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You have to really think about what resource your mutex is protecting, it was not clear what your intention is. I guess you have some protected process that you want to start/stop. In that case, it would look something like this:

Called from a thread A:

tm = nh.createTimer(ros::Duration(...), MyClass::timerCallback);

Called from a thread B:

my_mutex.lock();
// at this point, we are guaranteed that no protected process is running, but a timerCallback could be blocked
if (some_condition) {
  need_to_process = false
  my_mutex.unlock();

  // don't lock around timer because it has an internal mutex
  tm.stop(); // wait for finishing of the timerCallback

  // at this point in the code, the timer is timerCallback not running anymore
}
my_mutex.unlock();

Called from a thread C:

void MyClass::timerCallback(ros::TimerEvent ev) {
  std::lock_guard<std::mutex> lock(my_mutex);
  if (need_to_process)
    // protected process ...
}

You have to really think about what resource your mutex is protecting, it was not clear what your intention is. I guess you have some protected process that you want to start/stop. In that case, it would look something like this:

Called from a thread A:

tm = nh.createTimer(ros::Duration(...), MyClass::timerCallback);

Called from a thread B:

my_mutex.lock();
// at this point, we are guaranteed that no protected process is running, but a timerCallback could be blocked
if (some_condition) {
  need_to_process = false
  my_mutex.unlock();

  // don't lock around timer because it has an internal mutex
  tm.stop(); // wait for finishing of the timerCallback

  // at this point in the code, the timer is timerCallback not running anymore
} else {
  my_mutex.unlock();
}
my_mutex.unlock();

Called from a thread C:

void MyClass::timerCallback(ros::TimerEvent ev) {
  std::lock_guard<std::mutex> lock(my_mutex);
  if (need_to_process)
    // protected process ...
}