[media] media: dvb_ringbuffer: Add memory barriers

Implement memory barriers according to Documentation/circular-buffers.txt:
- use smp_store_release() to update ringbuffer read/write pointers
- use smp_load_acquire() to load write pointer on reader side
- use ACCESS_ONCE() to load read pointer on writer side

This fixes data stream corruptions observed e.g. on an ARM Cortex-A9
quad core system with different types (PCI, USB) of DVB tuners.

Signed-off-by: Soeren Moch <[e-mail]>
Cc: [e-mail] # 3.14+
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/dvb-core/dvb_ringbuffer.c

···
 
 int dvb_ringbuffer_empty(struct dvb_ringbuffer *rbuf)
 {
-	return (rbuf->pread==rbuf->pwrite);
+	/* smp_load_acquire() to load write pointer on reader side
+	 * this pairs with smp_store_release() in dvb_ringbuffer_write(),
+	 * dvb_ringbuffer_write_user(), or dvb_ringbuffer_reset()
+	 *
+	 * for memory barriers also see Documentation/circular-buffers.txt
+	 */
+	return (rbuf->pread == smp_load_acquire(&rbuf->pwrite));
 }
 
 
···
 {
 	ssize_t free;
 
-	free = rbuf->pread - rbuf->pwrite;
+	/* ACCESS_ONCE() to load read pointer on writer side
+	 * this pairs with smp_store_release() in dvb_ringbuffer_read(),
+	 * dvb_ringbuffer_read_user(), dvb_ringbuffer_flush(),
+	 * or dvb_ringbuffer_reset()
+	 */
+	free = ACCESS_ONCE(rbuf->pread) - rbuf->pwrite;
 	if (free <= 0)
 		free += rbuf->size;
 	return free-1;
···
 {
 	ssize_t avail;
 
-	avail = rbuf->pwrite - rbuf->pread;
+	/* smp_load_acquire() to load write pointer on reader side
+	 * this pairs with smp_store_release() in dvb_ringbuffer_write(),
+	 * dvb_ringbuffer_write_user(), or dvb_ringbuffer_reset()
+	 */
+	avail = smp_load_acquire(&rbuf->pwrite) - rbuf->pread;
 	if (avail < 0)
 		avail += rbuf->size;
 	return avail;
···
 
 void dvb_ringbuffer_flush(struct dvb_ringbuffer *rbuf)
 {
-	rbuf->pread = rbuf->pwrite;
+	/* dvb_ringbuffer_flush() counts as read operation
+	 * smp_load_acquire() to load write pointer
+	 * smp_store_release() to update read pointer, this ensures that the
+	 * correct pointer is visible for subsequent dvb_ringbuffer_free()
+	 * calls on other cpu cores
+	 */
+	smp_store_release(&rbuf->pread, smp_load_acquire(&rbuf->pwrite));
 	rbuf->error = 0;
 }
 EXPORT_SYMBOL(dvb_ringbuffer_flush);
 
 void dvb_ringbuffer_reset(struct dvb_ringbuffer *rbuf)
 {
-	rbuf->pread = rbuf->pwrite = 0;
+	/* dvb_ringbuffer_reset() counts as read and write operation
+	 * smp_store_release() to update read pointer
+	 */
+	smp_store_release(&rbuf->pread, 0);
+	/* smp_store_release() to update write pointer */
+	smp_store_release(&rbuf->pwrite, 0);
 	rbuf->error = 0;
 }
 
···
 			return -EFAULT;
 		buf += split;
 		todo -= split;
-		rbuf->pread = 0;
+		/* smp_store_release() for read pointer update to ensure
+		 * that buf is not overwritten until read is complete,
+		 * this pairs with ACCESS_ONCE() in dvb_ringbuffer_free()
+		 */
+		smp_store_release(&rbuf->pread, 0);
 	}
 	if (copy_to_user(buf, rbuf->data+rbuf->pread, todo))
 		return -EFAULT;
 
-	rbuf->pread = (rbuf->pread + todo) % rbuf->size;
+	/* smp_store_release() to update read pointer, see above */
+	smp_store_release(&rbuf->pread, (rbuf->pread + todo) % rbuf->size);
 
 	return len;
 }
···
 		memcpy(buf, rbuf->data+rbuf->pread, split);
 		buf += split;
 		todo -= split;
-		rbuf->pread = 0;
+		/* smp_store_release() for read pointer update to ensure
+		 * that buf is not overwritten until read is complete,
+		 * this pairs with ACCESS_ONCE() in dvb_ringbuffer_free()
+		 */
+		smp_store_release(&rbuf->pread, 0);
 	}
 	memcpy(buf, rbuf->data+rbuf->pread, todo);
 
-	rbuf->pread = (rbuf->pread + todo) % rbuf->size;
+	/* smp_store_release() to update read pointer, see above */
+	smp_store_release(&rbuf->pread, (rbuf->pread + todo) % rbuf->size);
 }
 
 
···
 		memcpy(rbuf->data+rbuf->pwrite, buf, split);
 		buf += split;
 		todo -= split;
-		rbuf->pwrite = 0;
+		/* smp_store_release() for write pointer update to ensure that
+		 * written data is visible on other cpu cores before the pointer
+		 * update, this pairs with smp_load_acquire() in
+		 * dvb_ringbuffer_empty() or dvb_ringbuffer_avail()
+		 */
+		smp_store_release(&rbuf->pwrite, 0);
 	}
 	memcpy(rbuf->data+rbuf->pwrite, buf, todo);
-	rbuf->pwrite = (rbuf->pwrite + todo) % rbuf->size;
+	/* smp_store_release() for write pointer update, see above */
+	smp_store_release(&rbuf->pwrite, (rbuf->pwrite + todo) % rbuf->size);
 
 	return len;
 }
···
 			return len - todo;
 		buf += split;
 		todo -= split;
-		rbuf->pwrite = 0;
+		/* smp_store_release() for write pointer update to ensure that
+		 * written data is visible on other cpu cores before the pointer
+		 * update, this pairs with smp_load_acquire() in
+		 * dvb_ringbuffer_empty() or dvb_ringbuffer_avail()
+		 */
+		smp_store_release(&rbuf->pwrite, 0);
 	}
 	status = copy_from_user(rbuf->data+rbuf->pwrite, buf, todo);
 	if (status)
 		return len - todo;
-	rbuf->pwrite = (rbuf->pwrite + todo) % rbuf->size;
+	/* smp_store_release() for write pointer update, see above */
+	smp_store_release(&rbuf->pwrite, (rbuf->pwrite + todo) % rbuf->size);
 
 	return len;
 }