USB: dummy-hcd: Fix interrupt synchronization error

USB: dummy-hcd: Fix interrupt synchronization error

This fixes an error in synchronization in the dummy-hcd driver.  The
error has a somewhat involved history.  The synchronization mechanism
was introduced by commit 7dbd8f4cabd9 ("USB: dummy-hcd: Fix erroneous
synchronization change"), which added an emulated "interrupts enabled"
flag together with code emulating synchronize_irq() (it waits until
all current handler callbacks have returned).

But the emulated interrupt-disable occurred too late, after the driver
containing the handler callback routines had been told that it was
unbound and no more callbacks would occur.  Commit 4a5d797a9f9c ("usb:
gadget: dummy_hcd: fix gpf in gadget_setup") tried to fix this by
moving the synchronize_irq() emulation code from dummy_stop() to
dummy_pullup(), which runs before the unbind callback.

There still were races, though, because the emulated interrupt-disable
still occurred too late.  It couldn't be moved to dummy_pullup(),
because that routine can be called for reasons other than an impending
unbind.  Therefore commits 7dc0c55e9f30 ("USB: UDC core: Add
udc_async_callbacks gadget op") and 04145a03db9d ("USB: UDC: Implement
udc_async_callbacks in dummy-hcd") added an API allowing the UDC core
to tell dummy-hcd exactly when emulated interrupts and their callbacks
should be disabled.

That brings us to the current state of things, which is still wrong
because the emulated synchronize_irq() occurs before the emulated
interrupt-disable!  That's no good, beause it means that more emulated
interrupts can occur after the synchronize_irq() emulation has run,
leading to the possibility that a callback handler may be running when
the gadget driver is unbound.

To fix this, we have to move the synchronize_irq() emulation code yet
again, to the dummy_udc_async_callbacks() routine, which takes care of
enabling and disabling emulated interrupt requests.  The
synchronization will now run immediately after emulated interrupts are
disabled, which is where it belongs.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Fixes: 04145a03db9d ("USB: UDC: Implement udc_async_callbacks in dummy-hcd")
Cc: stable <stable@kernel.org>
Link: https://patch.msgid.link/c7bc93fe-4241-4d04-bd56-27c12ba35c97@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>