linux-kernel - Re: [PATCH] scsi: sd: add runtime pm to open / release

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Message-ID: <20200810141343.GA299045@rowland.harvard.edu>
Date:   Mon, 10 Aug 2020 10:13:43 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Martin Kepplinger <martin.kepplinger@...i.sm>
Cc:     James Bottomley <James.Bottomley@...senpartnership.com>,
        Bart Van Assche <bvanassche@....org>,
        Can Guo <cang@...eaurora.org>, martin.petersen@...cle.com,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...i.sm
Subject: Re: [PATCH] scsi: sd: add runtime pm to open / release

On Mon, Aug 10, 2020 at 02:03:17PM +0200, Martin Kepplinger wrote:
> On 09.08.20 17:26, Alan Stern wrote:
> > This is a somewhat fragile approach.  You don't know for certain that 
> > scsi_noretry_cmd will be called.  Also, scsi_noretry_cmd can be called 
> > from other places.
> > 
> > It would be better to clear the expecting_media_change flag just before 
> > returning from scsi_decide_disposition.  That way its use is localized 
> > to one routine, not spread out between two.
> > 
> > Alan Stern
> > 
> 
> Hi Alan,
> 
> maybe you're right. I initially just thought that I'd allow for specific
> error codes in scsi_noretry_cmd() to return non-NULL (BUS_BUSY, PARITY,
> ERROR) despite having the flag set.
> 
> The below version works equally fine for me but I'm not sure if it's
> actually more safe.
> 
> James, when exposing a new writable sysfs option like
> "suspend_no_media_change"(?) that drivers can check before setting the
> new "expecting_media_change" flag (during resume), would this addition
> make sense to you?
> 
> thanks,
> 
>                       martin
> 
> 
> 
> --- a/drivers/scsi/scsi_error.c
> +++ b/drivers/scsi/scsi_error.c
> @@ -565,6 +565,18 @@ int scsi_check_sense(struct scsi_cmnd *scmd)
>  				return NEEDS_RETRY;
>  			}
>  		}
> +		if (scmd->device->expecting_media_change) {
> +			if (sshdr.asc == 0x28 && sshdr.ascq == 0x00) {
> +				/*
> +				 * clear the expecting_media_change in
> +				 * scsi_decide_disposition() because we
> +				 * need to catch possible "fail fast" overrides
> +				 * that block readahead can cause.
> +				 */
> +				return NEEDS_RETRY;
> +			}
> +		}
> +
>  		/*
>  		 * we might also expect a cc/ua if another LUN on the target
>  		 * reported a UA with an ASC/ASCQ of 3F 0E -
> @@ -1944,9 +1956,19 @@ int scsi_decide_disposition(struct scsi_cmnd *scmd)
>  	 * the request was not marked fast fail.  Note that above,
>  	 * even if the request is marked fast fail, we still requeue
>  	 * for queue congestion conditions (QUEUE_FULL or BUSY) */
> -	if ((++scmd->retries) <= scmd->allowed
> -	    && !scsi_noretry_cmd(scmd)) {
> -		return NEEDS_RETRY;
> +	if ((++scmd->retries) <= scmd->allowed) {
> +		/*
> +		 * but scsi_noretry_cmd() cannot override the
> +		 * expecting_media_change flag.
> +		 */
> +		if (!scsi_noretry_cmd(scmd) ||
> +		    scmd->device->expecting_media_change) {
> +			scmd->device->expecting_media_change = 0;
> +			return NEEDS_RETRY;
> +		} else {
> +			/* marked fast fail and not expected. */
> +			return SUCCESS;
> +		}
>  	} else {

This may not matter...  but it's worth pointing out that 
expecting_media_change doesn't get cleared if ++scmd->retries > 
scmd->allowed.

It also doesn't get cleared in cases where the device _doesn't_ 
report a Unit Attention.

Alan Stern