USB: gadget: dummy-hcd: Fix locking bug in RT-enabled kernels

USB: gadget: dummy-hcd: Fix locking bug in RT-enabled kernels

Yunseong Kim and the syzbot fuzzer both reported a problem in
RT-enabled kernels caused by the way dummy-hcd mixes interrupt
management and spin-locking.  The pattern was:

	local_irq_save(flags);
	spin_lock(&dum->lock);
	...
	spin_unlock(&dum->lock);
	...		// calls usb_gadget_giveback_request()
	local_irq_restore(flags);

The code was written this way because usb_gadget_giveback_request()
needs to be called with interrupts disabled and the private lock not
held.

While this pattern works fine in non-RT kernels, it's not good when RT
is enabled.  RT kernels handle spinlocks much like mutexes; in particular,
spin_lock() may sleep.  But sleeping is not allowed while local
interrupts are disabled.

To fix the problem, rewrite the code to conform to the pattern used
elsewhere in dummy-hcd and other UDC drivers:

	spin_lock_irqsave(&dum->lock, flags);
	...
	spin_unlock(&dum->lock);
	usb_gadget_giveback_request(...);
	spin_lock(&dum->lock);
	...
	spin_unlock_irqrestore(&dum->lock, flags);

This approach satisfies the RT requirements.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: stable <stable@kernel.org>
Fixes: b4dbda1a22d2 ("USB: dummy-hcd: disable interrupts during req->complete")
Reported-by: Yunseong Kim <ysk@kzalloc.com>
Closes: <https://lore.kernel.org/linux-usb/5b337389-73b9-4ee4-a83e-7e82bf5af87a@kzalloc.com/>
Reported-by: syzbot+8baacc4139f12fa77909@syzkaller.appspotmail.com
Closes: <https://lore.kernel.org/linux-usb/68ac2411.050a0220.37038e.0087.GAE@google.com/>
Tested-by: syzbot+8baacc4139f12fa77909@syzkaller.appspotmail.com
CC: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
CC: stable@vger.kernel.org
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/bb192ae2-4eee-48ee-981f-3efdbbd0d8f0@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>