On 11/11, Dmitry Torokhov wrote: > > On Tue, Nov 11, 2008 at 06:20:50PM +0100, Oleg Nesterov wrote: > > On 11/11, Dmitry Torokhov wrote: > > > > > But let me repeat, if queue_delayed_work() fails becuase this work is > > already queued we (in this particular case) need mb(), not wmb(). Or > > atkbd_schedule_event_work() can miss a bit in ->event_mask. So I think > > this wmb() is misleading. > > Could you please explain why wmb() is not enough and full mb() is > needed in this case? I thought that if write happens before we decide > whether to schedule event_work or not it would be enough. Yes, but how we decide whether to schedule or not? Let's suppose we do this without mb(). say, queue_work() starts with if (test_bit(WORK_STRUCT_PENDING)) // no barrier semantics return; In that case the code in atkbd_schedule_event_work() set_bit(event_bit, &atkbd->event_mask); wmb(); schedule_delayed_work(atkbd->event_work); can be reordered (if ->event_work is queued) as schedule_delayed_work(atkbd->event_work); set_bit(event_bit, &atkbd->event_mask); wmb() can only serialize STOREs, not STORE vs LOAD. The result of set_bit() can be "delayed". Now, run_workqueue() does // again, no barrier semantics, but this doesn't matter clear_bit(WORK_STRUCT_PENDING); call atkbd_schedule_event_work() if (test_and_clear_bit(atkbd->event_mask)) atkbd_set_xxx(); and we can miss an event. > > And unneeded because queue_work() implies mb(), > > but this is not really documented. > > It would be great if we can get it documented and then i'd drop *mb() > from atkbd. It is not easy document the current behaviour. Actually, perhaps run_workqueue() needs smp_mb__after_clear_bit()... But for this particular case this doesn't matter. Note that atkbd_event_work() does test_and_clear_bit(), it can't be re-ordered with clear_bit(WORK_STRUCT_PENDING), otherwise even mb() can't help. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-input" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
