Re: [PATCH v3 1/3] xen/blkfront: read response from backend only once

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@epam.com>]

Hi, Juergen!

On 30.07.21 13:38, Juergen Gross wrote:
> In order to avoid problems in case the backend is modifying a response
> on the ring page while the frontend has already seen it, just read the
> response into a local buffer in one go and then operate on that buffer
> only.
>
> Signed-off-by: Juergen Gross <jgross@suse.com>
> Reviewed-by: Jan Beulich <jbeulich@suse.com>
> Acked-by: Roger Pau Monné <roger.pau@citrix.com>
> ---
>   drivers/block/xen-blkfront.c | 35 ++++++++++++++++++-----------------
>   1 file changed, 18 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index d83fee21f6c5..15e840287734 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -1496,7 +1496,7 @@ static bool blkif_completion(unsigned long *id,
>   static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>   {
>   	struct request *req;
> -	struct blkif_response *bret;
> +	struct blkif_response bret;
>   	RING_IDX i, rp;
>   	unsigned long flags;
>   	struct blkfront_ring_info *rinfo = (struct blkfront_ring_info *)dev_id;
> @@ -1513,8 +1513,9 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>   	for (i = rinfo->ring.rsp_cons; i != rp; i++) {
>   		unsigned long id;
>   
> -		bret = RING_GET_RESPONSE(&rinfo->ring, i);
> -		id   = bret->id;
> +		RING_COPY_RESPONSE(&rinfo->ring, i, &bret);

As per my understanding copying is still not an atomic operation as the request/response

are multi-byte structures in general. IOW, what prevents the backend from modifying the ring while

we are copying?

Thanks,

Oleksandr

> +		id = bret.id;
> +
>   		/*
>   		 * The backend has messed up and given us an id that we would
>   		 * never have given to it (we stamp it up to BLK_RING_SIZE -
> @@ -1522,39 +1523,39 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>   		 */
>   		if (id >= BLK_RING_SIZE(info)) {
>   			WARN(1, "%s: response to %s has incorrect id (%ld)\n",
> -			     info->gd->disk_name, op_name(bret->operation), id);
> +			     info->gd->disk_name, op_name(bret.operation), id);
>   			/* We can't safely get the 'struct request' as
>   			 * the id is busted. */
>   			continue;
>   		}
>   		req  = rinfo->shadow[id].request;
>   
> -		if (bret->operation != BLKIF_OP_DISCARD) {
> +		if (bret.operation != BLKIF_OP_DISCARD) {
>   			/*
>   			 * We may need to wait for an extra response if the
>   			 * I/O request is split in 2
>   			 */
> -			if (!blkif_completion(&id, rinfo, bret))
> +			if (!blkif_completion(&id, rinfo, &bret))
>   				continue;
>   		}
>   
>   		if (add_id_to_freelist(rinfo, id)) {
>   			WARN(1, "%s: response to %s (id %ld) couldn't be recycled!\n",
> -			     info->gd->disk_name, op_name(bret->operation), id);
> +			     info->gd->disk_name, op_name(bret.operation), id);
>   			continue;
>   		}
>   
> -		if (bret->status == BLKIF_RSP_OKAY)
> +		if (bret.status == BLKIF_RSP_OKAY)
>   			blkif_req(req)->error = BLK_STS_OK;
>   		else
>   			blkif_req(req)->error = BLK_STS_IOERR;
>   
> -		switch (bret->operation) {
> +		switch (bret.operation) {
>   		case BLKIF_OP_DISCARD:
> -			if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
> +			if (unlikely(bret.status == BLKIF_RSP_EOPNOTSUPP)) {
>   				struct request_queue *rq = info->rq;
>   				printk(KERN_WARNING "blkfront: %s: %s op failed\n",
> -					   info->gd->disk_name, op_name(bret->operation));
> +					   info->gd->disk_name, op_name(bret.operation));
>   				blkif_req(req)->error = BLK_STS_NOTSUPP;
>   				info->feature_discard = 0;
>   				info->feature_secdiscard = 0;
> @@ -1564,15 +1565,15 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>   			break;
>   		case BLKIF_OP_FLUSH_DISKCACHE:
>   		case BLKIF_OP_WRITE_BARRIER:
> -			if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
> +			if (unlikely(bret.status == BLKIF_RSP_EOPNOTSUPP)) {
>   				printk(KERN_WARNING "blkfront: %s: %s op failed\n",
> -				       info->gd->disk_name, op_name(bret->operation));
> +				       info->gd->disk_name, op_name(bret.operation));
>   				blkif_req(req)->error = BLK_STS_NOTSUPP;
>   			}
> -			if (unlikely(bret->status == BLKIF_RSP_ERROR &&
> +			if (unlikely(bret.status == BLKIF_RSP_ERROR &&
>   				     rinfo->shadow[id].req.u.rw.nr_segments == 0)) {
>   				printk(KERN_WARNING "blkfront: %s: empty %s op failed\n",
> -				       info->gd->disk_name, op_name(bret->operation));
> +				       info->gd->disk_name, op_name(bret.operation));
>   				blkif_req(req)->error = BLK_STS_NOTSUPP;
>   			}
>   			if (unlikely(blkif_req(req)->error)) {
> @@ -1585,9 +1586,9 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>   			fallthrough;
>   		case BLKIF_OP_READ:
>   		case BLKIF_OP_WRITE:
> -			if (unlikely(bret->status != BLKIF_RSP_OKAY))
> +			if (unlikely(bret.status != BLKIF_RSP_OKAY))
>   				dev_dbg(&info->xbdev->dev, "Bad return from blkdev data "
> -					"request: %x\n", bret->status);
> +					"request: %x\n", bret.status);
>   
>   			break;
>   		default: