linux-kernel - Re: [PATCH v3 1/4] remoteproc: core: Move cdev add before device add

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Date:   Tue, 15 Jun 2021 12:03:26 -0700
From:   Siddharth Gupta <sidgup@...eaurora.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     bjorn.andersson@...aro.org, ohad@...ery.com,
        linux-remoteproc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, psodagud@...eaurora.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v3 1/4] remoteproc: core: Move cdev add before device add


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