sd: notify block layer when using temporary change to cache_type

This is a fix for commit 39c60a0948cc06139e2fbfe084f83cb7e7deae3b

  "sd: fix array cache flushing bug causing performance problems"

We must notify the block layer via q->flush_flags after a temporary change
of the cache_type to write through.  Without this, a SYNCHRONIZE CACHE
command will still be generated.  This patch factors out a helper that
can be called from sd_revalidate_disk and cache_type_store.

Signed-off-by: Vaughan Cao <vaughan.cao@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/scsi/sd.c

@@ -134,6 +134,19 @@ static const char *sd_cache_types[] = {
 	"write back, no read (daft)"
 };
 
+static void sd_set_flush_flag(struct scsi_disk *sdkp)
+{
+	unsigned flush = 0;
+
+	if (sdkp->WCE) {
+		flush |= REQ_FLUSH;
+		if (sdkp->DPOFUA)
+			flush |= REQ_FUA;
+	}
+
+	blk_queue_flush(sdkp->disk->queue, flush);
+}
+
 static ssize_t
 cache_type_store(struct device *dev, struct device_attribute *attr,
 		 const char *buf, size_t count)
@@ -177,6 +190,7 @@ cache_type_store(struct device *dev, struct device_attribute *attr,
 	if (sdkp->cache_override) {
 		sdkp->WCE = wce;
 		sdkp->RCD = rcd;
+		sd_set_flush_flag(sdkp);
 		return count;
 	}
 
@@ -2698,7 +2712,6 @@ static int sd_revalidate_disk(struct gendisk *disk)
 	struct scsi_disk *sdkp = scsi_disk(disk);
 	struct scsi_device *sdp = sdkp->device;
 	unsigned char *buffer;
-	unsigned flush = 0;
 
 	SCSI_LOG_HLQUEUE(3, sd_printk(KERN_INFO, sdkp,
 				      "sd_revalidate_disk\n"));
@@ -2744,13 +2757,7 @@ static int sd_revalidate_disk(struct gendisk *disk)
 	 * We now have all cache related info, determine how we deal
 	 * with flush requests.
 	 */
-	if (sdkp->WCE) {
-		flush |= REQ_FLUSH;
-		if (sdkp->DPOFUA)
-			flush |= REQ_FUA;
-	}
-
-	blk_queue_flush(sdkp->disk->queue, flush);
+	sd_set_flush_flag(sdkp);
 
 	set_capacity(disk, sdkp->capacity);
 	sd_config_write_same(sdkp);