On 05/13, Paul E. McKenney wrote: > > On Tue, May 13, 2014 at 04:17:48PM +0200, Peter Zijlstra wrote: > > > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > > index 46412bded104..dae5158c2382 100644 > > --- a/Documentation/memory-barriers.txt > > +++ b/Documentation/memory-barriers.txt > > @@ -1881,9 +1881,9 @@ The whole sequence above is available in various canned forms, all of which > > event_indicated = 1; > > wake_up_process(event_daemon); > > > > -A write memory barrier is implied by wake_up() and co. if and only if they wake > > -something up. The barrier occurs before the task state is cleared, and so sits > > -between the STORE to indicate the event and the STORE to set TASK_RUNNING: > > +A full memory barrier is implied by wake_up() and co. The barrier occurs > > Last I checked, the memory barrier was guaranteed 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)). > 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? And "if a wakeup actually occurred" is not clear to me too in this context. For example, suppose that ttwu() clears task->state but that task was not deactivated and it is going to check the condition, do we count this as "wakeup actually occurred" ? In this case that task still can see Y==0. > 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? > 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? > The write barrier occurs before the task state is > cleared, and so sits between the STORE to indicate the event and > the STORE to set TASK_RUNNING, and the read barrier after that: Plus: between the STORE to indicate the event and the LOAD which checks task->state, otherwise: > CPU 1 CPU 2 > =============================== =============================== > set_current_state(); STORE event_indicated > set_mb(); wake_up(); > STORE current->state <write barrier> > <general barrier> STORE current->state > LOAD event_indicated <read barrier> this code is still racy. In short: I am totally confused and most probably misunderstood you ;) Oleg. -- 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
