On Mon, 2015-04-27 at 16:25 -0400, Waiman Long wrote: > >> + /* > >> + * up_write() cleared the owner field before calling this function. > >> + * If that field is now set, a writer must have stolen the lock and > >> + * the wakeup operation should be aborted. > >> + */ > >> + if (rwsem_has_active_writer(sem)) > >> + goto out; > > We currently allow small races between rwsem owner and counter checks. > > And __rwsem_do_wake() can be called by checking the former -- and lock > > stealing is done with the counter as well. Please see below how we back > > out of such cases, as it is very much considered when granting the next > > reader. So nack to this as is, sorry. > > If the first one in the queue is a writer, wake_up_process() may be > called directly which can be quite expensive if the lock has already > been stolen as the task will have to sleep again.
But how can this occur? Lock stealing takes form in two places: 1) fastpath: only if the counter is 0 -- which, since we are discussing waking up waiter(s) code, obviously cannot occur. 2) With the cmpxchg() in rwsem_try_write_lock(), which is serialized with the wait_lock, so again this cannot occur. Which is why this is not considered in __rwsem_do_wake() when waking the writer fist in the queue. Thanks, Davidlohr -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
