staging: comedi: add rw_semaphore to protect against device detachment

The 'read' and 'write' file operations on comedi devices do not use the
main mutex in the `struct comedi_device` to avoid contention with ioctls
that may take a while to complete.  However, it is necessary to protect
against the device being detached while the operation is in progress.
Add member `struct rw_semaphore attach_lock` to `struct comedi_device`
for this purpose and initialize it on creation.

The actual locking and unlocking will be implemented by subsequent
patches.  Tasks that are attaching or detaching comedi devices will
write-acquire the new semaphore whilst also holding the main mutex in
the `struct comedi_device`.  Tasks that wish to protect against the
comedi device being detached need to acquire either the main mutex, or
read-acquire the new semaphore, or both in that order.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -91,6 +91,7 @@ static void comedi_device_init(struct comedi_device *dev)
 {
 	spin_lock_init(&dev->spinlock);
 	mutex_init(&dev->mutex);
+	init_rwsem(&dev->attach_lock);
 	dev->minor = -1;
 }
 

drivers/staging/comedi/comedidev.h

@@ -22,6 +22,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/mutex.h>
 #include <linux/spinlock_types.h>
+#include <linux/rwsem.h>
 
 #include "comedi.h"
 
@@ -184,6 +185,7 @@ struct comedi_device {
 	bool ioenabled:1;
 	spinlock_t spinlock;
 	struct mutex mutex;
+	struct rw_semaphore attach_lock;
 
 	int n_subdevices;
 	struct comedi_subdevice *subdevices;