USB: cdc-wdm: don't enable interrupts in USB-giveback

On 2018-06-12 12:43:01 [-0400], Alan Stern wrote:

On Tue, 12 Jun 2018, Sebastian Andrzej Siewior wrote:

quoted

In the code path
  __usb_hcd_giveback_urb()
  -> wdm_in_callback()
   -> service_outstanding_interrupt()

The function service_outstanding_interrupt() will unconditionally enable
interrupts during unlock and invoke usb_submit_urb() with GFP_KERNEL.
If the HCD completes in BH (like ehci does) then the context remains
atomic due local_bh_disable() and enabling interrupts does not change
this.

Add an argument to service_outstanding_interrupt() which decides
whether or not it is save to enable interrupts during unlocking and use
GFP_KERNEL or not.

Wouldn't it be easier just to use spin_lock_irqsave() and GFP_ATOMIC
all the time?  That's what people normally do with code that can be 
called in both process and interrupt contexts.

service_outstanding_interrupt() does unlock + lock instead lock +
unlock. If you want to have this "always" working (without the
argument), we could do the false case:
+               spin_unlock(&desc->iuspin);
+               rv = usb_submit_urb(desc->response, GFP_ATOMIC);
+               spin_lock(&desc->iuspin);

all the time. I wanted to preserve the GFP_KERNEL part which is probably
used more often but fell that this is not needed I could drop that part.

Alan Stern

Sebastian
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