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Subject[PATCH] tracing: Resets the trace buffer after a snapshot
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
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