linux-kernel - [PATCH 3/4] perf/ring-buffer: Always use {READ,WRITE}_ONCE() for rb->user

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Date:   Fri, 17 May 2019 13:52:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     peterz@...radead.org, mingo@...nel.org
Cc:     yabinc@...gle.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, mark.rutland@....com,
        linux-kernel@...r.kernel.org
Subject: [PATCH 3/4] perf/ring-buffer: Always use {READ,WRITE}_ONCE() for rb->user_page data

We must use {READ,WRITE}_ONCE() on rb->user_page data such that
concurrent usage will see whole values. A few key sites were missing
this.

Fixes:7b732a750477 ("perf_counter: new output ABI - part 1")
Suggested-by: Yabin Cui <yabinc@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/events/ring_buffer.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -100,7 +100,7 @@ static void perf_output_put_handle(struc
 	 * See perf_output_begin().
 	 */
 	smp_wmb(); /* B, matches C */
-	rb->user_page->data_head = head;
+	WRITE_ONCE(rb->user_page->data_head, head);
 
 	/*
 	 * We must publish the head before decrementing the nest count,
@@ -496,7 +496,7 @@ void perf_aux_output_end(struct perf_out
 		perf_event_aux_event(handle->event, aux_head, size,
 				     handle->aux_flags);
 
-	rb->user_page->aux_head = rb->aux_head;
+	WRITE_ONCE(rb->user_page->aux_head, rb->aux_head);
 	if (rb_need_aux_wakeup(rb))
 		wakeup = true;
 
@@ -528,7 +528,7 @@ int perf_aux_output_skip(struct perf_out
 
 	rb->aux_head += size;
 
-	rb->user_page->aux_head = rb->aux_head;
+	WRITE_ONCE(rb->user_page->aux_head, rb->aux_head);
 	if (rb_need_aux_wakeup(rb)) {
 		perf_output_wakeup(handle);
 		handle->wakeup = rb->aux_wakeup + rb->aux_watermark;