linux-kernel - Re: [PATCH 1/1] Drivers: scsi: Derive the FLUSH

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Date:	Fri, 4 Oct 2013 21:42:24 +0000
From:	James Bottomley <jbottomley@...allels.com>
To:	"K. Y. Srinivasan" <kys@...rosoft.com>
CC:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"ohering@...e.com" <ohering@...e.com>,
	"hch@...radead.org" <hch@...radead.org>,
	"eric@...estorage.com" <eric@...estorage.com>,
	"nab@...ux-iscsi.org" <nab@...ux-iscsi.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>
Subject: Re: [PATCH 1/1] Drivers: scsi: Derive the FLUSH_TIMEOUT from the
 basic I/O timeout

On Fri, 2013-10-04 at 12:38 -0700, K. Y. Srinivasan wrote:
> Rather than having a separate constant for specifying the timeout on FLUSH
> operations, use the basic I/O timeout value that is already configurable
> on a per target basis to derive the FLUSH timeout. Looking at the current
> definitions of these timeout values, the FLUSH operation is supposed to have
> a value that is twice the normal timeout value. This patch preserves this
> relationship while leveraging the flexibility of specifying the I/O timeout.
> 
> I would like to thank Eric Seppanen <eric@...estorage.com> and 
> Nicholas A. Bellinger <nab@...ux-iscsi.org> for their help in resolving
> this issue.
> 
> Signed-off-by: K. Y. Srinivasan <kys@...rosoft.com>
> ---
>  drivers/scsi/sd.c |    5 +++--
>  1 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index e62d17d..8aff306 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -829,7 +829,7 @@ static int sd_setup_write_same_cmnd(struct scsi_device *sdp, struct request *rq)
>  
>  static int scsi_setup_flush_cmnd(struct scsi_device *sdp, struct request *rq)
>  {
> -	rq->timeout = SD_FLUSH_TIMEOUT;
> +	rq->timeout *= 2;
>  	rq->retries = SD_MAX_RETRIES;
>  	rq->cmd[0] = SYNCHRONIZE_CACHE;
>  	rq->cmd_len = 10;
> @@ -1433,6 +1433,7 @@ static int sd_sync_cache(struct scsi_disk *sdkp)
>  {
>  	int retries, res;
>  	struct scsi_device *sdp = sdkp->device;
> +	unsigned int timeout = sdp->request_queue->rq_timeout;

The timeout is signed in the function prototype

>  	struct scsi_sense_hdr sshdr;
>  
>  	if (!scsi_device_online(sdp))
> @@ -1448,7 +1449,7 @@ static int sd_sync_cache(struct scsi_disk *sdkp)
>  		 * flush everything.
>  		 */
>  		res = scsi_execute_req_flags(sdp, cmd, DMA_NONE, NULL, 0,
> -					     &sshdr, SD_FLUSH_TIMEOUT,
> +					     &sshdr, timeout * 2,
>  					     SD_MAX_RETRIES, NULL, REQ_PM);
>  		if (res == 0)
>  			break;

Not like this, please: you now leave us with a dangling #define whose
name makes you think it should be related to flushing and a couple of
curious magic constants.  It's almost hand crafted to confuse people
reading the code.

Please do it like this instead: with a comment explaining what we're
doing above the #define and a name that clearly relates to the actions.

James

---

diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index e62d17d..5c9496d 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -829,7 +829,7 @@ static int sd_setup_write_same_cmnd(struct scsi_device *sdp, struct request *rq)
 
 static int scsi_setup_flush_cmnd(struct scsi_device *sdp, struct request *rq)
 {
-	rq->timeout = SD_FLUSH_TIMEOUT;
+	rq->timeout *= SD_FLUSH_TIMEOUT_MULTIPLIER;
 	rq->retries = SD_MAX_RETRIES;
 	rq->cmd[0] = SYNCHRONIZE_CACHE;
 	rq->cmd_len = 10;
@@ -1433,6 +1433,8 @@ static int sd_sync_cache(struct scsi_disk *sdkp)
 {
 	int retries, res;
 	struct scsi_device *sdp = sdkp->device;
+	const int timeout = sdp->request_queue->rq_timeout
+		* SD_FLUSH_TIMEOUT_MULTIPLIER;
 	struct scsi_sense_hdr sshdr;
 
 	if (!scsi_device_online(sdp))
@@ -1448,8 +1450,8 @@ static int sd_sync_cache(struct scsi_disk *sdkp)
 		 * flush everything.
 		 */
 		res = scsi_execute_req_flags(sdp, cmd, DMA_NONE, NULL, 0,
-					     &sshdr, SD_FLUSH_TIMEOUT,
-					     SD_MAX_RETRIES, NULL, REQ_PM);
+					     &sshdr, timeout, SD_MAX_RETRIES,
+					     NULL, REQ_PM);
 		if (res == 0)
 			break;
 	}
diff --git a/drivers/scsi/sd.h b/drivers/scsi/sd.h
index 7a049de..7f7999c 100644
--- a/drivers/scsi/sd.h
+++ b/drivers/scsi/sd.h
@@ -13,7 +13,11 @@
  */
 #define SD_TIMEOUT		(30 * HZ)
 #define SD_MOD_TIMEOUT		(75 * HZ)
-#define SD_FLUSH_TIMEOUT	(60 * HZ)
+/*
+ * Flush timeout is a multiplier over the standard device timeout which is
+ * user modifiable via sysfs but initially set to SD_TIMEOUT
+ */
+#define SD_FLUSH_TIMEOUT_MULTIPLIER	2
 #define SD_WRITE_SAME_TIMEOUT	(120 * HZ)
 
 /*

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