linux-kernel - Re: [PATCH] make cancel_rearming_delayed

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Message-ID: <20070515220024.GA615@tv-sign.ru>
Date:	Wed, 16 May 2007 02:00:24 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Tejun Heo <htejun@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Chinner <dgc@....com>,
	David Howells <dhowells@...hat.com>,
	Gautham Shenoy <ego@...ibm.com>,
	Jarek Poplawski <jarkao2@...pl>, Ingo Molnar <mingo@...e.hu>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [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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