Re: [PATCH] make cancel_rearming_delayed_work() reliable

[Oleg Nesterov <oleg@tv-sign.ru>]

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.