linux-kernel - Re: [PATCH] kobject: Make sure the parent does not get released before its children

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Date:   Sat, 23 May 2020 08:44:06 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        kernel test robot <rong.a.chen@...el.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH] kobject: Make sure the parent does not get released
 before its children

On 5/23/20 8:36 AM, Greg Kroah-Hartman wrote:
> On Wed, May 13, 2020 at 06:18:40PM +0300, Heikki Krogerus wrote:
>> In the function kobject_cleanup(), kobject_del(kobj) is
>> called before the kobj->release(). That makes it possible to
>> release the parent of the kobject before the kobject itself.
>>
>> To fix that, adding function __kboject_del() that does
>> everything that kobject_del() does except release the parent
>> reference. kobject_cleanup() then calls __kobject_del()
>> instead of kobject_del(), and separately decrements the
>> reference count of the parent kobject after kobj->release()
>> has been called.
>>
>> Reported-by: Naresh Kamboju <naresh.kamboju@...aro.org>
>> Reported-by: kernel test robot <rong.a.chen@...el.com>
>> Fixes: 7589238a8cf3 ("Revert "software node: Simplify software_node_release() function"")
>> Suggested-by: "Rafael J. Wysocki" <rafael@...nel.org>
>> Signed-off-by: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
>> Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>> Reviewed-by: Brendan Higgins <brendanhiggins@...gle.com>
>> Tested-by: Brendan Higgins <brendanhiggins@...gle.com>
>> Acked-by: Randy Dunlap <rdunlap@...radead.org>
>> ---
>>  lib/kobject.c | 30 ++++++++++++++++++++----------
>>  1 file changed, 20 insertions(+), 10 deletions(-)
> 
> Stepping back, now that it turns out this patch causes more problems
> than it fixes, how is everyone reproducing the original crash here?

Just load lib/test_printf.ko and boom!


> Is it just the KUNIT_DRIVER_PE_TEST that is causing the issue?
> 
> In looking at 7589238a8cf3 ("Revert "software node: Simplify
> software_node_release() function""), the log messages there look
> correct.  sysfs can't create a duplicate file, and so when your test is
> written to try to create software nodes, you always have to check the
> return value.  If you run the test in parallel, or before another test
> has had a chance to clean up, the function will fail, correctly.
> 
> So what real-world thing is this test "failure" trying to show?


-- 
~Randy