On Tue, Nov 01, 2011 at 06:52:11PM +0100, David Herrmann wrote: > Hi Greg > > On Tue, Nov 1, 2011 at 6:01 PM, Greg KH <gregkh@xxxxxxx> wrote: > > On Tue, Nov 01, 2011 at 04:41:40PM +0100, David Herrmann wrote: > >> Hi Dmitry and Greg > >> > >> It doesn't make sense to take a reference to our own module. When we call > >> module_put(THIS_MODULE) we cannot make sure that our module is still alive when > >> this function returns. Therefore, module_put() will return to invalid memory and > >> our input_dev_release() function is no longer available. > >> > >> It would be interesting if Greg could elaborate what else we could do to replace > >> this module-refcount as it is definitely needed here. However, "struct device" > >> doesn't provide an owner field so there is no way for us to let the device core > >> keep a reference to our module. > > > > For a bus module, yes, this is needed, so don't remove these calls, it's > > wrong to do so. > > > >> I have no clue what to do here but the current implementation is definitely > >> unsafe so this is marked as RFC. Currently, the device_attributes probably > >> already keep a reference to our module so applying this patch would probably not > >> break anything, however, this does not look like something we can trust on. > > > > Yes it is, why do you think it isn't? > > > >> My bug-thread kind of died (https://lkml.org/lkml/2011/10/29/75) so I now try to > >> show this with an example here. > > > > It died due to me traveling, sorry, I'll respond to them now. > > No problem. This is why I've resent this with an example. > > > I fail to see what the real problem you are trying to solve here is. Is > > there something with the way the kernel works today that you are having > > problems with? What is driving this? > > I am working on converting the hci stack to properly use sysfs APIs + > struct device. And my problem simply is the following: > > @@ -1417,8 +1417,6 @@ static void input_dev_release(struct device *device) > input_mt_destroy_slots(dev); > kfree(dev->absinfo); > kfree(dev); > - > - module_put(THIS_MODULE); > } > > If this module_put(THIS_MODULE) is needed as you said, then I can be > sure that this call does not release the last module-reference, can I? > Otherwise, this call may return to invalid memory. > > But, if I can be sure that this doesn't release the last reference, > why take this reference at all? > > The only reason I can think of is, that some other code calls > __get_module() after I called it, and it calls put_module() after I > call it. In all other cases, taking/releasing this reference is > needless as we can trust that our caller protects us. > > In other words, which code does this module_get/put() protect? It > cannot protect input_dev_release() because module_put(THIS_MODULE) is > called *inside* input_dev_release(). I need some way to protect the > input_dev_release() callback-code outside of this callback. > Or can I go sure that the caller of the input_dev_callback() takes a > reference to my module before calling this and releases it after? (But > then I wonder how does it know what module I am?) > > If this is the recommended way to protect the device_release > callbacks, I will just copy it into hci_dev, but currently I really > don't get why these are needed. > If you can tell me an example why the input-core breaks if this patch > is applied, I can probably better explain to you, why I think it still > breaks without this patch applied. > > > > For instance see my example: > > 1) > input-core-module is loaded > 2) > input-core-module creates a new input device and increases > module-refcnt inside input_allocate_device() > 4) > another subsystem grabs the "struct device" and increases its refcnt > (for any reason...) > 5) > input-core-module destroys the input-device but it still stays alive > until the other subsystem releases its refcnt of the "struct device". > 6) > input-core-module is unloaded > This doesn't succeed as the still living input-device has a module-refcnt > 7) > the other subsystem releases its refcnt of the input-device > 8) > The input-device is destroyed and its _release_ function is called > The release function destroys the input-device *and* frees the last > module-refcnt. Then *boom*, the release function cannot return as it > is no longer available as described above. The analysis is right except that this condition is very unlikely to trigger. By removing __module_get/module_put you are making this problem much much easier to hit. > > My solution: Some parent subsystem of us must take and release this > module-refcnt instead of us, so this bug doesn't occur. > Or: We simply wait for all these input_devices to be released before > exiting input_exit(). How would you know that all input devices are released in the sense that all threads left (as in exited) all code in input.c completely? -- Dmitry -- To unsubscribe from this list: send the line "unsubscribe linux-input" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
