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Date:   Tue, 31 Dec 2019 03:58:22 -0500
From:   "Frank A. Cancio Bello" <frank@generalsoftwareinc.com>
To:     Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org
Cc:     joel@joelfernandes.org, saiprakash.ranjan@codeaurora.org,
        nachukannan@gmail.com
Subject: [PATCH] tracing: Resets the trace buffer after a snapshot
Message-ID: <20191231085822.yxhph6wcguejb7al@frank-laptop>
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Currently, when a snapshot is taken the trace_buffer and the
max_buffer are swapped. After this swap, the "new" trace_buffer is
not reset. This produces an odd behavior: after a snapshot is taken
the previous snapshot entries become available to the next reader of
the trace_buffer as far as the reading occurs before the buffer is
refilled with new entries by a writer.

This patch resets the trace buffer after a snapshot is taken.

Signed-off-by: Frank A. Cancio Bello <frank@generalsoftwareinc.com>
---

The following commands illustrate this odd behavior:

# cd /sys/kernel/debug/tracing
# echo nop > current_tracer
# echo 1 > tracing_on
# echo m1 > trace_marker
# echo 1 > snapshot
# echo m2 > trace_marker
# echo 1 > snapshot
# cat trace
# tracer: nop
#
# entries-in-buffer/entries-written: 1/1   #P:2
#
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
            bash-550   [000] ....    50.479755: tracing_mark_write: m1


 kernel/trace/trace.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ddb7e7f5fe8d..58373b5ae0cf 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6867,10 +6867,13 @@ tracing_snapshot_write(struct file *filp, const char __user *ubuf, size_t cnt,
 			break;
 		local_irq_disable();
 		/* Now, we're going to swap */
-		if (iter->cpu_file == RING_BUFFER_ALL_CPUS)
+		if (iter->cpu_file == RING_BUFFER_ALL_CPUS) {
 			update_max_tr(tr, current, smp_processor_id(), NULL);
-		else
+			tracing_reset_online_cpus(&tr->trace_buffer);
+		} else {
 			update_max_tr_single(tr, current, iter->cpu_file);
+			tracing_reset_cpu(&tr->trace_buffer, iter->cpu_file);
+		}
 		local_irq_enable();
 		break;
 	default:
-- 
2.17.1