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From:   Ben Hutchings <ben@decadent.org.uk>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC:     akpm@linux-foundation.org, Denis Kirjanov <kda@linux-powerpc.org>,
        "Alexander Shishkin" <alexander.shishkin@linux.intel.com>,
        "Vince Weaver" <vincent.weaver@maine.edu>, namhyung@kernel.org,
        acme@kernel.org, "Jiri Olsa" <jolsa@redhat.com>,
        "Linus Torvalds" <torvalds@linux-foundation.org>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        "Arnaldo Carvalho de Melo" <acme@redhat.com>,
        "Stephane Eranian" <eranian@google.com>,
        "Ingo Molnar" <mingo@kernel.org>, mark.rutland@arm.com,
        "Peter Zijlstra" <peterz@infradead.org>,
        "Yabin Cui" <yabinc@google.com>
Date:   Wed, 02 Oct 2019 20:06:51 +0100
Message-ID: <lsq.1570043211.419547842@decadent.org.uk>
Subject: [PATCH 3.16 25/87] perf/ring_buffer: Add ordering to rb->nest
 increment
In-Reply-To: <lsq.1570043210.379046399@decadent.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
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3.16.75-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@infradead.org>

commit 3f9fbe9bd86c534eba2faf5d840fd44c6049f50e upstream.

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@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: acme@kernel.org
Cc: mark.rutland@arm.com
Cc: namhyung@kernel.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@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 kernel/events/ring_buffer.c | 9 +++++++++
 1 file changed, 9 insertions(+)

--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -47,6 +47,15 @@ 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);
 
 	/*