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

Martin Kepplinger
2020-08-11T07:56:39Z

On 10.08.20 16:13, Alan Stern wrote:
> 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.

absolutely worth pointing out and I'm not yet sure about that one.

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

true. but don't we set the flag for a future UA we don't yet know of? If
we would want to clear it outside of a UA, I think we'd need to keep
track of a suspend/resume cycle and if we see that we *had* successfully
"done requests" after resuming, we could clear it...

>
> Alan Stern
>