linux-kernel - Re: RFC: repeated insmod/rmmod and DEBUG_KOBJECT_RELEASE

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Date:   Mon, 13 Jun 2022 21:38:22 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:     Russell King <linux@...linux.org.uk>, linux-kernel@...r.kernel.org
Subject: Re: RFC: repeated insmod/rmmod and DEBUG_KOBJECT_RELEASE - do we
 care?

On Mon, Jun 13, 2022 at 07:02:39AM -0400, Paul Gortmaker wrote:
> If a person has CONFIG_DEBUG_KOBJECT_RELEASE enabled, and runs a rather
> questionable test suite doing a repeated modprobe followed by modprobe -r
> it will eventually trigger a duplicate sysfs name warning:
> 
> [ 1427.032646] kobject: 'usbserial_generic' (00000000c91a1c2c): kobject_release, parent 00000000890627c7 (delayed 4000)
> [...]
> [ 1430.110659] kobject: 'usbserial_generic' (00000000a633d9a5): kobject_add_internal: parent: 'drivers', set: 'drivers'
> [ 1430.110667] sysfs: cannot create duplicate filename '/bus/usb/drivers/usbserial_generic'
> [ 1430.110671] CPU: 2 PID: 1102 Comm: modprobe Not tainted 5.15.38 #7
> [ 1430.110678] Call Trace:
> [ 1430.110685]  dump_stack_lvl+0x33/0x42
> [ 1430.110693]  sysfs_warn_dup+0x51/0x60         <----------
> [ 1430.110699]  sysfs_create_dir_ns+0xb8/0xd0
> 
> For context to lkml readers, CONFIG_DEBUG_KOBJECT_RELEASE inserts a
> random delay of between 2 and 5 seconds before freeing a kobject.
> 
> In the above case, the free was delayed 4s by the DEBUG option, but we tried
> to reconstruct the same sysfs entry in just 3s elapsed and hence it triggered
> the namespace collision warning. 
> 
> I'm not convinced this warrants fixing, given the contrived nature of the
> root only test, but I did at least want to get it on record, so maybe it
> saves someone else some research time, given a similar report.

It's come up many many times in the past years, sorry you hit it again
without realizing it.

> We could I guess, within the CONFIG_DEBUG_KOBJECT_RELEASE ifdef'd code use
> kobject_rename() to add a "-zombie" suffix or something like that, but then
> it might mask name space collisions people *do* want to see?

That wouldn't work as you would have rename collisions as well.

And that warning is just that, a warning that you did something foolish
so the kernel is trying to say "don't do that".  And doing a
modprobe/rmmod constantly is a huge "don't do that" hint.

> As per above it was seen with usb-serial, but I suspect any driver with a
> sysfs kobject could reproduce the issue.  I also didn't reproduce on the
> latest kernel but I can't imagine anything has changed in this area.

Yes, the module handling code has changed a bit, so it might have solved
some of these things.  But the root cause is still there, you are doing
something odd/broken and the kernel warned you about it :)

thanks,

greg k-h