Thanks again Alan !
On Mon, May 22, 2023 at 8:55 AM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, May 22, 2023 at 12:48:39AM -0700, Badhri Jagan Sridharan wrote:
> > Hi Alan,
> >
> > Thanks for taking the time out to share more details !
> > +1 on your comment: " A big problem with the USB gadget
> > framework is that it does not clearly state which routines have to run
> > in process context and which have to run in interrupt/atomic context."
> >
> >
> > I started to work on allow_connect and other suggestions that you had made.
> > In one of the previous comments you had mentioned that the
> > connect_lock should be a spinlock and not a mutex.
>
> Yeah, I changed my mind about that.
>
> > Right now there are four conditions that seem to be deciding whether
> > pullup needs to be enabled or disabled through gadget->ops->pullup().
> > 1. Gadget not deactivated through usb_gadget_deactivate()
> > 2. Gadget has to be started through usb_gadget_udc_start().
> > soft_connect_store() can start/stop gadget.
> > 3. usb_gadget has been connected through usb_gadget_connect(). This is
> > assuming we are getting rid of usb_udc_vbus_handler.
> > 4. allow_connect is true
> >
> > I have so far identified two constraints here:
> > a. gadget->ops->pullup() can sleep in some implementations.
> > For instance:
> > BUG: scheduling while atomic: init/1/0x00000002
> > ..
> > [ 26.990631][ T1] Call trace:
> > [ 26.993759][ T1] dump_backtrace+0x104/0x128
> > [ 26.998281][ T1] show_stack+0x20/0x30
> > [ 27.002279][ T1] dump_stack_lvl+0x6c/0x9c
> > [ 27.006627][ T1] __schedule_bug+0x84/0xb4
> > [ 27.010973][ T1] __schedule+0x6f0/0xaec
> > [ 27.015147][ T1] schedule+0xc8/0x134
> > [ 27.019059][ T1] schedule_timeout+0x98/0x134
> > [ 27.023666][ T1] msleep+0x34/0x4c
> > [ 27.027317][ T1] dwc3_core_soft_reset+0xf0/0x354
> > [ 27.032273][ T1] dwc3_gadget_pullup+0xec/0x1d8
> > [ 27.037055][ T1] usb_gadget_pullup_update_locked+0xa0/0x1e0
> > [ 27.042967][ T1] udc_bind_to_driver+0x1e4/0x30c
> > [ 27.047835][ T1] usb_gadget_probe_driver+0xd0/0x178
> > [ 27.053051][ T1] gadget_dev_desc_UDC_store+0xf0/0x13c
> > [ 27.058442][ T1] configfs_write_iter+0x100/0x178
> > [ 27.063399][ T1] vfs_write+0x278/0x3c4
> > [ 27.067483][ T1] ksys_write+0x80/0xf4
>
> What kernel was this trace made with? I don't see udc_bind_to_driver
> appearing anywhere in 6.4-rc3.
Sorry, I was switching between devices running different kernel
versions, with the latest one running 6.1, and posted trace from an
older one by mistake.
>
>
> > b. gadget->ops->udc_start can also sleep in some implementations.
> > For example:
> > [ 28.024255][ T1] BUG: scheduling while atomic: init/1/0x00000002
> > ....
> > [ 28.324996][ T1] Call trace:
> > [ 28.328126][ T1] dump_backtrace+0x104/0x128
> > [ 28.332647][ T1] show_stack+0x20/0x30
> > [ 28.336645][ T1] dump_stack_lvl+0x6c/0x9c
> > [ 28.340993][ T1] __schedule_bug+0x84/0xb4
> > [ 28.345340][ T1] __schedule+0x6f0/0xaec
> > [ 28.349513][ T1] schedule+0xc8/0x134
> > [ 28.353425][ T1] schedule_timeout+0x4c/0x134
> > [ 28.358033][ T1] wait_for_common+0xac/0x13c
> > [ 28.362554][ T1] wait_for_completion_killable+0x20/0x3c
> > [ 28.368118][ T1] __kthread_create_on_node+0xe4/0x1ec
> > [ 28.373422][ T1] kthread_create_on_node+0x54/0x80
> > [ 28.378464][ T1] setup_irq_thread+0x50/0x108
> > [ 28.383072][ T1] __setup_irq+0x90/0x87c
> > [ 28.387245][ T1] request_threaded_irq+0x144/0x180
> > [ 28.392287][ T1] dwc3_gadget_start+0x50/0xac
> > [ 28.396866][ T1] udc_bind_to_driver+0x14c/0x31c
> > [ 28.401763][ T1] usb_gadget_probe_driver+0xd0/0x178
> > [ 28.406980][ T1] gadget_dev_desc_UDC_store+0xf0/0x13c
> > [ 28.412370][ T1] configfs_write_iter+0x100/0x178
> > [ 28.417325][ T1] vfs_write+0x278/0x3c4
> > [ 28.421411][ T1] ksys_write+0x80/0xf4
> >
> > static int dwc3_gadget_start(struct usb_gadget *g,
> > struct usb_gadget_driver *driver)
> > {
> > struct dwc3 *dwc = gadget_to_dwc(g);
> > ...
