On Tue, May 13, 2014 at 09:31:46PM +0200, Oleg Nesterov wrote: > On 05/13, Paul E. McKenney wrote: > > > > On Tue, May 13, 2014 at 08:18:52PM +0200, Oleg Nesterov wrote: > > > > > > I have to admit, I am confused. I simply do not understand what "memory > > > barrier" actually means in this discussion. > > > > > > To me, wake_up/ttwu should only guarantee one thing: all the preceding > > > STORE's should be serialized with all the subsequent manipulations with > > > task->state (even with LOAD(task->state)). > > > > I was thinking in terms of "everything done before the wake_up() is > > visible after the wait_event*() returns" -- but only if the task doing > > the wait_event*() actually sleeps and is awakened by that particular > > wake_up(). > > Hmm. The question is, visible to whom ;) To the woken task? > > Yes sure, and this is simply because both sleeper/waker take rq->lock. Yep, that was the thought. > > > > If there is a sleep-wakeup race, for example, > > > > between wait_event_interruptible() and wake_up(), then it looks to me > > > > that the following can happen: > > > > > > > > o Task A invokes wait_event_interruptible(), waiting for > > > > X==1. > > > > > > > > o Before Task A gets anywhere, Task B sets Y=1, does > > > > smp_mb(), then sets X=1. > > > > > > > > o Task B invokes wake_up(), which invokes __wake_up(), which > > > > acquires the wait_queue_head_t's lock and invokes > > > > __wake_up_common(), which sees nothing to wake up. > > > > > > > > o Task A tests the condition, finds X==1, and returns without > > > > locks, memory barriers, atomic instructions, or anything else > > > > that would guarantee ordering. > > > > > > > > o Task A then loads from Y. Because there have been no memory > > > > barriers, it might well see Y==0. > > > > > > Sure, but I can't understand "Because there have been no memory barriers". > > > > > > IOW. Suppose we add mb() into wake_up(). The same can happen anyway? > > > > If the mb() is placed just after the fastpath condition check, then the > > awakened task will be guaranteed to see Y=1. > > Of course. My point was, this has nothing to do with the barriers provided > by wake_up(), that is why I was confused. > > > > > On the other hand, if a wake_up() really does happen, then > > > > the fast-path out of wait_event_interruptible() is not taken, > > > > and __wait_event_interruptible() is called instead. This calls > > > > ___wait_event(), which eventually calls prepare_to_wait_event(), which > > > > in turn calls set_current_state(), which calls set_mb(), which does a > > > > full memory barrier. > > > > > > Can't understand this part too... OK, and suppose that right after that > > > the task B from the scenario above does > > > > > > Y = 1; > > > mb(); > > > X = 1; > > > wake_up(); > > > > > > After that task A checks the condition, sees X==1, and returns from > > > wait_event() without spin_lock(wait_queue_head_t->lock) (if it also > > > sees list_empty_careful() == T). Then it can see Y==0 again? > > > > Yes. You need the barriers to be paired, and in this case, Task A isn't > > executing a memory barrier. Yes, the mb() has forced Task B's CPU to > > commit the writes in order (or at least pretend to), but Task A might > > have speculated the read to Y. > > > > Or am I missing your point? > > I only meant that this case doesn't really differ from the scenario you > described above. Indeed, I was taking a bit of an exploratory approach to this. > > > > A read and a write memory barrier (-not- a full memory barrier) > > > > are implied by wake_up() and co. if and only if they wake > > > > something up. > > > > > > Now this looks as if you document that, say, > > > > > > X = 1; > > > wake_up(); > > > Y = 1; > > > > > > doesn't need wmb() before "Y = 1" if wake_up() wakes something up. Do we > > > really want to document this? Is it fine to rely on this guarantee? > > > > That is an excellent question. It would not be hard to argue that we > > should either make the guarantee unconditional by adding smp_mb() to > > the wait_event*() paths or alternatively just saying that there isn't > > a guarantee to begin with. > > I'd vote for "no guarantees". I would have no objections to that. Other than the large number of those things in the kernel! The thing is that I am having a hard time imagining how you guarantee that a wakeup actually happened. I am betting that there are a lot of bugs related to this weak guarantee... > > > In short: I am totally confused and most probably misunderstood you ;) > > > > Oleg, if it confuses you, it is in desperate need of help! ;-) > > Thanks, this helped ;) Glad to help! ;-) Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
