linux-kernel - Re: [PATCH v11 2/4] uacce: add uacce driver

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Message-ID: <750bb4b6-20a9-96b0-5801-5b8bff8cc3b5@linaro.org>
Date:   Wed, 15 Jan 2020 22:11:56 +0800
From:   zhangfei <zhangfei.gao@...aro.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        jonathan.cameron@...wei.com, dave.jiang@...el.com,
        grant.likely@....com, jean-philippe <jean-philippe@...aro.org>,
        Jerome Glisse <jglisse@...hat.com>,
        ilias.apalodimas@...aro.org, francois.ozog@...aro.org,
        kenneth-lee-2012@...mail.com, Wangzhou <wangzhou1@...ilicon.com>,
        "haojian . zhuang" <haojian.zhuang@...aro.org>,
        guodong.xu@...aro.org, linux-accelerators@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        iommu@...ts.linux-foundation.org,
        Kenneth Lee <liguozhu@...ilicon.com>,
        Zaibo Xu <xuzaibo@...wei.com>
Subject: Re: [PATCH v11 2/4] uacce: add uacce driver



On 2020/1/15 下午8:02, Greg Kroah-Hartman wrote:
> On Wed, Jan 15, 2020 at 07:18:34PM +0800, zhangfei wrote:
>> Hi, Greg
>>
>> On 2020/1/14 下午10:59, Greg Kroah-Hartman wrote:
>>> On Mon, Jan 13, 2020 at 11:34:55AM +0800, zhangfei wrote:
>>>> Hi, Greg
>>>>
>>>> Thanks for the review.
>>>>
>>>> On 2020/1/12 上午3:40, Greg Kroah-Hartman wrote:
>>>>> On Sat, Jan 11, 2020 at 10:48:37AM +0800, Zhangfei Gao wrote:
>>>>>> +static int uacce_fops_open(struct inode *inode, struct file *filep)
>>>>>> +{
>>>>>> +	struct uacce_mm *uacce_mm = NULL;
>>>>>> +	struct uacce_device *uacce;
>>>>>> +	struct uacce_queue *q;
>>>>>> +	int ret = 0;
>>>>>> +
>>>>>> +	uacce = xa_load(&uacce_xa, iminor(inode));
>>>>>> +	if (!uacce)
>>>>>> +		return -ENODEV;
>>>>>> +
>>>>>> +	if (!try_module_get(uacce->parent->driver->owner))
>>>>>> +		return -ENODEV;
>>>>> Why are you trying to grab the module reference of the parent device?
>>>>> Why is that needed and what is that going to help with here?
>>>>>
>>>>> This shouldn't be needed as the module reference of the owner of the
>>>>> fileops for this module is incremented, and the "parent" module depends
>>>>> on this module, so how could it be unloaded without this code being
>>>>> unloaded?
>>>>>
>>>>> Yes, if you build this code into the kernel and the "parent" driver is a
>>>>> module, then you will not have a reference, but when you remove that
>>>>> parent driver the device will be removed as it has to be unregistered
>>>>> before that parent driver can be removed from the system, right?
>>>>>
>>>>> Or what am I missing here?
>>>> The refcount here is preventing rmmod "parent" module after fd is opened,
>>>> since user driver has mmap kernel memory to user space, like mmio, which may
>>>> still in-use.
>>>>
>>>> With the refcount protection, rmmod "parent" module will fail until
>>>> application free the fd.
>>>> log like: rmmod: ERROR: Module hisi_zip is in use
>>> But if the "parent" module is to be unloaded, it has to unregister the
>>> "child" device and that will call the destructor in here and then you
>>> will tear everything down and all should be good.
>>>
>>> There's no need to "forbid" a module from being unloaded, even if it is
>>> being used.  Look at all networking drivers, they work that way, right?
>> Thanks Greg for the kind suggestion.
>>
>> I still have one uncertainty.
>> Does uacce has to block process continue accessing the mmapped area when
>> remove "parent" module?
>> Uacce can block device access the physical memory when parent module call
>> uacce_remove.
>> But application is still running, and suppose it is not the kernel driver's
>> responsibility to call unmap.
>>
>> I am looking for some examples in kernel,
>> looks vfio does not block process continue accessing when
>> vfio_unregister_iommu_driver either.
>>
>> In my test, application will keep waiting after rmmod parent, until ctrl+c,
>> when unmap is called.
>> During the process, kernel does not report any error.
>>
>> Do you have any advice?
> Is there no way for the kernel to invalidate the memory and tell the
> process to stop?  tty drivers do this for when they are removed from the
> system.
>
> Anyway, this is all very rare, no kernel module is ever unloaded on a
> real system, that is only for when developers are working on them, so
> it's probably not that big of an issue, right?
>
Thanks Greg, will update a new version while ignoring this first.

Thanks