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

From: Peter Zijlstra <peterz@infradead.org>
To: peterz@infradead.org, mingo@kernel.org
Cc: yabinc@google.com, acme@kernel.org,
	alexander.shishkin@linux.intel.com, jolsa@redhat.com,
	namhyung@kernel.org, mark.rutland@arm.com,
	linux-kernel@vger.kernel.org
Subject: [PATCH 2/4] perf/ring_buffer: Add ordering to rb->nest increment
Date: Fri, 17 May 2019 13:52:32 +0200	[thread overview]
Message-ID: <20190517115418.309516009@infradead.org> (raw)
In-Reply-To: 20190517115230.437269790@infradead.org

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@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.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);
 
 	/*


