Re: iSCSI: response to second login (with same ISID)

[Santosh Rao <santoshr@cup.hp.com>]

> However if two copies of an application attempt to open sessions to the same
> iSCSI URL from the same host at the same time, the second should not disrupt
> the first.  I'm not sure what will force them to have different ISID's, or
> how they would even try to each pick a unique one.


The above scenario should not typically result in the HBA driver sending
out 2 logins on the wire, unless its open logic is broken. The open
paths are typically single-threaded and the first of the 2 applications
entering the driver open code would cause a login to go out on the wire
with the 2nd one just bumping an open_count or ref_count of some kind
within the HBA driver.

The cases when an initiator would send out 2 logins on the wire would be
ones like :

a) Bug in the initiator driver open_count logic or open locking logic.
b) Initiator's state out of sync with target's. (initiator thinks it is
logged out with the target, but target thinks the initiator is still
logged in.)
c) Prior logout operation failed at the target.
d) initiator failed to logout all sessions prior to reboot.

In cases (b), (c) & (d), the initiator does want to re-login, and logout
any stale session if active. In case (a) the driver is broken and not
much forward progress will occur with such a version.

The login semantics should be optimized for :

a) Minimal time for initiator login operation. (Should not require some
iterations of login to get a session going, while the initiator is in a
"guess an ISID" game with the target. Also, should not require a login
-> login reject -> logout of old session -> login of new session).

b) Allow a scheme for clean up of stale session resources.

Regards,
Santosh
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