Re: [RFC PATCH 0/2] futex: how to solve the robust_list race condition?

* Mathieu Desnoyers:

Trying to find a backward compatible way to solve this may be tricky.
Here is one possible approach I have in mind: Introduce a new syscall,
e.g. sys_cleanup_robust_list(void *addr)

This system call would be invoked on pthread_mutex_destroy(3) of
robust mutexes, and do the following:

- Calculate the offset of @addr within its mapping,
- Iterate on all processes which map the backing store which contain
  the lock address @addr.
  - Iterate on each thread sibling within each of those processes,
    - If the thread has a robust list, and its list_op_pending points
      to the same offset within the backing store mapping, clear the
      list_op_pending pointer.

The overhead would be added specifically to pthread_mutex_destroy(3),
and only for robust mutexes.

Would we have to do this for pthread_mutex_destroy only, or also for
pthread_join?  It is defined to exit a thread with mutexes still locked,
and the pthread_join call could mean that the application can determine
by its own logic that the backing store can be deallocated.

Thanks,
Florian