Re: [PATCH v2] scsi: core: clear driver private data when retry request

[John Garry <john.g.garry@oracle.com>]

On 14/02/2025 01:16, Ye Bin wrote:
> From: Ye Bin <yebin10@huawei.com>
> 
> After commit 1bad6c4a57ef
> ("scsi: zero per-cmd private driver data for each MQ I/O"),
> xen-scsifront/virtio_scsi/snic driver remove code that zeroes
> driver-private command data. If request do retry will lead to
> driver-private command data remains. Before commit 464a00c9e0ad
> ("scsi: core: Kill DRIVER_SENSE") if virtio_scsi do capacity
> expansion, first request may return UA then request will do retry,
> as driver-private command data remains, request will return UA
> again. As a result, the request keeps retrying, and the request
> times out and fails.
> So zeroes driver-private command data when request do retry.
> 
> Fixes: f7de50da1479 ("scsi: xen-scsifront: Remove code that zeroes driver-private command data")
> Fixes: c2bb87318baa ("scsi: virtio_scsi: Remove code that zeroes driver-private command data")
> Fixes: c3006a926468 ("scsi: snic: Remove code that zeroes driver-private command data")
> Signed-off-by: Ye Bin <yebin10@huawei.com>
> ---
>   drivers/scsi/scsi_lib.c | 14 +++++++-------
>   1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index be0890e4e706..c876b1c82153 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -1669,13 +1669,6 @@ static blk_status_t scsi_prepare_cmd(struct request *req)
>   	if (in_flight)
>   		__set_bit(SCMD_STATE_INFLIGHT, &cmd->state);
>   
> -	/*
> -	 * Only clear the driver-private command data if the LLD does not supply
> -	 * a function to initialize that data.
> -	 */
> -	if (!shost->hostt->init_cmd_priv)
> -		memset(cmd + 1, 0, shost->hostt->cmd_size);
> -
>   	cmd->prot_op = SCSI_PROT_NORMAL;
>   	if (blk_rq_bytes(req))
>   		cmd->sc_data_direction = rq_dma_dir(req);
> @@ -1842,6 +1835,13 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
>   	if (!scsi_host_queue_ready(q, shost, sdev, cmd))
>   		goto out_dec_target_busy;
>   
> +	/*
> +	 * Only clear the driver-private command data if the LLD does not supply
> +	 * a function to initialize that data.
> +	 */
> +	if (!shost->hostt->init_cmd_priv && shost->hostt->cmd_size)

Some drivers do not set shost->hostt->cmd_size. And none that don't set 
shost->hostt->cmd_size also do set shost->hostt->init_cmd_priv, AFAICS.

So for those we are always needlessly checking both 
shost->hostt->init_cmd_priv and shost->hostt->cmd_size.

So how about re-order the checks:
	if (shost->hostt->cmd_size && !shost->hostt->init_cmd_priv)

> +		memset(cmd + 1, 0, shost->hostt->cmd_size);
> +
>   	if (!(req->rq_flags & RQF_DONTPREP)) {
>   		ret = scsi_prepare_cmd(req);
>   		if (ret != BLK_STS_OK)