linux-kernel - [PATCH 2/4] perf/ring_buffer: Add ordering to rb->nest increment

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Message-Id: <20190517115418.309516009@infradead.org>
Date:   Fri, 17 May 2019 13:52:32 +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 2/4] perf/ring_buffer: Add ordering to rb->nest increment

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 <yabinc@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 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);
 
 	/*