linux-kernel - Re: [PATCH] PCI: Do not touch siblings in pci_assign_unassigned_bridge

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Date:	Tue, 17 Jun 2014 16:16:20 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Andreas Noever <andreas.noever@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	yinghai@...nel.org
Subject: Re: [PATCH] PCI: Do not touch siblings in
 pci_assign_unassigned_bridge_resources

[+cc Yinghai]

On Mon, Jun 09, 2014 at 10:45:30PM +0200, Andreas Noever wrote:
> pci_assign_unassigned_bridge_resources is used to assign resources below
> a hotplug bridge. If the first attempt fails it will release some
> resources and try again. If a resource allocation on the hotplug bridge
> itself fails then pci_assign_unassigned_bridge_resources will invoke
> pci_bus_release_bridge_resources on the parent bus of the hotplug
> bridge. This in turn will release resources assigned to siblings of the
> hotplug bridge which may already be in use.
> 
> This patch checks for this case and prevents
> pci_bus_release_bridge_resources to be invoked on the parent bus.
> 
> The problem can be reproduced by having two sibling hotplug bridges A
> and B. The problem will occour if the parent of A and B does not have
> enough resources to satisfy window allocations for B during a hotplug
> event.
> 
> Signed-off-by: Andreas Noever <andreas.noever@...il.com>
> ---
> 
> I must admit that I do not fully understand how
> pci_assign_unassigned_bridge_resources works. Under which scenario would the
> second allocation attempt be successful?

I don't understand how all this works either.  Yinghai?

We definitely don't want to release resources that are already in use.  Can
you review and ack or nack this?

>  drivers/pci/setup-bus.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> index 138bdd6..2e418d6 100644
> --- a/drivers/pci/setup-bus.c
> +++ b/drivers/pci/setup-bus.c
> @@ -1560,6 +1560,12 @@ void __init pci_assign_unassigned_resources(void)
>  		pci_assign_unassigned_root_bus_resources(root_bus);
>  }
>  
> +/**
> + * pci_assign_unassigned_bridge_resources - greenfield resource allocation
> + *
> + * Try to assign io and memory resources on and below @bridge. The caller must
> + * ensure that no device below @bridge is active.
> + */
>  void pci_assign_unassigned_bridge_resources(struct pci_dev *bridge)
>  {
>  	struct pci_bus *parent = bridge->subordinate;
> @@ -1567,7 +1573,7 @@ void pci_assign_unassigned_bridge_resources(struct pci_dev *bridge)
>  					want additional resources */
>  	int tried_times = 0;
>  	LIST_HEAD(fail_head);
> -	struct pci_dev_resource *fail_res;
> +	struct pci_dev_resource *fail_res, *tmp;
>  	int retval;
>  	unsigned long type_mask = IORESOURCE_IO | IORESOURCE_MEM |
>  				  IORESOURCE_PREFETCH;
> @@ -1594,10 +1600,21 @@ again:
>  	 * Try to release leaf bridge's resources that doesn't fit resource of
>  	 * child device under that bridge
>  	 */
> -	list_for_each_entry(fail_res, &fail_head, list)
> +	list_for_each_entry_safe(fail_res, tmp, &fail_head, list) {
> +		 /*
> +		  * The allocation of the mem/io window of the top level bridge
> +		  * can fail.  Without the following check we would release our
> +		  * siblings' resources.
> +		  */
> +		if (fail_res->dev == bridge) {
> +			list_del(&fail_res->list);
> +			kfree(fail_res);
> +			continue;
> +		}
>  		pci_bus_release_bridge_resources(fail_res->dev->bus,
>  						 fail_res->flags & type_mask,
>  						 whole_subtree);
> +	}
>  
>  	/* restore size and flags */
>  	list_for_each_entry(fail_res, &fail_head, list) {
> -- 
> 2.0.0
> 
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