linux-kernel - Re: [PATCH] Use ida_simple for SCSI iSCSI transport session id

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Message-ID: <56BE5915.9070308@cs.wisc.edu>
Date:	Fri, 12 Feb 2016 16:13:41 -0600
From:	Mike Christie <michaelc@...wisc.edu>
To:	open-iscsi@...glegroups.com, Lee Duncan <lduncan@...e.com>,
	linux-scsi@...r.kernel.org
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Use ida_simple for SCSI iSCSI transport session id

On 02/12/2016 11:54 AM, James Bottomley wrote:
> On Fri, 2016-02-12 at 09:38 -0800, Lee Duncan wrote:
>> The scsi_transport_iscsi module already uses the ida_simple
>> routines for managing the target ID, if requested to do
>> so. This change replaces an ever-increasing atomic integer
>> that tracks the session ID itself with the ida_simple
>> family of routines. This means that the session ID
>> will be reclaimed and can be reused when the session
>> is freed.
> 
> Is reusing session ID's really a good idea?

I think it is if we do it by the iscsi rfc. We have two issues.

1. For iSCSI's iSID we need a 24 bit id. The iSID + initiator name is
the SCSI initiator port ID, so targets want the iSID to be reused for
things like persistent reservation tracking.

There are a couple red hat bugzillas. I am not sure if they are open
though. Here is a KB

https://access.redhat.com/solutions/66861

for more info.

We have been lazy and did not implement this and were just reusing part
of the session->sid.

2. For the kobject/sysfs name we want a unique name/id. Linux boxes stay
up a long time and some users login and logout of sessions a lot. We
roll over and have collisions.

Lee's patch was only meant to fix this. The reuse part was not meant to
fix #1, and so it does not handle every case like you suspect.

To fix everything how about this:

- Use Lee's patch to fix #2. We do not need the iSID to match the sysfs
name. Userspace does not do any type of iSCSI iSID <-> kernel session id
matching.

- For #1, in the userespace node db code we can implement some iSID
allocation/management code that follows the spec. We can also add a
blacklist there in case we find broken targets.