linux-kernel - Re: [PATCH] Don't change direction flags in struct request.

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Date:	Tue, 02 Mar 2010 10:56:58 +0530
From:	James Bottomley <James.Bottomley@...e.de>
To:	Hugh Daschbach <hdasch@...adcom.com>
Cc:	Chandra Seetharaman <sekharan@...ibm.com>,
	Eddie Williams <Eddie.Williams@...eleye.com>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Don't change direction flags in struct request.

On Thu, 2010-03-18 at 22:31 -0700, Hugh Daschbach wrote:
> The EMC multipath device handler should not change the I/O direction
> flags of its trespass command request.
> 
> The CFQ elevator may BUG if the direction flags on an I/O request are
> changed after allocation.  cfq_set_request() and cfq_put_request()
> count READ and WRITE requests separately.  Changing the I/O request
> direction after blk_get_request() allocates the request throws off
> this CFQ accounting.

This description doesn't really match what the problem seems to be
below:

> Signed-off-by: Hugh Daschbach <hdasch@...adcom.com>
> ---
>  drivers/scsi/device_handler/scsi_dh_emc.c |    8 ++++----
>  1 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/scsi/device_handler/scsi_dh_emc.c b/drivers/scsi/device_handler/scsi_dh_emc.c
> index 6196675..3709342 100644
> --- a/drivers/scsi/device_handler/scsi_dh_emc.c
> +++ b/drivers/scsi/device_handler/scsi_dh_emc.c
> @@ -269,10 +269,12 @@ static struct request *get_req(struct scsi_device *sdev, int cmd,
>  				unsigned char *buffer)
>  {
>  	struct request *rq;
> +	int mode = READ;
>  	int len = 0;
>  
> -	rq = blk_get_request(sdev->request_queue,
> -			(cmd == MODE_SELECT) ? WRITE : READ, GFP_NOIO);
> +	if (cmd == MODE_SELECT || cmd == MODE_SELECT_10)
> +		mode = WRITE;
> +	rq = blk_get_request(sdev->request_queue, mode, GFP_NOIO);

So the actual bug is failure to set WRITE for MODE_SELECT_10.

>  	if (!rq) {
>  		sdev_printk(KERN_INFO, sdev, "get_req: blk_get_request failed");
>  		return NULL;
> @@ -284,12 +286,10 @@ static struct request *get_req(struct scsi_device *sdev, int cmd,
>  	switch (cmd) {
>  	case MODE_SELECT:
>  		len = sizeof(short_trespass);
> -		rq->cmd_flags |= REQ_RW;

And this is just cosmetic ... the flags already having been set by the
allocation, they don't need to be set again.

>  		rq->cmd[1] = 0x10;
>  		break;
>  	case MODE_SELECT_10:
>  		len = sizeof(long_trespass);
> -		rq->cmd_flags |= REQ_RW;
>  		rq->cmd[1] = 0x10;
>  		break;
>  	case INQUIRY:

Chandra, can you tidy this up and take it, please?

Thanks,

James


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