Messages in this thread | | | Date | Thu, 2 Oct 2014 23:21:08 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 02/11] wait: Provide infrastructure to deal with nested blocking |
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On 10/02, Peter Zijlstra wrote: > > On Mon, Sep 29, 2014 at 11:02:21PM +0200, Oleg Nesterov wrote: > > On 09/24, Peter Zijlstra wrote: > > > > > > +int woken_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key) > > > +{ > > > + /* > > > + * Although this function is called under waitqueue lock, LOCK > > > + * doesn't imply write barrier and the users expects write > > > + * barrier semantics on wakeup functions. The following > > > + * smp_wmb() is equivalent to smp_wmb() in try_to_wake_up() > > > + * and is paired with set_mb() in wait_woken(). > > > + */ > > > + smp_wmb(); /* C */ > > > + wait->flags |= WQ_FLAG_WOKEN; > > > > Perhaps it is just me, but I was a bit confused by the comment above wmb(). > > Afaics, it is not that "users expects write barrier semantics", just we > > need to ensure that > > > > CONDITION = true; > > wait->flags |= WQ_FLAG_WOKEN; > > > > can't be reordered (and this differs from smp_wmb() in try_to_wake_up()). > > Otherwise we can obviously race with > > > > // wait_woken() -> set_mb() > > wait->flags &= ~WQ_FLAG_WOKEN; > > mb(); > > > > if (CONDITION) > > break; > > > > Yes, that comment could be clearer. It is however, to me, the 'same' as > a regular wakeup in that we need to separate whatever state changes > before the wakeup (CONDITION=true typically) from whatever writes are > required to affect the wakeup (->state=TASK_RUNNING typically,
Not really, ttwu() needs to serialize CONDITION=true and the reading of task->state. And for the waiter its state is write only, it doesn't need to check it.
While in this case we need to separate CONDITION and WQ_FLAG_WOKEN, and the waiter needs to check them in the right order.
But please forget, the code looks clear with or without the comment, and "paired with set_mb() in wait_woken()" should explain the intent anyway.
Oleg.
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