we've
+ * incremented @rb->nest.
+ *
+ * Otherwise we can observe a @rb->head value before one published
+ * by an IRQ/NMI happening between the load and the increment.
+ */
+ barrier();
head = local_read(&rb->head);
/*
