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    Subject[PATCH 3.16 091/328] tracing: Do not call start/stop() functions when tracing_on does not change
    3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

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

    From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

    commit f143641bfef9a4a60c57af30de26c63057e7e695 upstream.

    Currently, when one echo's in 1 into tracing_on, the current tracer's
    "start()" function is executed, even if tracing_on was already one. This can
    lead to strange side effects. One being that if the hwlat tracer is enabled,
    and someone does "echo 1 > tracing_on" into tracing_on, the hwlat tracer's
    start() function is called again which will recreate another kernel thread,
    and make it unable to remove the old one.

    Link: http://lkml.kernel.org/r/1533120354-22923-1-git-send-email-erica.bugden@linutronix.de

    Fixes: 2df8f8a6a897e ("tracing: Fix regression with irqsoff tracer and tracing_on file")
    Reported-by: Erica Bugden <erica.bugden@linutronix.de>
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    kernel/trace/trace.c | 4 +++-
    1 file changed, 3 insertions(+), 1 deletion(-)

    --- a/kernel/trace/trace.c
    +++ b/kernel/trace/trace.c
    @@ -6180,7 +6180,9 @@ rb_simple_write(struct file *filp, const

    if (buffer) {
    mutex_lock(&trace_types_lock);
    - if (val) {
    + if (!!val == tracer_tracing_is_on(tr)) {
    + val = 0; /* do nothing */
    + } else if (val) {
    tracer_tracing_on(tr);
    if (tr->current_trace->start)
    tr->current_trace->start(tr);
    \
     
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