linux-kernel - Re: [PATCH 5/5] OF: Introduce utility helper functions

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Date:	Thu, 14 Nov 2013 10:44:20 +0900
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Pantelis Antoniou <panto@...oniou-consulting.com>
Cc:	Rob Herring <robherring2@...il.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Matt Porter <matt.porter@...aro.org>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Alison Chaiken <Alison_Chaiken@...tor.com>,
	Dinh Nguyen <dinh.linux@...il.com>,
	Jan Lubbe <jluebbe@...net.de>,
	Alexander Sverdlin <alexander.sverdlin@....com>,
	Michael Stickel <ms@...able.de>,
	Guenter Roeck <linux@...ck-us.net>,
	Dirk Behme <dirk.behme@...il.com>,
	Alan Tull <delicious.quinoa@...il.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Michael Bohan <mbohan@...eaurora.org>,
	Ionut Nicu <ioan.nicu.ext@....com>,
	Michal Simek <monstr@...str.eu>,
	Matt Ranostay <mranostay@...il.com>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] OF: Introduce utility helper functions

On Wed, 13 Nov 2013 10:03:37 +0100, Pantelis Antoniou <panto@...oniou-consulting.com> wrote:
> On Nov 13, 2013, at 2:34 AM, Grant Likely wrote:
> > On Tue, 12 Nov 2013 11:39:08 +0100, Pantelis Antoniou <panto@...oniou-consulting.com> wrote:
> >>> On Tue,  5 Nov 2013 19:50:16 +0200, Pantelis Antoniou <panto@...oniou-consulting.com> wrote:
> >>>> +	} else {
> >>>> +		pr_warn("%s: node %p cannot be freed; memory is gone\n",
> >>>> +				__func__, node);
> >>>> +	}
> >>>> +}
> >>> 
> >>> All of the above is potentially dangerous. There is no way to determine
> >>> if anything still holds a reference to a node. The proper way to handle
> >>> removal of properties is to have a release method when the last
> >>> of_node_put is called.
> >>> 
> >> 
> >> This is safe, and expected to be called only on a dynamically created tree,
> >> that's what all the checks against OF_DYNAMIC guard against.
> >> 
> >> It is not ever meant to be called on an arbitrary tree, created by unflattening
> >> a blob.
> > 
> > I am talking about when being used on a dynamic tree. The problem is
> > when a driver or other code holds a reference to a dynamic nodes, but
> > doesn't release it correctly. The memory must not be freed until all of
> > the references are relased. OF_DYNAMIC doesn't actually help in that
> > case, and it is the reason for of_node_get()/of_node_put()
> > 
> 
> I know, but even that is not enough. of_node_get()/of_node_put() handles the
> case of references to the nodes, but not what happens with references to
> properties. deadprops is mitigating the problem somewhat, but if we're going
> to go to all the trouble of kobjectification let's do the props as well.
> 
> of_get_property could be modified to return a devm_kmalloced copy of the real
> property and that would deal with most of the callers. Of course for
> the small sized scalar data we can avoid the copy.
> 
> By using the devm_* interface we also avoid having to mess too much with the callers.
> 
> I.e. what about something like devm_of_get_property()?

Reference counting is already a horrible pain to keep correct. I don't
see a better way to handle it in the dynamic case, so we're stuck with
it, but I don't want to make it any harder. Adding ref counting to
properties will make it harder than it already is to get the code right.
I'm absolutely fine with a little bit of wasted memory in the form of
deadprops when the alternative is so horrible. References at the node
level is enough granularity.

I don't think kduping the property is the solution either. I strongly
suspect that will be far more expensive than the deadprop solution.

> >>>> +{
> >>>> +	struct device_node *node;
> >>>> +
> >>>> +	node = kzalloc(sizeof(*node), flags);
> >>>> +	if (node == NULL)
> >>>> +		return NULL;
> >>>> +
> >>>> +	node->name = kstrdup(name, flags);
> >>>> +	if (node->name == NULL)
> >>>> +		goto err_return;
> >>>> +
> >>>> +	node->type = kstrdup(type, flags);
> >>>> +	if (node->type == NULL)
> >>>> +		goto err_return;
> >>>> +
> >>>> +	node->full_name = kstrdup(full_name, flags);
> >>>> +	if (node->type == NULL)
> >>>> +		goto err_return;
> >>> 
> >>> Again, who do you expect the user of this function to be? If it is part
> >>> of unflattening an overlay tree, is there a reason that the passed in
> >>> names cannot be used directly instead of kmallocing them?
> >>> 
> >> 
> >> I want to be able to get rid of the blob eventually; I don't need to keep
> >> dragging it around.
> > 
> > Why? It really doesn't hurt and it means data does not need to be
> > copied.
> 
> Copying data lead to less problems that having to drag that blob around. 
> That's just preference, so not a big issue.

Can you elaborate? What problems do you foresee being created by keeping
the blob?

g.
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