linux-kernel - Re: [PATCH linux-next] PCI: Fix the order in unregister path

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Date:   Tue, 31 Aug 2021 02:41:42 +0000
From:   yajun.deng@...ux.dev
To:     "Rob Herring" <robh@...nel.org>
Cc:     "Bjorn Helgaas" <bhelgaas@...gle.com>,
        "Arnd Bergmann" <arnd@...db.de>,
        "Lorenzo Pieralisi" <lorenzo.pieralisi@....com>,
        "PCI" <linux-pci@...r.kernel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH linux-next] PCI: Fix the order in unregister path

August 30, 2021 10:55 PM, "Rob Herring" <robh@...nel.org> wrote:

> On Thu, Aug 26, 2021 at 9:39 PM <yajun.deng@...ux.dev> wrote:
> 
>> August 26, 2021 8:01 PM, "Rob Herring" <robh@...nel.org> wrote:
>> 
>> On Wed, Aug 25, 2021 at 10:57 PM <yajun.deng@...ux.dev> wrote:
>> 
>> August 25, 2021 9:55 PM, "Rob Herring" <robh@...nel.org> wrote:
>> 
>> On Wed, Aug 25, 2021 at 3:34 AM Yajun Deng <yajun.deng@...ux.dev> wrote:
>> 
>> device_del() should be called first and then called put_device() in
>> unregister path, becase if that the final reference count, the device
>> will be cleaned up via device_release() above. So use device_unregister()
>> instead.
>> 
>> Fixes: 9885440b16b8 (PCI: Fix pci_host_bridge struct device release/free handling)
>> Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
>> ---
>> drivers/pci/probe.c | 4 +---
>> 1 file changed, 1 insertion(+), 3 deletions(-)
>> 
>> NAK.
>> 
>> The current code is correct. Go read the comments for device_add/device_del.
>> 
>> But the device_unregister() is only contains device_del() and put_device(). It just put
>> device_del() before put_device().
>> 
>> And that is the wrong order as we want to undo what the code above
>> did. The put_device here is for the get_device we did. The put_device
>> in device_unregister is for the get_device that device_register did
>> (on success only).
>> 
>> Logically, it is wrong too to call unregister if register failed. That
>> would be like doing this:
> 
> You are right that the register and unregister are different devices.
> However, your change is still wrong. The device_register is actually
> irrelevant.
> 
OK, the original order is right, it was my mistake.

>> p = malloc(1);
>> if (!p)
>> free(p);
>> 
>> This is the raw code:
>> err = device_register(&bus->dev);
>> if (err)
>> goto unregister;
>> unregister:
>> put_device(&bridge->dev);
>> device_del(&bridge->dev);
> 
> The pertinent parts are this:
> 
> err = device_add(&bridge->dev); // which calls get_device() itself,
> so there's the first ref
> if (err) {
> put_device(&bridge->dev);
> goto free;
> }
> bus->bridge = get_device(&bridge->dev); // This is the 2nd ref which
> the PCI core holds
> ...
> unregister:
> put_device(&bridge->dev); // This is the put for the get_device
> just above here.
> device_del(&bridge->dev); // Then this does the 2nd put.
> 
> The get_device and put_device are paired, and the device_add and
> device_del are paired.
> 
> As I said earlier, go read the kerneldoc for device_add. For your
> convenience, here's the important part:
> 
> device_add:
> * Rule of thumb is: if device_add() succeeds, you should call
> * device_del() when you want to get rid of it. If device_add() has
> * *not* succeeded, use *only* put_device() to drop the reference
> * count.
> 
> device_del:
> * NOTE: this should be called manually _iff_ device_add() was
> * also called manually.
> 
> Rob