On Wed, 20 May 2020 at 09:44, Rasmus Villemoes <r...@rasmusvillemoes.dk> wrote: > > Hi > > The condition variable implementation added in commit 806dd0472f56fd > seems to fall into the trap(s) pointed out in the paper > > http://birrell.org/andrew/papers/ImplementingCVs.pdf > > in particular, the "if no thread is currently waiting, signal should not > leave a full semaphore so a following _wait will just fall through". > > Now, libgcc/gthr.h doesn't really spell out whether __gthread condition > variables are supposed to behave as POSIX condition variables, so I > don't know if it's a problem in practice.
The libstdc++ std::condition_variable assumes POSIX-ish semantics for __gthread_cond_t. It certainly assumes that a signal/broadcast will only wake threads that are currently waiting, not affect waits that haven't started yet. So it seems gthr-vxworks.h should not set __GTHREADS_CXX0X, or libstdc++ needs a different condition_variable implementation for VxWorks. Libstdc++ also assumes this requirement of pthread_cond_destroy is met: "It shall be safe to destroy an initialized condition variable upon which no threads are currently blocked." This means it's OK for another thread to destroy the CV after the semTake (*cond, WAIT_FOREVER) statement in __ghtread_cond_wait, even if the semTake (*mutex, WAIT_FOREVER) statement hasn't happened. The code looks OK as far as that requirement goes. I'm not sure how many other implicit assumptions libstdc++ makes about POSIX-ish semantics. As an aside, it looks like there are enough libgcc/config/*vx* files to justify creating a libgcc/config/vxworks sub-directory.
