On Wed, Jun 16, 2021 at 11:47:01AM -0700, Siddharth Gupta wrote: > > On 6/15/2021 10:58 PM, Greg KH wrote: > > On Tue, Jun 15, 2021 at 12:03:26PM -0700, Siddharth Gupta wrote: > > > 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. > > It isn't an issue of systemd/ueventd, those do not control /dev on a > > normal system, that is what devtmpfs is for. > I am not fully aware of when devtmpfs is enabled or not, but in > case it is not - systemd/ueventd will create these files with > mknod, right? No, systemd does not create device nodes, and neither does udev. Hasn't done so for well over 10 years now. > I was even manually able to call mknod from the > terminal when some of the remoteproc character device entries > showed up (using major number from there, and minor number being > the remoteproc id), and that allowed me to boot up the > remoteprocs as well. Yes, that is fine, but that also means that this was not working from the very beginning :( > > And devtmpfs nodes are only created if you create a struct device > > somewhere with a proper major/minor, which you were not doing here, so > > you must have had a static /dev on your test systems, right? > I am not sure of what you mean by a static /dev? Could you > explain? In case you mean the character device would be > non-functional, that is not the case. They have been working > for us since the beginning. /dev on modern systems is managed by devtmpfs, which knows to create the device nodes when you properly register the device with the driver core. A "static" /dev is managed by mknod from userspace, like you did "by hand", and that is usually only done by older systems. thanks, greg k-h
