* Paul E. McKenney ([email protected]) wrote: > On Sat, Oct 30, 2010 at 10:57:47AM +0200, Paolo Bonzini wrote: > > >+static void *call_rcu_thread(void *arg) > > >+{ > > >+ [...] > > >+ else { > > >+ call_rcu_lock(&crdp->mtx); > > >+ crdp->flags &= ~URCU_CALL_RCU_RUNNING; > > >+ if (&cbs->next != cbs_tail&& > > >+ pthread_cond_wait(&crdp->cond,&crdp->mtx) != 0) { > > >+ perror("pthread_cond_wait"); > > >+ exit(-1); > > >+ } else > > >+ poll(NULL, 0, 10); > > >+ crdp->flags |= URCU_CALL_RCU_RUNNING; > > >+ call_rcu_unlock(&crdp->mtx); > > >+ } > > >+ } > > >+ return NULL; /* NOTREACHED */ > > >+} > > > > Given the way you handle URCU_CALL_RCU_RUNNING above, the flag will > > be reset in call_rcu iff call_rcu sees contention on the lock. > > Doesn't pthread_cond_wait() release the mutex for the duration of the > wait? Ah, are you worried about the poll() under the lock? I am > moving this out from under the lock. > > > >+ call_rcu_lock(&crdp->mtx); > > >+ if (!(crdp->flags& URCU_CALL_RCU_RUNNING)) { > > >+ if (pthread_cond_signal(&crdp->cond) != 0) { > > >+ perror("pthread_cond_signal"); > > >+ exit(-1); > > >+ } > > >+ } > > >+ call_rcu_unlock(&crdp->mtx); > > >+ } > > >+} > > > > So, the mutex is basically unnecessary if some futex magic replaces > > the condition variable. For example, in the thread: > > > > else { > > retry: > > flags = crdp->flags; > > if ((flags & URCU_CALL_RCU_REQUESTED)) > > continue; > > if (cmpxchg (&crdp->flags, flags, > > flags & ~URCU_CALL_RCU_RUNNING) != flags) > > goto retry; > > > > futex_wait (&crdp->flags, > > flags & ~URCU_CALL_RCU_RUNNING); > > } > > > > and in call_rcu: > > > > mb (); > > /* If the thread is not blocked, it will see our request. */ > > do { > > flags = crdp->flags; > > /* If there's already a request pending, no need to > > wake up the process. If the thread is running, no > > need to do anything, it'll pick up our request. */ > > if (flags & > > (URCU_CALL_RCU_REQUESTED | URCU_CALL_RCU_RUNNING)) > > return; > > } while (cmpxchg (&crdp->flags, flags, > > flags | URCU_CALL_RCU_REQUESTED) != flags); > > futex_wake (&crdp->flags, 1); > > OK, sounds like a nice optimization, though a bit Linux-specific. > I will stick with the POSIX stuff for the moment, and once I am > convinced that it really is working, I might consider doing futexes > if running on Linux.
Paul, this is why I created urcu-futex.h. It offers the futex API, but with a polling or pthread-cond-based fallback for non-Linux platforms. So please feel free to implement this using futex_noasync() (this is the version using a pthread_cond() fallback, which is not signal-safe, but does not require polling). Thanks, Mathieu -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com _______________________________________________ ltt-dev mailing list [email protected] http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev
