Noel Grandin wrote in cygwin-patches: > Pardon my ignorance, but why not rather use the Windows SRWLock functionality? > https://learn.microsoft.com/en-us/windows/win32/sync/slim-reader-writer--srw--locks > > SRW locks are very fast, only require a single pointer-sized storage area, > can be statically initialised, and do not > need to be destroyed, so there is no possibibilty of memory leakage. > > Then the implementation simply becomes > > int pthread::once (pthread_once_t *once_control, void (*init_routine) (void)) > { > AcquireSRWLockExclusive(once_control->lock); > if (!once_control->state) > { > init_routine() > once_control->state = 1; > } > ReleaseSRWLockExclusive(once_control->lock); > }
SRW locks are spin-locks. Since they are only pointer-sized, ReleaseSRWLockExclusive cannot notify other threads — unlike CRITICAL_SECTION. Therefore, AcquireSRWLockExclusive must busy-loop when the lock is already held. But busy-looping is a bad practice for pthread_once. In the case, for example, that the init_routine reads a multi-megabyte data structure into memory and parses it, the other threads that wait on the result of this operation would by busy-looping, eating full CPU time. Bruno -- Problem reports: https://cygwin.com/problems.html FAQ: https://cygwin.com/faq/ Documentation: https://cygwin.com/docs.html Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple
