linux-kernel - Re: [PATCH v2 06/10] driver core: Avoid endless recursion if device has more than one link

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Date:   Wed, 07 Sep 2016 01:09:36 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org,
        linux-samsung-soc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Joerg Roedel <joro@...tes.org>,
        Inki Dae <inki.dae@...sung.com>, Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <k.kozlowski@...sung.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2 06/10] driver core: Avoid endless recursion if device has more than one link

On Friday, June 17, 2016 08:26:56 AM Marek Szyprowski wrote:
> This patch fixes endless recursion, which happends when device has
> more than one link.
> 
> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> ---
>  drivers/base/core.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 215cd44de761..4e778539b750 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -57,7 +57,8 @@ static int device_reorder_to_tail(struct device *dev, void *not_used)
>  	device_pm_move_last(dev);
>  	device_for_each_child(dev, NULL, device_reorder_to_tail);
>  	list_for_each_entry(link, &dev->consumer_links, c_node)
> -		device_reorder_to_tail(link->consumer, NULL);
> +		if (link->consumer != dev)
> +			device_reorder_to_tail(link->consumer, NULL);
>  
>  	return 0;
>  }
> 

If I'm not mistaken, this should not be necessary unless dev has a link
pointing to itself as a consumer.  That would be a bug, though.

I can add a WARN_ON() to catch this case, but then if there's a link from
a consumer of dev pointing back to dev as a consumer, that still will loop
forever.

I guess we need to detect circular dependencies and fail link creation in
such cases.  Oh well.

Thanks,
Rafael