linux-kernel - Re: [PATCH 20/43] workqueue: reimplement work flushing using linked works

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Message-ID: <20100301185153.GA18607@redhat.com>
Date:	Mon, 1 Mar 2010 19:51:53 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	torvalds@...ux-foundation.org, mingo@...e.hu, peterz@...radead.org,
	awalls@...ix.net, linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com, avi@...hat.com,
	johannes@...solutions.net, andi@...stfloor.org
Subject: Re: [PATCH 20/43] workqueue: reimplement work flushing using linked
	works

On 03/02, Tejun Heo wrote:
>
> On 03/01/2010 11:53 PM, Oleg Nesterov wrote:
> >
> > and in this case we put this barrier at the head of ->scheduled list.
> >
> > This means, this barrier will run after that work W, not before it?
>
> Yes, the barrier will run after the target work as it should.

You are right. it will run after target work == current_work and before
the next pending work W.

Because,

> > Hmm. And what if there are no pending works but ->current_work == target ?
> > Again, we add the barrier to ->scheduled, but in this case worker_thread()
> > can't even notice ->scheduled is not empty because it only checks ->worklist?
>
> A worker always checks ->scheduled after a work is finished.

Yes! I missed this, thanks.



> > insert_wq_barrier() also does:
> >
> > 	unsigned long *bits = work_data_bits(target);
> > 	...
> > 	*bits |= WORK_STRUCT_LINKED;
> >
> > perhaps this needs atomic_long_set(), although I am not sure this really
> > matters.
>
> Yeah, well, work->data access is pretty messed up.  At this point,
> there's no reason for atomic_long_t to begin with.

grep, grep, grep... arch/sparc/lib/atomic32.c uses spinlocks for
atomic_set() and ___set_bit(). Probably that is why atomic_long_set()
is really needed to avoid the race with test_and_set_bit(PENDING).

Oleg.

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