V4L/DVB (9054): implement proper locking in the dvb ca en50221 driver

Concurrent access to a single DVB CA 50221 interface slot is generally
discouraged. The underlying drivers (budget-av, budget-ci) do not implement
proper locking and thus two transactions could (and do) interfere with on
another.

This fixes the following problems seen by others and myself:

 - sudden i/o errors when writing to the ci device which usually would
   result in an undefined state of the hw and require a software restart

 - errors about the CAM trying to send a buffer larger than the agreed size
   usually also resulting in an undefined state of the hw

Due the to design of the DVB CA 50221 driver, implementing the locks in the
underlying drivers would not be enough and still leave some race conditions,
even though they were harder to trigger.

Signed-off-by: Matthias Dahl <devel@mortal-soul.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/dvb/dvb-core/dvb_ca_en50221.c

@@ -93,6 +93,9 @@ struct dvb_ca_slot {
 	/* current state of the CAM */
 	int slot_state;
 
+	/* mutex used for serializing access to one CI slot */
+	struct mutex slot_lock;
+
 	/* Number of CAMCHANGES that have occurred since last processing */
 	atomic_t camchange_count;
 
@@ -711,14 +714,20 @@ static int dvb_ca_en50221_write_data(struct dvb_ca_private *ca, int slot, u8 * b
 	dprintk("%s\n", __func__);
 
 
-	// sanity check
+	/* sanity check */
 	if (bytes_write > ca->slot_info[slot].link_buf_size)
 		return -EINVAL;
 
-	/* check if interface is actually waiting for us to read from it, or if a read is in progress */
+	/* it is possible we are dealing with a single buffer implementation,
+	   thus if there is data available for read or if there is even a read
+	   already in progress, we do nothing but awake the kernel thread to
+	   process the data if necessary. */
 	if ((status = ca->pub->read_cam_control(ca->pub, slot, CTRLIF_STATUS)) < 0)
 		goto exitnowrite;
 	if (status & (STATUSREG_DA | STATUSREG_RE)) {
+		if (status & STATUSREG_DA)
+			dvb_ca_en50221_thread_wakeup(ca);
+
 		status = -EAGAIN;
 		goto exitnowrite;
 	}
@@ -987,6 +996,8 @@ static int dvb_ca_en50221_thread(void *data)
 		/* go through all the slots processing them */
 		for (slot = 0; slot < ca->slot_count; slot++) {
 
+			mutex_lock(&ca->slot_info[slot].slot_lock);
+
 			// check the cam status + deal with CAMCHANGEs
 			while (dvb_ca_en50221_check_camstatus(ca, slot)) {
 				/* clear down an old CI slot if necessary */
@@ -1122,7 +1133,7 @@ static int dvb_ca_en50221_thread(void *data)
 
 			case DVB_CA_SLOTSTATE_RUNNING:
 				if (!ca->open)
-					continue;
+					break;
 
 				// poll slots for data
 				pktcount = 0;
@@ -1146,6 +1157,8 @@ static int dvb_ca_en50221_thread(void *data)
 				}
 				break;
 			}
+
+			mutex_unlock(&ca->slot_info[slot].slot_lock);
 		}
 	}
 
@@ -1181,13 +1194,15 @@ static int dvb_ca_en50221_io_do_ioctl(struct inode *inode, struct file *file,
 	switch (cmd) {
 	case CA_RESET:
 		for (slot = 0; slot < ca->slot_count; slot++) {
+			mutex_lock(&ca->slot_info[slot].slot_lock);
 			if (ca->slot_info[slot].slot_state != DVB_CA_SLOTSTATE_NONE) {
 				dvb_ca_en50221_slot_shutdown(ca, slot);
 				if (ca->flags & DVB_CA_EN50221_FLAG_IRQ_CAMCHANGE)
 					dvb_ca_en50221_camchange_irq(ca->pub,
 								     slot,
 								     DVB_CA_EN50221_CAMCHANGE_INSERTED);
 			}
+			mutex_unlock(&ca->slot_info[slot].slot_lock);
 		}
 		ca->next_read_slot = 0;
 		dvb_ca_en50221_thread_wakeup(ca);
@@ -1308,7 +1323,9 @@ static ssize_t dvb_ca_en50221_io_write(struct file *file,
 				goto exit;
 			}
 
+			mutex_lock(&ca->slot_info[slot].slot_lock);
 			status = dvb_ca_en50221_write_data(ca, slot, fragbuf, fraglen + 2);
+			mutex_unlock(&ca->slot_info[slot].slot_lock);
 			if (status == (fraglen + 2)) {
 				written = 1;
 				break;
@@ -1664,6 +1681,7 @@ int dvb_ca_en50221_init(struct dvb_adapter *dvb_adapter,
 		ca->slot_info[i].slot_state = DVB_CA_SLOTSTATE_NONE;
 		atomic_set(&ca->slot_info[i].camchange_count, 0);
 		ca->slot_info[i].camchange_type = DVB_CA_EN50221_CAMCHANGE_REMOVED;
+		mutex_init(&ca->slot_info[i].slot_lock);
 	}
 
 	if (signal_pending(current)) {

drivers/media/dvb/dvb-core/dvb_ca_en50221.h

@@ -45,8 +45,10 @@ struct dvb_ca_en50221 {
 	/* the module owning this structure */
 	struct module* owner;
 
-	/* NOTE: the read_*, write_* and poll_slot_status functions must use locks as
-	 * they may be called from several threads at once */
+	/* NOTE: the read_*, write_* and poll_slot_status functions will be
+	 * called for different slots concurrently and need to use locks where
+	 * and if appropriate. There will be no concurrent access to one slot.
+	 */
 
 	/* functions for accessing attribute memory on the CAM */
 	int (*read_attribute_mem)(struct dvb_ca_en50221* ca, int slot, int address);