linux-kernel - Re: [PATCH 3/5] writeback: bdi_writeback iteration must not skip dying ones

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Date:	Fri, 2 Oct 2015 14:19:10 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Jan Kara <jack@...e.cz>
Cc:	axboe@...nel.dk, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, tytso@....edu, dedekind1@...il.com,
	decui@...rosoft.com, kernel-team@...com
Subject: Re: [PATCH 3/5] writeback: bdi_writeback iteration must not skip
 dying ones

On Fri, Oct 02, 2015 at 04:12:12PM +0200, Jan Kara wrote:
> > +		if (last_wb) {
> > +			wb_put(last_wb);
> > +			last_wb = NULL;
> > +		}
> 
> But you seem to forget to drop last_wb reference in case this was the last
> wb in the list, don't you?

You're right.  Will update the patch.

> > @@ -686,6 +691,9 @@ static void cgwb_bdi_destroy(struct backing_dev_info *bdi)
> >  	radix_tree_for_each_slot(slot, &bdi->cgwb_tree, &iter, 0)
> >  		cgwb_kill(*slot);
> >  
> > +	/* wb may get released after @bdi is freed, sever list head */
> > +	list_del(&bdi->wb_list);
> > +
> 
> But we wait for bdi->usage_cnt to drop to 0 which means there's no wb,
> don't we? What am I missing?

And I forgot that we were doing that.  This isn't necessary.

> > @@ -764,15 +772,22 @@ static void cgwb_bdi_destroy(struct backing_dev_info *bdi) { }
> >  
> >  int bdi_init(struct backing_dev_info *bdi)
> >  {
> > +	int ret;
> > +
> >  	bdi->dev = NULL;
> >  
> >  	bdi->min_ratio = 0;
> >  	bdi->max_ratio = 100;
> >  	bdi->max_prop_frac = FPROP_FRAC_BASE;
> >  	INIT_LIST_HEAD(&bdi->bdi_list);
> > +	INIT_LIST_HEAD(&bdi->wb_list);
> >  	init_waitqueue_head(&bdi->wb_waitq);
> >  
> > -	return cgwb_bdi_init(bdi);
> > +	ret = cgwb_bdi_init(bdi);
> > +
> > +	list_add_tail_rcu(&bdi->wb.bdi_node, &bdi->wb_list);
> 
> Won't this be more logical in cgwb_bdi_init()?

bdi->wb_list exists whether cgwb is enabled or not, so if we move this
to cgwb_bdi_init(), we'll be duplicating it in both paths.

Thanks.

-- 
tejun
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