On Tue, Nov 02, 2021 at 03:51:33PM +0100, Petr Mladek wrote: > On Tue 2021-11-02 15:15:19, Petr Mladek wrote: > > On Tue 2021-10-26 23:37:30, Ming Lei wrote: > > > On Tue, Oct 26, 2021 at 10:48:18AM +0200, Petr Mladek wrote: > > > > Below are more details about the livepatch code. I hope that it will > > > > help you to see if zram has similar problems or not. > > > > > > > > We have kobject in three structures: klp_func, klp_object, and > > > > klp_patch, see include/linux/livepatch.h. > > > > > > > > These structures have to be statically defined in the module sources > > > > because they define what is livepatched, see > > > > samples/livepatch/livepatch-sample.c > > > > > > > > The kobject is used there to show information about the patch, patched > > > > objects, and patched functions, in sysfs. And most importantly, > > > > the sysfs interface can be used to disable the livepatch. > > > > > > > > The problem with static structures is that the module must stay > > > > in the memory as long as the sysfs interface exists. It can be > > > > solved in module_exit() callback. It could wait until the sysfs > > > > interface is destroyed. > > > > > > > > kobject API does not support this scenario. The relase() callbacks > > > > > > kobject_delete() is for supporting this scenario, that is why we don't > > > need to grab module refcnt before calling show()/store() of the > > > kobject's attributes. > > > > > > kobject_delete() can be called in module_exit(), then any show()/store() > > > will be done after kobject_delete() returns. > > > > I am a bit confused. I do not see kobject_delete() anywhere in kernel > > sources. > > > > I see only kobject_del() and kobject_put(). AFAIK, they do _not_ > > guarantee that either the sysfs interface was destroyed or > > the release callbacks were called. For example, see > > schedule_delayed_work(&kobj->release, delay) in kobject_release(). > > Grr, I always get confused by the code. kobject_del() actually waits > until the sysfs interface gets destroyed. This is why there is > the deadlock. Right. > > But kobject_put() is _not_ synchronous. And the comment above > kobject_add() repeat 3 times that kobject_put() must be called > on success: > > * Return: If this function returns an error, kobject_put() must be > * called to properly clean up the memory associated with the > * object. Under no instance should the kobject that is passed > * to this function be directly freed with a call to kfree(), > * that can leak memory. > * > * If this function returns success, kobject_put() must also be called > * in order to properly clean up the memory associated with the object. > * > * In short, once this function is called, kobject_put() MUST be called > * when the use of the object is finished in order to properly free > * everything. > > and similar text in Documentation/core-api/kobject.rst > > After a kobject has been registered with the kobject core successfully, it > must be cleaned up when the code is finished with it. To do that, call > kobject_put(). > > > If I read the code correctly then kobject_put() calls kref_put() > that might call kobject_delayed_cleanup(). This function does a lot > of things and need to access struct kobject. Yes, then what is the problem here wrt. kobject_put() which may not be synchronous? > > > IMHO, kobject API does not support static structures and module > > removal. > > If kobject_put() has to be called also for static structures then > module_exit() must explicitly wait until the clean up is finished. Right, that is exactly how klp_patch kobject is implemented. klp_patch kobject has to be disabled first, then module refcnt can be dropped after the klp_patch kobject is released. Then module_exit() is possible. Thanks, Ming
