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    Subject[PATCH 5.5 033/367] tracing: Fix sched switch start/stop refcount racy updates
    Date
    From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

    commit 64ae572bc7d0060429e40e1c8d803ce5eb31a0d6 upstream.

    Reading the sched_cmdline_ref and sched_tgid_ref initial state within
    tracing_start_sched_switch without holding the sched_register_mutex is
    racy against concurrent updates, which can lead to tracepoint probes
    being registered more than once (and thus trigger warnings within
    tracepoint.c).

    [ May be the fix for this bug ]
    Link: https://lore.kernel.org/r/000000000000ab6f84056c786b93@google.com

    Link: http://lkml.kernel.org/r/20190817141208.15226-1-mathieu.desnoyers@efficios.com

    Cc: stable@vger.kernel.org
    CC: Steven Rostedt (VMware) <rostedt@goodmis.org>
    CC: Joel Fernandes (Google) <joel@joelfernandes.org>
    CC: Peter Zijlstra <peterz@infradead.org>
    CC: Thomas Gleixner <tglx@linutronix.de>
    CC: Paul E. McKenney <paulmck@linux.ibm.com>
    Reported-by: syzbot+774fddf07b7ab29a1e55@syzkaller.appspotmail.com
    Fixes: d914ba37d7145 ("tracing: Add support for recording tgid of tasks")
    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/trace/trace_sched_switch.c | 4 +++-
    1 file changed, 3 insertions(+), 1 deletion(-)

    --- a/kernel/trace/trace_sched_switch.c
    +++ b/kernel/trace/trace_sched_switch.c
    @@ -89,8 +89,10 @@ static void tracing_sched_unregister(voi

    static void tracing_start_sched_switch(int ops)
    {
    - bool sched_register = (!sched_cmdline_ref && !sched_tgid_ref);
    + bool sched_register;
    +
    mutex_lock(&sched_register_mutex);
    + sched_register = (!sched_cmdline_ref && !sched_tgid_ref);

    switch (ops) {
    case RECORD_CMDLINE:

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