Re: [PATCH] mtd: partitions: fix of_node_get/put balance in parser

[Boris Brezillon <boris.brezillon@xxxxxxxxxxx>]

Hi Miquel,

On Fri, 7 Sep 2018 16:38:24 +0200
Miquel Raynal <miquel.raynal@xxxxxxxxxxx> wrote:

> Hello,
> 
> I forgot to add Rafal which I know worked a lot on the parsers.
> 
> Miquel Raynal <miquel.raynal@xxxxxxxxxxx> wrote on Fri,  7 Sep 2018
> 16:35:54 +0200:
> 
> > While at first mtd_part_of_parse() would just call
> > of_get_chil_by_name(), it has been edited to first try to get the OF
> > node thanks to mtd_get_of_node() and fallback on
> > of_get_child_by_name().
> > 
> > A of_node_put() was a bit below in the code, to balance the
> > of_get_child_by_name(). However, despite its name, mtd_get_of_node()
> > does not take a reference on the OF node.

That's probably something we should patch at some point, but that
implies patching all mtd_get_of_node() users at the same time, so let's
keep that for later.

BTW, if mtd_get_of_node() was actually retaining a reference, you
would miss an of_node_put() in the !mtd_is_partition(master) case.

> > It is a simple helper hiding
> > some pointer logic to retrieve the OF node related to an MTD
> > device. People often used it this way:
> > 
> >     of_node_put(mtd_get_of_node(<mtd>)).

I don't get your point. Are you saying other places in the code are
doing the wrong thing? Should we fix them too?

> > 
> > The direct effect of such unbalanced reference counting is visible by
> > rmmod'ing any module that would have added MTD partitions:
> > 
> >     OF: ERROR: Bad of_node_put() on <of_path_to_partition>
> > 
> > As it seems normal to get a reference on the OF node during the
> > of_property_for_each_string() that follows, add a call to
> > of_node_get() when relevant.
> > 
> > Fixes: 76a832254ab0 ("mtd: partitions: use DT info for parsing partitions with "compatible" prop")
> > Cc: stable@xxxxxxxxxxxxxxx

You can drop the above line since this patch is fixing a bug introduced
in 4.19-rc1.

> > Signed-off-by: Miquel Raynal <miquel.raynal@xxxxxxxxxxx>
> > ---
> >  drivers/mtd/mtdpart.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/mtd/mtdpart.c b/drivers/mtd/mtdpart.c
> > index 52e2cb35fc79..99c460facd5e 100644
> > --- a/drivers/mtd/mtdpart.c
> > +++ b/drivers/mtd/mtdpart.c
> > @@ -873,8 +873,11 @@ static int mtd_part_of_parse(struct mtd_info *master,
> >  	int ret, err = 0;
> >  
> >  	np = mtd_get_of_node(master);
> > -	if (!mtd_is_partition(master))
> > +	if (mtd_is_partition(master))
> > +		of_node_get(np);
> > +	else
> >  		np = of_get_child_by_name(np, "partitions");
> > +
> >  	of_property_for_each_string(np, "compatible", prop, compat) {
> >  		parser = mtd_part_get_compatible_parser(compat);
> >  		if (!parser)  

The patch itself looks correct, but I'd like some clarification about
the commit message before applying it.

Thanks,

Boris

-- 
Boris Brezillon, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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