> > irq = dwc->irq_gadget;
> > ret = request_threaded_irq(irq, dwc3_interrupt, dwc3_thread_interrupt,
> > IRQF_SHARED, "dwc3", dwc->ev_buf);
> >
> > Given that "1016fc0c096c USB: gadget: Fix obscure lockdep violation
> > for udc_mutex" has been there for a while and no one has reported
> > issues so far, perhaps ->disconnect() callback is no longer being
> > invoked in atomic context and the documentation is what that needs to
> > be updated ?
>
> That's part of what I'm trying to figure out. However, some UDC drivers
> call ->disconnect() directly when they detect loss of VBUS power,
> instead of going through the core. So disconnect handlers will have
> remain capable of running in interrupt context until those UDC drivers
> are changed.
>
> Getting back to your first point, it looks like we need to assume any
> routine that needs to communicate with the UDC hardware (such as the
> ->pullup callback used in usb_gadget_{dis}connect()) must always be
> called in process context. This means that usb_udc_connect_control()
> always has to run in process context, since it will do either a connect
> or a disconnect.
>
> On the other hand, some routines -- in particular,
> usb_udc_vbus_handler() -- may be called by a UDC driver's interrupt
> handler and therefore may run in interrupt context. (This fact should
> be noted in that routine's kerneldoc, by the way.)
>
> So here's the problem: usb_udc_vbus_handler() running in interrupt
> context calls usb_udc_connect_control(), which has to run in process
> context. And this is not just a simple issue caused by the
> ->disconnect() callback or use of mutexes; it's more fundamental.
>
> I'm led to conclude that you were right to offload part of
> usb_udc_vbus_handler()'s job to a workqueue. It's an awkward thing to
> do, because you have to make sure to cancel the work item at times when
> it's no longer needed. But there doesn't seem to be any other choice.
>
> Here's two related problems for you to think about:
>
> 1. Once gadget_unbind_driver() has called usb_gadget_disconnect(),
> we don't want a VBUS change to cause usb_udc_vbus_handler()'s
> work routine to turn the pullup back on. How can we prevent
> this?
>
> 2. More generally, suppose usb_udc_vbus_handler() gets called at
> exactly the same time that some other pathway (either
> gadget_bind_driver() or soft_connect_store()) tries to do a
> connect or disconnect. What should happen then?
I believe I can solve the above races by protecting the flags set by
each of them with connect_lock and not pulling up unless all of them
are true.
The caller will hold connect_lock, update the respective flag and
invoke the below usb_gadget_pullup_update_locked function(shown
below).
Code stub:
/* Internal version of usb_gadget_connect needs to be called with
connect_lock held. */
static int usb_gadget_pullup_update_locked(struct usb_gadget *gadget)
__must_hold(&gadget->udc->connect_lock)
{
int ret = 0;
bool connect = !gadget->deactivated && gadget->udc->started &&
gadget->udc->vbus &&
gadget->udc->allow_connect;
if (!gadget->ops->pullup) {
ret = -EOPNOTSUPP;
goto out;
}
if (connect != gadget->connected) {
ret = gadget->ops->pullup(gadget, connect);
if (!ret)
gadget->connected = connect;
if (!connect) {
mutex_lock(&udc_lock);
if (gadget->udc->driver)
gadget->udc->driver->disconnect(gadget);
mutex_unlock(&udc_lock);
}
out:
trace_usb_gadget_connect(gadget, ret);
return ret;
}
However, while auditing the code again, I noticed another potential
race as well:
Looks like usb_del_gadget() can potentially race against
usb_udc_vbus_handler() and call device_unregister.
This implies usb_udc can be freed while usb_udc_vbus_handler() or the
work item is executing.
void usb_del_gadget(struct usb_gadget *gadget)
{
struct usb_udc *udc = gadget->udc;
..
...
device_unregister(&udc->dev);
}
EXPORT_SYMBOL_GPL(usb_del_gadget);
Does this look like a valid concern to you or am I misunderstanding this ?
If so, I am afraid that the only way to solve this is by synchronizing
usb_udc_vbus_handler() against usb_del_gadget() through a mutex as
device_unregister() can sleep.
So offloading usb_udc_vbus_handler() cannot work either.
usb_udc_vbus_hander() seems to be invoked from the following drivers:
1. drivers/usb/chipidea/udc.c:
usb_udc_vbus_hander() is called from a non-atomic context.
2. drivers/usb/gadget/udc/max3420_udc.c
usb_udc_vbus_hander() is called from the interrupt handler.
However, all the events are processed from max3420_thread kthread.
So I am thinking of making them threaded irq handlers instead.
3. drivers/usb/gadget/udc/renesas_usbf.c
This one looks more invasive. However, still attempting to move them
to threaded irq handlers.
As always, I'm looking forward to your feedback !
Thanks,
Badhri
>
> Alan Stern
