[PATCH RESEND v2] scsi: ignore Synchronize Cache command failures to keep using drives not supporting it

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Many obsolete hard drives do not support the Synchronize Cache SCSI
command. Such command is generally issued during fsync() calls which
at the moment therefore fail with the ILLEGAL_REQUEST sense key.

Since this failure is currently treated as critical in the kernel SCSI
disk driver, such obsolete hard drives cannot be used anymore: they
cannot be formatted, mounted and/or checked using tools such as e2fsprogs.

Because there is nothing which can be done if the drive does not support
such command, such ILLEGAL_REQUEST should be treated as non-critical so
that the underlying operation does not fail and the obsolete hard drive
can be used normally.

This patch disables the Write Cache feature as a precaution on hard
drives which do not support the Synchronize Cache command and therefore
the cache flushing functionality.

Fixes bug: https://bugzilla.kernel.org/show_bug.cgi?id=203635

Signed-off-by: Guido Trentalancia <guido@xxxxxxxxxxxxxxxx>
---
 drivers/scsi/sd.c |   29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff -pru a/drivers/scsi/sd.c b/drivers/scsi/sd.c
--- a/drivers/scsi/sd.c	2019-03-17 18:22:04.822720851 +0100
+++ b/drivers/scsi/sd.c	2019-03-20 17:41:44.526957307 +0100
@@ -22,6 +22,10 @@
  *	 - Badari Pulavarty <pbadari@xxxxxxxxxx>, Matthew Wilcox 
  *	   <willy@xxxxxxxxxx>, Kurt Garloff <garloff@xxxxxxx>: 
  *	   Support 32k/1M disks.
+ *	 - Guido Trentalancia <guido@xxxxxxxxxxxxxxxx> ignore Synchronize
+ *	   Cache command failures on hard-drives that do not support it
+ *	   and disable the Write Cache functionality on such devices as a
+ *	   precaution: this allows to keep using several obsolete drives.
  *
  *	Logging policy (needs CONFIG_SCSI_LOGGING defined):
  *	 - setting up transfer: SCSI_LOG_HLQUEUE levels 1 and 2
@@ -1633,6 +1637,20 @@ static int sd_sync_cache(struct scsi_dis
 	}
 
 	if (res) {
+		/*
+		 * sshdr.sense_key == ILLEGAL_REQUEST means this drive
+		 * doesn't support sync. There's not much to do and
+		 * sync shouldn't fail.
+		 */
+		if (sshdr->sense_key == ILLEGAL_REQUEST && sshdr->asc == 0x20) {
+			if (sdkp->WCE) {
+				sdkp->WCE = 0;
+				sd_printk(KERN_NOTICE, sdkp, "Drive does not support Synchronize Cache(10) command: disabling write cache.\n");
+				sd_set_flush_flag(sdkp);
+			}
+			return 0;
+		}
+
 		sd_print_result(sdkp, "Synchronize Cache(10) failed", res);
 
 		if (driver_byte(res) == DRIVER_SENSE)
@@ -2022,6 +2040,17 @@ static int sd_done(struct scsi_cmnd *SCp
 					req->rq_flags |= RQF_QUIET;
 				}
 				break;
+			case SYNCHRONIZE_CACHE:
+				if (sshdr.asc == 0x20) {
+					if (sdkp->WCE) {
+						sdkp->WCE = 0;
+						sd_printk(KERN_NOTICE, sdkp, "Drive does not support Synchronize Cache(10) command: disabling write cache.\n");
+						sd_set_flush_flag(sdkp);
+					}
+					SCpnt->result = 0;
+					good_bytes = scsi_bufflen(SCpnt);
+				}
+				break;
 			}
 		}
 		break;



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