Re: [PATCH 0/7] HID: picoLCD updates

On Sun, 19 Aug 2012, Bruno Prémont wrote:

quoted

I don't know bout hid_hw_close().  Certainly no more reports should be 
submitted following usbhid_stop().

Ok, I did just that, prevent new calls to usbhid_submit_report(), after
calling hid_hw_close(), fixed one bug in my code that triggers the NULL
pointer dereference (calling hid_set_drvdata(hdev, NULL) too early).

Now I'm still seeing the bad paging request in _mmx_memcpy(), though rather
sporadically.
The last ones I saw were during remove() around the time of calling hid_hw_close()
and hid_hw_stop(). Adding a printk() between the two (at least while picoLCD
is hosting fbcon) makes it very improbably for the bad page to happen.

It looks like low-level driver did free memory in hid_hw_close() for some
in-flight URB and thus things break in following USB interrupt.

No, memory is not freed in usbhid_close().  It is freed in 
usbhid_stop().

From mapping trace information to source it seems:
usbhid/hid-core.c:
static int hid_submit_out(struct hid_device *hid)
{
        struct hid_report *report;
        char *raw_report;
        struct usbhid_device *usbhid = hid->driver_data;
        int r;

        report = usbhid->out[usbhid->outtail].report;
        raw_report = usbhid->out[usbhid->outtail].raw_report;

        usbhid->urbout->transfer_buffer_length = ((report->size - 1) >> 3) +
                                                 1 + (report->id > 0);
        usbhid->urbout->dev = hid_to_usb_dev(hid);
        if (raw_report) {
                memcpy(usbhid->outbuf, raw_report,
                                usbhid->urbout->transfer_buffer_length);
                ^^^^^^^^^^^^^^^_ this is exploding

Right.  Like I said, usbhid->outbuf is freed in hid_free_buffers(), 
which is called from usbhid_stop().

Forget about usbhid_close().  The race is between hid_submit_out() and 
usbhid_stop().

Alan Stern