linux-kernel - Re: [PATCH v2 6/6] vfio: platform: move get/put reset at open/release

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Date:	Thu, 22 Oct 2015 14:27:16 +0200
From:	Eric Auger <eric.auger@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	eric.auger@...com, alex.williamson@...hat.com,
	b.reynal@...tualopensystems.com,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
	kvm@...r.kernel.org, christoffer.dall@...aro.org,
	linux-kernel@...r.kernel.org, patches@...aro.org
Subject: Re: [PATCH v2 6/6] vfio: platform: move get/put reset at open/release

On 10/22/2015 02:05 PM, Arnd Bergmann wrote:
> On Thursday 22 October 2015 13:40:16 Eric Auger wrote:
>> On 10/22/2015 12:29 PM, Arnd Bergmann wrote:
>>> On Thursday 22 October 2015 11:42:02 Eric Auger wrote:
>>>> Currently reset lookup is done on probe. This introduces a
>>>> race with new registration mechanism in the case where the
>>>> vfio-platform driver is bound to the device before its module
>>>> is loaded: on the load, the probe happens which triggers the
>>>> reset module load which itself attempts to get the symbol for
>>>> the registration function (vfio_platform_register_reset). The
>>>> symbol is not yet available hence the lookup fails. In case we
>>>> do the lookup in the first open we are sure the vfio-platform
>>>> module is loaded and vfio_platform_register_reset is available.
>>>>
>>>> Signed-off-by: Eric Auger <eric.auger@...aro.org>
>>>
>>> I don't understand the explanation. I would expect the request_module()
>>> call to block until the module is actually loaded. Is this not
>>> what happens?
>>
>> Again many thanks for this new review.
>>
>> My understanding is the following
>> 1) vfio-platform is attached to the device through the override mechanism
>> 2) vfio-platform get loaded through modprobe:
>> 2-1) the platform driver init function eventually calls the
>> vfio-platform probe function.
>> 2-2) request_module of vfio-platform-calxedaxgmac gets called.
>> 3) The init of  vfio-platform-calxedaxgmac looks for
>> vfio_platform_register_reset. Unfortunately at that stage the init of
>> vfio-platform is not completed so the symbol is not available
>> 3-1) in case symbol_get is used in vfio_platform_calxedaxgmac init, as
>> of today, this latter simply returns -EINVAL. Reset registration failed
>> but no stall.
>> 3-2) in case symbol_get is *not* used, I think the module loader
>> attempts to load vfio-platform, which is already under load and this
>> causes a stall.
>>
>> Let me know if you think this understanding is not correct.
> 
> I was expecting the vfio_platform_register_reset() function to be
> available by the time of step 2-2.
> 
> What I think is going on here is that we are still inside of the
> module_init() function of the vfio-platform driver at the time that
> we call the request_module(), and the symbols are not made visible
> to other modules until the module_init function returns with success.
> This is a side-effect of the probe function for the platform driver
> getting called from deep inside of the platform_driver_register()
> function for all devices that are already present.
yes we share the same understanding, this is what I meant.
> 
> I think it already works for the AMBA case, which uses separate modules,
> but we need to restructure the platform_device case slightly to do
> the same:

OK I will test the new module structure below and eventually remove the
symbol_get (I got the link issue). Thanks for the hint!

Eric
> 
> diff --git a/drivers/vfio/platform/Makefile b/drivers/vfio/platform/Makefile
> index 9ce8afe28450..a00a44814255 100644
> --- a/drivers/vfio/platform/Makefile
> +++ b/drivers/vfio/platform/Makefile
> @@ -1,10 +1,12 @@
>  
> -vfio-platform-y := vfio_platform.o vfio_platform_common.o vfio_platform_irq.o
> +vfio-platform-base-y := vfio_platform_common.o vfio_platform_irq.o
>  
> -obj-$(CONFIG_VFIO_PLATFORM) += vfio-platform.o
> +obj-$(CONFIG_VFIO_PLATFORM) += vfio_platform.o
> +obj-$(CONFIG_VFIO_PLATFORM) += vfio-platform-base.o
>  obj-$(CONFIG_VFIO_PLATFORM) += reset/
>  
>  vfio-amba-y := vfio_amba.o
>  
>  obj-$(CONFIG_VFIO_AMBA) += vfio-amba.o
> +obj-$(CONFIG_VFIO_AMBA) += vfio-platform-base.o
>  obj-$(CONFIG_VFIO_AMBA) += reset/
> 
> 
> 	Arnd
> --
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