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SubjectRe: [PATCH] make cancel_rearming_delayed_work() reliable
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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