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Subject: Re: [PATCH v3 1/4] remoteproc: core: Move cdev add before device add
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     bjorn.andersson@linaro.org, ohad@wizery.com,
        linux-remoteproc@vger.kernel.org, linux-kernel@vger.kernel.org,
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        linux-arm-kernel@lists.infradead.org, psodagud@codeaurora.org,
        stable@vger.kernel.org
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 <YMgy7eg3wde0eVfe@kroah.com>
From:   Siddharth Gupta <sidgup@codeaurora.org>
Message-ID: <0a196786-f624-d9bb-8ef9-55c04ed57497@codeaurora.org>
Date:   Tue, 15 Jun 2021 12:03:26 -0700
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On 6/14/2021 9:56 PM, Greg KH wrote:
> On Mon, Jun 14, 2021 at 07:21:08PM -0700, Siddharth Gupta wrote:
>> When cdev_add is called after device_add has been called there is no
>> way for the userspace to know about the addition of a cdev as cdev_add
>> itself doesn't trigger a uevent notification, or for the kernel to
>> know about the change to devt. This results in two problems:
>>   - mknod is never called for the cdev and hence no cdev appears on
>>     devtmpfs.
>>   - sysfs links to the new cdev are not established.
>>
>> The cdev needs to be added and devt assigned before device_add() is
>> called in order for the relevant sysfs and devtmpfs entries to be
>> created and the uevent to be properly populated.
> So this means no one ever ran this code on a system that used devtmpfs?
>
> How was it ever tested?
My testing was done with toybox + Android's ueventd ramdisk.
As I mentioned in the discussion, the race became evident
recently. I will make sure to test all such changes without
systemd/ueventd in the future.

Thanks,
Sid