linux-kernel - Re: [PATCH v3 00/22] PCI: Iterate pci host bridge instead of pci root bus

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Date:	Sat, 2 Feb 2013 21:05:53 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Jiang Liu <jiang.liu@...wei.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Taku Izumi <izumi.taku@...fujitsu.com>,
	Toshi Kani <toshi.kani@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-pci@...r.kernel.org, Russell King <linux@....linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 00/22] PCI: Iterate pci host bridge instead of pci root bus

[Trim down CC list, otherwise lkml will drop it, Thanks for David
Miller to bring attention to me ]

On Sat, Feb 2, 2013 at 1:50 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> On Sun, Jan 27, 2013 at 12:23 PM, Yinghai Lu <yinghai@...nel.org> wrote:
>> Now we have pci_root_buses list, and there is lots of iteration with
>> list_of_each of it, that is not safe after we add pci root bus hotplug
>> support after booting stage.
>>
>> Add pci_get_next_host_bridge and use bus_find_device in driver core to
>> iterate host bridge and the same time get root bus.
>>
>> We replace searching root bus with searching host_bridge,
>> as host_bridge->bus is the root bus.
>> After those replacing, we even could kill pci_root_buses list.
>
> These are the problems I think you're fixing:
>
> 1) pci_find_next_bus() is not safe because even though it holds
> pci_bus_sem while walking the pci_root_buses list, it doesn't hold a
> reference on the bus it returns.  The bus could be removed while the
> caller is using it.
>
> 2) "list_for_each_entry(bus, &pci_root_buses, node)" is not safe
> because hotplug might modify the pci_root_buses list.  Replacing that
> with for_each_pci_host_bridge() solves that problem by using
> bus_find_device(), which is built on klists, which are designed for
> that problem.
>
> 3) pci_find_next_bus() claims to iterate through all known PCI buses,
> but in fact only iterates through root buses.
>
> So far, so good.  Those are problems we need to fix.
>
> Your solution is to introduce for_each_pci_host_bridge() as an
> iterator through the known host bridges.  There are two scenarios
> where we use something like this:
>
> 1) We want to perform an operation on every known host bridge.
>
> 2) We want to initialize something for every host bridge.
>
> In my opinion, the only instance of scenario 1) is bus_rescan_store(),
> where we want to rescan all PCI host bridges.
>
> In every other case, we're doing some kind of initialization of all
> the host bridges.  For these cases, for_each_pci_host_bridge() is the
> wrong solution because it doesn't work for hot-added bridges.  I think
> these cases should be changed to use pcibios_root_bridge_prepare() or
> something something else called in the host bridge add path.

Yes, will check if those for_pci_host_bridge could be converted to
adding calling in pcibios_root_bridge_prepare one by one.

Thanks

Yinghai
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