On 09/01, Boqun Feng wrote: > > But I'm still a little confused at Oleg's words: > > "What is really important is that we have a barrier before we _read_ the > task state." > > I read is as "What is really important is that we have a barrier before > we _read_ the task state and _after_ we write the CONDITION", if I don't > misunderstand Oleg, this means a STORE-barrier-LOAD sequence,
Yes, exactly. Let's look at this trivial code again, CONDITION = 1; wake_up_process(); note that try_to_wake_up() does if (!(p->state & state)) goto out; If this LOAD could be reordered with STORE(CONDITION) above we can obviously race with set_current_state(...); if (!CONDITION) schedule(); See the comment at the start of try_to_wake_up(). And again, again, please note that initially the only documented behaviour of smp_mb__before_spinlock() was the STORE - LOAD serialization. This is what try_to_wake_up() needs, it doesn't actually need the write barrier after STORE(CONDITION). And just in case, wake_up() differs in a sense that it doesn't even need that STORE-LOAD barrier in try_to_wake_up(), we can rely on wait_queue_head_t->lock. Assuming that wake_up() pairs with the "normal" wait_event()-like code. > which IIUC > can't pair with anything. It pairs with the barrier implied by set_current_state(). Oleg. -- 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/