linux-kernel - Re: [RFC PATCH 2/6] pci: Set pci_dev->is_added before calling device

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Message-ID: <20180817162534.GD128050@bhelgaas-glaptop.roam.corp.google.com>
Date:   Fri, 17 Aug 2018 11:25:34 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     linux-pci@...r.kernel.org, Hari Vyas <hari.vyas@...adcom.com>,
        Ray Jui <ray.jui@...adcom.com>,
        Srinath Mannam <srinath.mannam@...adcom.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Jens Axboe <axboe@...nel.dk>, Lukas Wunner <lukas@...ner.de>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Marta Rybczynska <mrybczyn@...ray.eu>,
        Pierre-Yves Kerbrat <pkerbrat@...ray.eu>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/6] pci: Set pci_dev->is_added before calling
 device_add

On Fri, Aug 17, 2018 at 02:48:58PM +1000, Benjamin Herrenschmidt wrote:
> This re-fixes the bug reported by Hari Vyas <hari.vyas@...adcom.com>
> after my revert of his commit but in a much simpler way.
> 
> The main issues is that is_added was being set after the driver
> got bound and started, and thus setting it could race with other
> changes to struct pci_dev.

The "bind driver, then set dev->added = 1" order seems to have been
there since the beginning of dev->is_added:

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=8a1bc9013a03

This patch seems reasonable, but I'm a little dubious about the
existence of "is_added" in the first place.  As far as I can tell, the
only other buses with something similar are the MEN Chameleon bus and
the Intel Management Engine Interface.

The PCI uses of "is_added" don't seem *that* critical or unique to
PCI, so I'm not 100% convinced we need it at all.  But I haven't
looked into it enough to be able to propose an alternative.

> This fixes it by setting the flag first, which also has the
> advantage of matching the fact that we are clearing it *after*
> unbinding in the remove path, thus the flag is now symtetric
> and always set while the driver code is running.
> 
> Signed-off-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> ---
>  drivers/pci/bus.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c
> index 35b7fc87eac5..48ae63673aa8 100644
> --- a/drivers/pci/bus.c
> +++ b/drivers/pci/bus.c
> @@ -321,16 +321,16 @@ void pci_bus_add_device(struct pci_dev *dev)
>  	pci_proc_attach_device(dev);
>  	pci_bridge_d3_update(dev);
>  
> +	dev->is_added = 1;
>  	dev->match_driver = true;
>  	retval = device_attach(&dev->dev);
>  	if (retval < 0 && retval != -EPROBE_DEFER) {
> +		dev->is_added = 0;
>  		pci_warn(dev, "device attach failed (%d)\n", retval);
>  		pci_proc_detach_device(dev);
>  		pci_remove_sysfs_dev_files(dev);
>  		return;
>  	}
> -
> -	dev->is_added = 1;
>  }
>  EXPORT_SYMBOL_GPL(pci_bus_add_device);
>  
> -- 
> 2.17.1
>