On 12/27, Manfred Spraul wrote: > > >I _think_ that > > > > wait_event_whatever(WQ, CONDITION); > >vs > > > > CONDITION = 1; > > if (wq_has_sleeper(WQ)) > > wake_up_xxx(WQ, ...); > > > >is fine. > > This pattern is documented in wait.h: > > https://elixir.bootlin.com/linux/v6.12.6/source/include/linux/wait.h#L96 > > Thus if there an issue, then the documentation should be updated. Agreed, basically the same pattern, prepare_to_wait_event() is similar to prepare_to_wait(). > But I do not understand this comment (from 2.6.0) > > https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/kernel/fork.c?h=v2.6.0&id=e220fdf7a39b54a758f4102bdd9d0d5706aa32a7 > > >/* * Note: we use "set_current_state()" _after_ the wait-queue add, * > >because we need a memory barrier there on SMP, so that any * wake-function > >that tests for the wait-queue being active * will be guaranteed to see > >waitqueue addition _or_ subsequent * tests in this thread will see the > >wakeup having taken place. * * The spin_unlock() itself is semi-permeable > >and only protects * one way (it only protects stuff inside the critical > >region and * stops them from bleeding out - it would still allow > >subsequent * loads to move into the the critical region). */ ... > set_current_state() now uses smp_store_mb(), which is a memory barrier > _after_ the store. And afaics this is what we actually need. > Thus I do not see what enforces that the store happens > before the store for the __add_wait_queue(). IIUC this is fine, no need to serialize list_add() and STORE(tsk->__state), they can be reordered. But we need mb() between __add_wait_queue + __set_current_state (in any order) and the subsequent "if (CONDITION)" check. > --- a/kernel/sched/wait.c > +++ b/kernel/sched/wait.c > @@ -124,6 +124,23 @@ static int __wake_up_common_lock(struct wait_queue_head *wq_head, unsigned int m > int __wake_up(struct wait_queue_head *wq_head, unsigned int mode, > int nr_exclusive, void *key) > { > + if (list_empty(&wq_head->head)) { > + struct list_head *pn; > + > + /* > + * pairs with spin_unlock_irqrestore(&wq_head->lock); > + * We actually do not need to acquire wq_head->lock, we just > + * need to be sure that there is no prepare_to_wait() that > + * completed on any CPU before __wake_up was called. > + * Thus instead of load_acquiring the spinlock and dropping > + * it again, we load_acquire the next list entry and check > + * that the list is not empty. > + */ > + pn = smp_load_acquire(&wq_head->head.next); > + > + if(pn == &wq_head->head) > + return 0; > + } Too subtle for me ;) I have some concerns, but I need to think a bit more to (try to) actually understand this change. Oleg.
