Re: [PATCH 03/11] target/core: Release SPC-2 reservation upon initiator logout

[Mike Christie <mchristi@redhat.com>]

On 04/08/2019 06:17 PM, Bart Van Assche wrote:
> On Mon, 2019-04-08 at 17:41 -0500, Mike Christie wrote:
>> On 04/08/2019 03:17 PM, Bart Van Assche wrote:
>>> On Mon, 2019-04-08 at 15:04 -0500, Mike Christie wrote:
>>>> Was the original code done for iscsi? We have this in
>>>> https://tools.ietf.org/html/rfc7143:
>>>>
>>>> 4.4.3.2. Reservations
>>>>
>>>> ....
>>>>
>>>> In contrast, [SPC2] does not specify detailed persistence
>>>> requirements for reserve/release reservation state after an I_T nexus
>>>> failure. Nonetheless, when reserve/release reservations are
>>>> supported by an iSCSI target, the preferred implementation approach
>>>> is to preserve reserve/release reservation state for iSCSI session
>>>> reinstatement (see Section 6.3.5) or session continuation (see
>>>> Section 6.3.6).
>>>>
>>>>
>>>> So for example for session reinstatement if you pull a network cable
>>>> then plug it back in the iscsi target can do a
>>>> iscsit_close_session->transport_deregister_session ->
>>>> transport_free_session. Above we will now release the reservation.
>>>
>>> Hello Mike,
>>>
>>> Was that paragraph from the iSCSI RFC perhaps written before it was made clear
>>> in SPC-2 that reserve/release reservations do not persist? I think the name of
>>
>> Do you know when that was added to SPC 2?
>>
>> The iscsi text above is from RFC 7143 which I think was ratified in
>> 2014. I think it was added in that version to clarify the issue. I am
>> not sure though.
>>
>> The original 3720 ratified in 2004 did not have that chunk and only had
>> limited references/info on reservations.
> 
> Hello Mike,
> 
> From spc2r01 (1997-11-13): "Reservations managed using the Reserve/Release
> method do not persist across some recovery actions (e.g., hard resets), so
> most systems require significant reinitialization after a failure that
> results in a hard reset. Reserve/Release managed reservations are retained
> by the device server until released or until reset by mechanisms specified
> in this standard."
> 
> Although the words differ from later versions of SPC2, the intention has not
> changed.
> 
> Other vendors seem to agree with what has been defined in SPC2. From
> https://kb.netapp.com/app/answers/answer_view/a_id/1001463/~/what-are-scsi-reservations-and-scsi-persistent-reservations%3F-:
> "Thus, a SCSI bus reset performed due to an error recovery would cause the
> reservation to be released."
> 
> Does this mean that the authors of the iSCSI RFC chose behavior that
> contradicts established behavior for the SCSI reserve and release commands?
> 

I don't know.

For the hard reset/reset event case, if we go by SAM 2 which iscsi RFC
7143 and 3720 say they are based on at the beginning of their
definitions then it sounds like it depends on the transport spec for
what is a hard reset. We have this from SAM 2:

5.9.6 Hard reset:

A hard reset is a SCSI target port action in response to a reset event
within the service delivery subsystem. A wakeup event (see 3.1.132) is a
reset event. The definition of additional reset events is SCSI transport
protocol specific. Each SCSI transport protocol standard that defines
reset events shall specify the SCSI target port’s action
in response to reset events.

--------

That might sound like a wake event is a hard reset defined by SAM 2, and
the transport defines others. In the iscsi RFC the only mention of reset
events is when handling the iSCSI TARGET COLD RESET TMF. However, the
RFC does define the session reinstatement case as a I_T nexus loss
event, so if that is considered a hard reset by the SAM 2 definition
then I think your patch is correct. I have no idea if other software
writers read the specs/rfcs like maybe Nick did though.