staging: comedi: use attach_lock semaphore during attach and detach
Acquire the `attach_lock` semaphore in the `struct comedi_device` while
modifying the `attached` flag. This is a "write" acquire. Note that
the main mutex in the `struct comedi_device` is also held at this time.
Tasks wishing to check the device is attached will need to either
acquire the main mutex, or "read" acquire the `attach_lock` semaphore,
or both in that order.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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