"benign" data race: stop request flag, write once, read periodically,

Joshua Maurice <joshuamaurice@googlemail.com>
Sat, 18 Apr 2015 06:37:12 CST
{ edited by mod to shorten lines to ~70 characters. -mod }

I have a question about a common code pattern that is cooperative task
(or thread) cancellation. Here's a crude definition off the cuff:
a task is cancellable if the controller of the task and the cancellable
task have a shared boolean flag which is used to signal or notify that
the task is to be canceled. The controller code exists in a separate
thread. If the controller code wants the task to stop, it sets the stop
flag to true. It is not a requirement for the task to notice the
request right away, or ever. It is acceptable behavior for a stop
request to be issued before the task completes execution, and for the
task to report successful, non-stopped execution. The requirement is
merely that the task make a best-effort attempt to notice the stop
request in a timely manner and to stop, interrupt, and cancel its
execution when it notices a stop request.

Consider the following pseudo C++ code which is an example of this


#include <thread>

class MyTask
  MyTask() : stopFlag(false) {}

  bool stopFlag;

  void operator() ()
    for (;;)
      if (stopFlag)

      //do some (bounded) work, then check the stopFlag again


int main()
  MyTask task;
  std::thread mythread(task);
  task.stopFlag = true;


It's just an example of basic userspace cooperative task / thread

I know this is a formal race condition according to the C++11 standard,
and I know that the simplest fix would be to change stopFlag to this:
    std::atomic<bool> stopFlag;
I also remember some other dedicated standard type for just this
purpose or something.

I also know that I could change stopFlag to this:
  std::atomic<bool> stopFlag;
and I could get away with making the load and store both

My question concerns common implementations. It would be nice ammo to
convince my colleagues and friends that all race conditions are harmful,
and there is no such thing as a benign race condition. I am aware of
the excellent paper "How to miscompile programs with "benign" data
races" by Hans-J. Boehm. However, it doesn't cover this very simple
example, and it would be nice if someone had an example of plausible
code in this pattern plus a plausible implementation which would
produce actual undesired behavior.

I also have more than enough ammo to make the case that changing the
read and write into atomic std::memory_order_relaxed will pose
basically zero overhead and will solve any possible lingering doubts
about real-world real-implementation correctness.

Finally, I recall seeing a real academic paper which covered this very
specific example and actually gave plausible user code and a plausible
reason why the implementation would break even this. However, I did
not save the paper, and I do not remember enough to find this paper.
(I spent the last several hours trying.) I am posting this to ask if
anyone knows of this paper and can point me towards it. I would be
most grateful if someone could post a link or enough information to
find this paper. I hope the paper is not a figment of my imagination -
it might be.

Thanks for your time!

      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]

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