scsi: sr: get rid of sr global mutex

When replacing the Big Kernel Lock in commit 2a48fc0ab242 ("block:
autoconvert trivial BKL users to private mutex"), the lock was replaced
with a sr-wide lock.

This causes very poor performance when using multiple sr devices, as the sr
driver was not able to execute more than one command to one drive at any
given time, even when there were many CD drives available.

Replace the global mutex with per-sr-device mutex.

Someone tried this patch at the time, but it never made it upstream, due to
possible concerns with race conditions, but it's not clear the patch
actually caused those:

https://www.spinics.net/lists/linux-scsi/msg63706.html
https://www.spinics.net/lists/linux-scsi/msg63750.html

Also see

http://lists.xiph.org/pipermail/paranoia/2019-December/001647.html

Link: https://lore.kernel.org/r/20200218143918.30267-1-merlijn@archive.org
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Merlijn Wajer <merlijn@archive.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/drivers/scsi/sr.h b/drivers/scsi/sr.h
index a2bb7b8bace5..339c624e04d8 100644
--- a/drivers/scsi/sr.h
+++ b/drivers/scsi/sr.h
@@ -20,6 +20,7 @@
 
 #include <linux/genhd.h>
 #include <linux/kref.h>
+#include <linux/mutex.h>
 
 #define MAX_RETRIES	3
 #define SR_TIMEOUT	(30 * HZ)
@@ -51,6 +52,7 @@ typedef struct scsi_cd {
 	bool ignore_get_event:1;	/* GET_EVENT is unreliable, use TUR */
 
 	struct cdrom_device_info cdi;
+	struct mutex lock;
 	/* We hold gendisk and scsi_device references on probe and use
 	 * the refs on this kref to decide when to release them */
 	struct kref kref;