Messages in this thread | | | Date | Mon, 6 Dec 2021 10:13:53 +0800 | From | Ming Lei <> | Subject | Re: [PATCH V2 2/2] kobject: wait until kobject is cleaned up before freeing module |
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On Fri, Dec 03, 2021 at 04:07:39PM +0100, Greg Kroah-Hartman wrote: > On Mon, Nov 29, 2021 at 11:45:09AM +0800, Ming Lei wrote: > > kobject_put() may become asynchronously because of > > CONFIG_DEBUG_KOBJECT_RELEASE, so once kobject_put() returns, the caller may > > expect the kobject is released after the last refcnt is dropped, however > > CONFIG_DEBUG_KOBJECT_RELEASE just schedules one delayed work function > > for cleaning up the kobject. > > The caller should NOT expect the kobject to be released. That's the > whole point of dynamic reference counted objects, you never "know" when > the last object is released. This option just makes it obvious so that > you know when to fix up code that has this assumption.
Yes, so CONFIG_DEBUG_KOBJECT_RELEASE needs to be fixed.
> > > Inside the cleanup handler, kobj->ktype and kobj->ktype->release are > > required. > > Yes. Is that a problem?
Of course for CONFIG_DEBUG_KOBJECT_RELEASE, which delays to call ->release after random time, when the module for storing ->ktype and ->ktype->release has been unloaded.
As I mentioned, the issue can be triggered 100% by 'modprobe -r kset-example' when CONFIG_DEBUG_KOBJECT_RELEASE is enabled if the 1st patch is applied.
> > > It is supposed that no activity is on kobject itself any more since > > module_exit() is started, so it is reasonable for the kobject user or > > driver to expect that kobject can be really released in the last run of > > kobject_put() in module_exit() code path. Otherwise, it can be thought as > > one driver's bug since the module is going away. > > Why is module_exit() somehow special here? What is so odd about that?
After module_exit() is done, the module will be unloaded, then any code or data stored in the module can't be referred.
> > > When the ->ktype and ->ktype->release are allocated as module static > > variable, it can cause trouble because the delayed cleanup handler may > > be run after the module is unloaded. > > Why is ktype and release part of module code?
Lots of driver defines ktype and ktype->release in its module static variable.
> > What module kobject is causing this problem?
Any modules which defines its ktype and ktype->release in its module static variable, which is pretty common.
> > > Fixes the issue by flushing scheduled kobject cleanup work before > > freeing module. > > Why are modules special here? > > And if you enable this option, and then start unloading kernel modules, > yes, things can go wrong, but that's not what this kernel option is for > at all. > > This feels like a hack for not a real problem.
I think it is caused by CONFIG_DEBUG_KOBJECT_RELEASE, that is why this patch is posted. Otherwise I'd suggest to remove CONFIG_DEBUG_KOBJECT_RELEASE, which supposes to not panic kernel since there isn't anything wrong from driver side.
Thanks, Ming
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