linux-kernel - [tip:perf/urgent] perf/ring_buffer: Add ordering to rb->nest increment

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Message-ID: <tip-3f9fbe9bd86c534eba2faf5d840fd44c6049f50e@git.kernel.org>
Date:   Fri, 24 May 2019 01:08:55 -0700
From:   tip-bot for Peter Zijlstra <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     yabinc@...gle.com, eranian@...gle.com, mingo@...nel.org,
        jolsa@...hat.com, linux-kernel@...r.kernel.org, acme@...hat.com,
        tglx@...utronix.de, vincent.weaver@...ne.edu, peterz@...radead.org,
        torvalds@...ux-foundation.org, hpa@...or.com,
        alexander.shishkin@...ux.intel.com
Subject: [tip:perf/urgent] perf/ring_buffer: Add ordering to rb->nest
 increment

Commit-ID:  3f9fbe9bd86c534eba2faf5d840fd44c6049f50e
Gitweb:     https://git.kernel.org/tip/3f9fbe9bd86c534eba2faf5d840fd44c6049f50e
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Fri, 17 May 2019 13:52:32 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 24 May 2019 09:00:10 +0200

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.

Suggested-by: Yabin Cui <yabinc@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Jiri Olsa <jolsa@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Stephane Eranian <eranian@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vince Weaver <vincent.weaver@...ne.edu>
Cc: acme@...nel.org
Cc: mark.rutland@....com
Cc: namhyung@...nel.org
Fixes: ef60777c9abd ("perf: Optimize the perf_output() path by removing IRQ-disables")
Link: http://lkml.kernel.org/r/20190517115418.309516009@infradead.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/events/ring_buffer.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 009467a60578..4b5f8d932400 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -48,6 +48,15 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
 	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);
 
 	/*