linux-kernel - Re: [PATCH v3 1/2] usb: udc: core: hide struct usb_gadget

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Date:   Mon, 21 Jun 2021 09:37:34 +0800
From:   linyyuan@...eaurora.org
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Felipe Balbi <balbi@...nel.org>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jack Pham <jackp@...eaurora.org>
Subject: Re: [PATCH v3 1/2] usb: udc: core: hide struct usb_gadget_driver to
 gadget driver

On 2021-06-20 21:47, Alan Stern wrote:
> On Sun, Jun 20, 2021 at 11:53:18AM +0800, linyyuan@...eaurora.org 
> wrote:
>> On 2021-06-20 11:46, linyyuan@...eaurora.org wrote:
>> > On 2021-06-20 10:13, Alan Stern wrote:
>> > > On Sat, Jun 19, 2021 at 11:43:08PM +0800, Linyu Yuan wrote:
>> > > > currently most gadget driver have a pointer to save
>> > > > struct usb_gadget_driver from upper layer,
>> > > > it allow upper layer set and unset of the pointer.
>> > > >
>> > > > there is race that upper layer unset the pointer first,
>> > > > but gadget driver use the pointer later,
>> > > > and it cause system crash due to NULL pointer access.
>> > >
>> > > This race has already been fixed in Greg's usb-next branch.  See
>> > > commit
>> > > 7dc0c55e9f30 ("USB: UDC core: Add udc_async_callbacks gadget op") and
>> > > following commits 04145a03db9d ("USB: UDC: Implement
>> > > udc_async_callbacks in dummy-hcd") and b42e8090ba93 ("USB: UDC:
>> > > Implement udc_async_callbacks in net2280").
>> > >
>> > thanks, this is better, lower driver only need change several places.
>> > > You just need to write a corresponding patch implementing the
>> > > async_callbacks op for dwc3.
>> > yes, i will do.
>> > >
>> Alan, i want to discuss your suggestion again in b42e8090ba93 ("USB: 
>> UDC:
>> Implement udc_async_callbacks in net2280")
>> 
>> +                       if (dev->async_callbacks) { ----> if CPU1 saw 
>> this
>> is true
>> +                               spin_unlock(&dev->lock); ---> CPU2 get 
>> lock
>> after this unlock,
>> it will set async_callbacks to false, then follow call also crash, 
>> right ?
>> +                               tmp = dev->driver->setup(&dev->gadget,
>> &u.r);
>> +                               spin_lock(&dev->lock);
>> +                       }
> 
> No, this is okay.  The reason is because usb_gadget_remove_driver (CPU2
> in your example) does this:
> 
>         usb_gadget_disable_async_callbacks(udc);
>         if (udc->gadget->irq)
>                 synchronize_irq(udc->gadget->irq);
>         udc->driver->unbind(udc->gadget);
>         usb_gadget_udc_stop(udc);
> 
> The synchronize_irq call will make CPU2 wait until CPU1 has finished
> handling the interrupt for the setup packet.  The system won't crash,
> because dev->driver->setup will be called before unbind and udc_stop
> instead of after.
still several question,
1. how about suspend calll dev->driver->suspend ?
2. will 04145a03db9d ("USB: UDC: Implement udc_async_callbacks in 
dummy-hcd") backport to LTS branch ?
3. how about coding style ? so following code
if (foo->gadget_driver && foo->gadget_driver->resume)
change to
if (foo->asnyc_callbacks && foo->gadget_driver->resume)
> 
> Alan Stern