Unlocking a robust mutex in a cleanup handler

[Dimitri Staessens]

Dear devs,

first of all, thank you for supporting robust mutexes in FreeBSD 11.

I'm having some issues with a thread that holds a robust mutex (residing 
in a POSIX shared memory (shm) segment) in conjunctions with a condition 
variable (also in that POSIX shm) on which that thread is blocked via a 
pthread_cond_wait() call. pthread_cond_wait tries to retake the mutex 
after the thread cancellation signal is received, and there is a 
pthread_mutex_unlock pushed to the cleanup stack to unlock that robust 
mutex in case of a cancellation.

Cancelling that thread works fine on Linux, however, on FreeBSD 
11.0-RELEASE, if I pthread_cancel that thread I can't get past the 
following check and resulting PANIC call:
https://github.com/freebsd/freebsd/blob/master/lib/libthr/thread/thr_mutex.c#L187

After removing the check and recompiling libthr everything seems to work 
fine.

Could this be a bug in libthr or am I missing a nuance in the use of a 
shared robust mutexes in conjunction with condition variables?

Kind regards,

Dimitri

-- 
Dimitri Staessens
Ghent University - imec
Dept. of Information Technology (INTEC)
Internet Based Communication Networks and Services
Technologiepark 15
9052 Zwijnaarde
T: +32 9 331 48 70
F: +32 9 331 48 99