linux-kernel - Re: [PATCH 1/2] mtd: use put_device() if device

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Message-ID: <20180314153600.458c0fe4@bbrezillon>
Date:   Wed, 14 Mar 2018 15:36:00 +0100
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     Arvind Yadav <arvind.yadav.cs@...il.com>
Cc:     dwmw2@...radead.org, computersforpeace@...il.com,
        boris.brezillon@...e-electrons.com, marek.vasut@...il.com,
        richard@....at, cyrille.pitchen@...ev4u.fr, dedekind1@...il.com,
        linux-kernel@...r.kernel.org, linux-mtd@...ts.infradead.org
Subject: Re: [PATCH 1/2] mtd: use put_device() if device_register fail

On Fri,  9 Mar 2018 16:20:48 +0530
Arvind Yadav <arvind.yadav.cs@...il.com> wrote:

> if device_register() returned an error! Always use put_device()
> to give up the reference initialized.
> 
> Signed-off-by: Arvind Yadav <arvind.yadav.cs@...il.com>
> ---
>  drivers/mtd/mtdcore.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
> index 28553c8..4d77ca2 100644
> --- a/drivers/mtd/mtdcore.c
> +++ b/drivers/mtd/mtdcore.c
> @@ -586,6 +586,7 @@ int add_mtd_device(struct mtd_info *mtd)
>  	return 0;
>  
>  fail_added:
> +	put_device(&mtd->dev);

Not sure this is a good idea: the put_device() call will trigger
an mtd_devtype->release(), which will in turn call device_destroy() on
something that does not exist yet. Not sure if this is a real problem,
but it does not look like the right thing to do.

>  	of_node_put(mtd_get_of_node(mtd));

You're referencing an object that is supposed to have been
freed/released by the put_device() call. Again, it's not really a
problem because in our case ->release() does not free the mtd object
(as is usually done in other parts of the kernel), but it still looks
wrong. It's probably better to move the of_node_put() and the below
idr_remove() call in the ->release() hook if you want to use
put_device().

>  	idr_remove(&mtd_idr, i);

>  fail_locked:

-- 
Boris Brezillon, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com