linux-kernel - Re: [PATCH 3/3] xen-blkfront: If no barrier or flush is supported, use invalid operation.

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Date:	Mon, 12 Sep 2011 09:01:25 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>
Cc:	<jeremy.fitzhardinge@...rix.com>, <axboe@...nel.dk>,
	<stable@...nel.org>, <xen-devel@...ts.xensource.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] xen-blkfront: If no barrier or flush is
	 supported, use invalid operation.

>>> On 09.09.11 at 20:31, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
> By default we would set the info->flush_op=0, and zero maps
> to BLKIF_OP_READ request code. So if the backend did not support
> either barrier ('feature-barrier') or flush ('feature-flush-cache')
> we would end up using that opcode for the flush/barrier request, meaning
> we actually do a READ request. Fortunatly for us, __generic_make_request
> checks q->flush_flags and realizes that we don't do FLUSH and removes
> the REQ_FLUSH | REQ_FUA so we never end up issuing a READ request
> for a flush request. However, other third party implementations of
> __make_request might not be so smart, so lets fix this up.

Wouldn't it be better to simply have blkif_queue_request() fail in that
case (and then it doesn't matter whether flush_op is set to 0 or -1)?
Not the least because older (forward-port) backends stall the incoming
queue and are possibly verbose for invalid requests seen.

Jan

> CC: stable@...nel.org 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  drivers/block/xen-blkfront.c |    8 ++++----
>  1 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index e205d91..c4aa9da 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -97,7 +97,7 @@ struct blkfront_info
>  	struct blk_shadow shadow[BLK_RING_SIZE];
>  	unsigned long shadow_free;
>  	unsigned int feature_flush;
> -	unsigned int flush_op;
> +	int flush_op;
>  	unsigned int feature_discard;
>  	unsigned int discard_granularity;
>  	unsigned int discard_alignment;
> @@ -774,7 +774,7 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>  				if (error == -EOPNOTSUPP)
>  					error = 0;
>  				info->feature_flush = 0;
> -				info->flush_op = 0;
> +				info->flush_op = -1; /* 0 is BLKIF_OP_READ */
>  				xlvbd_flush(info);
>  			}
>  			/* fall through */
> @@ -1207,7 +1207,7 @@ static void blkfront_connect(struct blkfront_info 
> *info)
>  	}
>  
>  	info->feature_flush = 0;
> -	info->flush_op = 0;
> +	info->flush_op = -1; /* 0 is BLKIF_OP_READ */
>  
>  	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
>  			    "feature-barrier", "%d", &barrier,
> @@ -1234,7 +1234,7 @@ static void blkfront_connect(struct blkfront_info 
> *info)
>  	if (!err && flush)
>  		info->flush_op = BLKIF_OP_FLUSH_DISKCACHE;
>  
> -	if (info->flush_op)
> +	if (info->flush_op > 0)
>  		info->feature_flush = REQ_FLUSH | REQ_FUA;
>  
>  	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,




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