Messages in this thread | | | Date | Mon, 14 May 2007 23:44:46 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH] make cancel_rearming_delayed_work() reliable |
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On 05/13, Tejun Heo wrote: > > Oleg Nesterov wrote: > > Heh. I thought about another bit-in-pointer too. I can't explain this, > > but I personally hate these bits even more than barriers. > > I'm the other way around but it's like saying "I like donkey poo better > than horse poo". Let's accept that we have different tastes in poos.
Yes sure :)
> >> * run_workqueue() clears the cwq pointer and VALID bit while holding > >> lock before executing the work. > > > > We should not clear cwq. I guess you meant "does list_del() and clears > > VALID bit". > > Yeap, I've used the term 'clearing' as more of a logical term - making > the pointer invalid, but is there any reason why we can't clear the > pointer itself?
Because this breaks cancel_work_sync(work), it needs get_wq_data(work) for wait_on_work(work).
> >> * try_to_grab_pending() checks VALID && pointers equal after grabbing > >> cwq->lock. > > > > We don't even need to check the pointers. VALID bit is always changed > > under cwq->lock. So, if we see VALID under cwq->lock, we have a right > > pointer. > > But there are multiple cwq->lock's. VALID can be set with one of other > cwq->lock's locked.
Yes, you are right, thanks.
So, try_to_grab_pending() should check "VALID && pointers equal" atomically. We can't do "if (VALID && cwq == get_wq_data(work))". We should do something like this
(((long)cwq) | VALID | PENDING) == atomic_long_read(&work->data)
Yes? I need to think more about this.
> I didn't really get the smp_mb__before_spinlock() thing. How are you > planning to use it? spinlock() is already a read barrier. What do you > gain by making it also a write barrier?
As I said, we can shift set_wq_data() from insert_work() to __queue_work(),
static void insert_work(struct cpu_workqueue_struct *cwq, struct work_struct *work, int tail) { if (tail) list_add_tail(&work->entry, &cwq->worklist); else list_add(&work->entry, &cwq->worklist); wake_up(&cwq->more_work); }
static void __queue_work(struct cpu_workqueue_struct *cwq, struct work_struct *work) { unsigned long flags;
set_wq_data(work, cwq);
smp_mb_before_spinlock(); spin_lock_irqsave(&cwq->lock, flags); insert_work(cwq, work, 1); spin_unlock_irqrestore(&cwq->lock, flags); }
this needs very minor changes.
BTW, in _theory_, spinlock() is not a read barrier, yes? From Documentation/memory-barriers.txt
Memory operations that occur before a LOCK operation may appear to happen after it completes.
Oleg.
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