Messages in this thread | | | Date | Wed, 16 May 2007 02:00:24 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH] make cancel_rearming_delayed_work() reliable |
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On 05/15, Tejun Heo wrote: > > Oleg Nesterov wrote: > > > 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 don't think the test for PENDING should be atomic too. cwq pointer > and VALID is one package. PENDING lives its own life as a atomic bit > switch.
Yes sure, it should not be atomic. But (VALID && !PENDING) == BUG, so we can't just "kill" PENDING form the check above.
> > BTW, in _theory_, spinlock() is not a read barrier, yes? > > It actually is. > > > From Documentation/memory-barriers.txt > > > > Memory operations that occur before a LOCK operation may appear to happen > > after it completes. > > Which means that spin_lock() isn't a write barrier.
yes, it is not very clear which "Memory operations" memory-barriers.txt describes.
> lock is read > barrier, unlock is write barrier.
(To avoid a possible confusion: I am not arguing, I am trying to understand, and please also note "in _theory_" above)
Is it so? Shoudn't we document this if it is true?
> Otherwise, locking doesn't make much > sense.
Why? Could you please give a code example we have which relies on this?
> If we're going the barrier way, I think we're better off with > explicit smp_wmb(). It's only barrier() on x86/64.
Yes. But note that we don't have any reason to do set_wq_data() under cwq->lock (this is also true for wake_up(->more_work) btw), so it makes sense to do this change anyway. And "wmb + spin_lock" looks a bit strange, I _suspect_ spin_lock() means a full barrier on most platforms.
Could you also look at http://marc.info/?t=116275561700001&r=1
and, in particular, http://marc.info/?l=linux-kernel&m=116281136122456
Thanks!
Oleg.
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