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

[Florian Weimer]

* 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