[PATCH] scsi/sd: release scan_mutex during sync_cache and start_stop

[PATCH] scsi/sd: release scan_mutex during sync_cache and start_stop

[Song Liu]

When a device is deleted through sysfs handle "delete", the code
locks shost->scan_mutex. If multiple devices are deleted at the
same time, these deletes will be handled in series.

On the other hand, sd_shutdown() sometimes issues long latency
commands: sync cache and start_stop. It is not necessary for these
commands to run in series.

To reduce latency of parallel "delete" requests, this patch unlock
shost->scan_mutex before long latency commands and relock the mutex
after the command.

Signed-off-by: Song Liu <songliubraving@fb.com>
---
 drivers/scsi/sd.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 2ea3568..39aa209 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -3241,6 +3241,8 @@ static int sd_start_stop_device(struct scsi_disk *sdkp, int start)
 static void sd_shutdown(struct device *dev)
 {
 	struct scsi_disk *sdkp = dev_get_drvdata(dev);
+	struct scsi_device *sdev = sdkp->device;
+	struct Scsi_Host *shost = sdev->host;
 
 	if (!sdkp)
 		return;         /* this can happen */
@@ -3249,13 +3251,17 @@ static void sd_shutdown(struct device *dev)
 		return;
 
 	if (sdkp->WCE && sdkp->media_present) {
+		mutex_unlock(&shost->scan_mutex);
 		sd_printk(KERN_NOTICE, sdkp, "Synchronizing SCSI cache\n");
 		sd_sync_cache(sdkp);
+		mutex_lock(&shost->scan_mutex);
 	}
 
 	if (system_state != SYSTEM_RESTART && sdkp->device->manage_start_stop) {
+		mutex_unlock(&shost->scan_mutex);
 		sd_printk(KERN_NOTICE, sdkp, "Stopping disk\n");
 		sd_start_stop_device(sdkp, 0);
+		mutex_lock(&shost->scan_mutex);
 	}
 }
 
-- 
2.9.3