Staging: hv: Introduce a function to wait to drain outgoing I/O

Rather than busy loop waiting to drain I/O, introduce a function
that does not burn CPU cycles waiting.
Signed-off-by: NK. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: NHaiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: NAbhishek Kane <v-abkane@microsoft.com>
Signed-off-by: NHank Janssen <hjanssen@microsoft.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/hv/storvsc.c

@@ -47,6 +47,7 @@ static inline struct storvsc_device *alloc_stor_device(struct hv_device *device)
 	/* (ie get_stor_device() and must_get_stor_device()) to proceed. */
 	atomic_cmpxchg(&stor_device->ref_count, 0, 2);
 
+	init_waitqueue_head(&stor_device->waiting_to_drain);
 	stor_device->device = device;
 	device->ext = stor_device;
 
@@ -332,7 +333,10 @@ static void stor_vsc_on_io_completion(struct hv_device *device,
 
 	request->on_io_completion(request);
 
-	atomic_dec(&stor_device->num_outstanding_req);
+	if (atomic_dec_and_test(&stor_device->num_outstanding_req) &&
+		stor_device->drain_notify)
+		wake_up(&stor_device->waiting_to_drain);
+
 
 	put_stor_device(device);
 }

drivers/staging/hv/storvsc_api.h

@@ -26,6 +26,7 @@
 #define _STORVSC_API_H_
 
 #include <linux/kernel.h>
+#include <linux/wait.h>
 #include "vstorage.h"
 #include "vmbus_api.h"
 #include "vmbus.h"
@@ -108,8 +109,11 @@ struct storvsc_device {
 	/* 0 indicates the device is being destroyed */
 	atomic_t ref_count;
 
+	bool	 drain_notify;
 	atomic_t num_outstanding_req;
 
+	wait_queue_head_t waiting_to_drain;
+
 	/*
 	 * Each unique Port/Path/Target represents 1 channel ie scsi
 	 * controller. In reality, the pathid, targetid is always 0
@@ -161,6 +165,14 @@ struct storvsc_driver *drv_to_stordrv(struct device_driver *d)
 	return hvdr_to_stordr(hvdrv);
 }
 
+static inline void storvsc_wait_to_drain(struct storvsc_device *dev)
+{
+	dev->drain_notify = true;
+	wait_event(dev->waiting_to_drain,
+		   atomic_read(&dev->num_outstanding_req) == 0);
+	dev->drain_notify = false;
+}
+
 /* Interface */
 
 int storvsc_dev_add(struct hv_device *device,