Messages in this thread | | | Date | Mon, 7 Sep 2015 19:06:52 +0200 | From | Oleg Nesterov <> | Subject | Re: wake_up_process implied memory barrier clarification |
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Sorry for delay,
On 09/02, Boqun Feng wrote: > > On Tue, Sep 01, 2015 at 06:39:23PM +0200, Oleg Nesterov wrote: > > On 09/01, Boqun Feng wrote: > > > > > > On Tue, Sep 01, 2015 at 11:59:23AM +0200, Oleg Nesterov wrote: > > > > > > > > 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. > > > > Looks like, you have missed this part of my previous email. See below. > > I guess I need to think through this, though I haven't found any problem > in wake_up() if we remove the STORE-LOAD barrier in try_to_wake_up(). > And I know that in wake_up(), try_to_wake_up() will be called with > holding wait_queue_head_t->lock, however, only part of wait_event() > holds the same lock, I can't figure out why the barrier is not needed > because of the lock..
This is very simple. __wait_event() does
for (;;) { prepare_to_wait_event(WQ, ...); // takes WQ->lock
if (CONDITION) break;
schedule(); }
and we have
CONDITION = 1; wake_up(WQ); // takes WQ->lock
on another side.
Suppose that __wait_event() wins and takes WQ->lock first. It can block then. In this case wake_up() must see the result of set_current_state() and list_add() when it takes the same lock, otherwise spin_lock() would be simply buggy. So it will wake the waiter up.
At the same time, if __wait_event() takes this lock after wake_up(), it can not miss CONDITION = 1. It must see it after it takes the lock, and of course after it drops the lock too.
So I am not sure I understand your concerns in this case...
Oleg.
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