On 11/18/15, 5:30 AM, Or Gerlitz wrote:
On Mon, Nov 16, 2015 at 7:30 PM, Michael Christie <micha...@cs.wisc.edu> wrote:
On Nov 15, 2015, at 4:10 AM, Or Gerlitz <gerlitz...@gmail.com> wrote:
On Fri, Nov 13, 2015 at 6:51 PM, Mike Christie <micha...@cs.wisc.edu> wrote:
On 11/13/2015 09:06 AM, Or Gerlitz wrote:
After the locking change, adding a task to any of the connection
mgmtqueue, cmdqueue, or requeue lists is under the session forward lock.
Removing tasks from any of these lists in iscsi_data_xmit is under
the session forward lock and **before** calling down to the transport
to handle the task.
The iscsi_complete_task helper was added by Mike's commit
3bbaaad95fd38ded "[SCSI] libiscsi: handle cleanup task races"
and is indeed typically called under the backward lock && has this section
+ if (!list_empty(&task->running))
+ list_del_init(&task->running);
which per my reading of the code never comes into play, can you comment?
I had sent this to Sagi and your mellanox email the other day:
The bug occurs when a target completes a command while we are still
processing it. If we are doing a WRITE and the iscsi_task
is on the cmdqueue because we are handling a R2T.
Mike,
I failed to find how an iscsi_task can be added again to the cmdqueue list,
nor how it can be added to the requeue list without the right locking, nor how
can an iscsi_task be on either of these lists when iscsi_complete_task
is invoked.
Are you only considering normal execution or also the cc case I
mentioned in the last mail? For normal execution we are ok.
Specifically, my reading of the code says that these are the events over time:
t1. queuecommand :: we put the task on the cmdqueue list
(libiscsi.c:1741) - under fwd lock
t2. iscsi_data_xmit :: we remove the task from the cmdqueue list
(libiscsi.c:1537) - under fwd lock
when the R2T flow happens, we do the following
t3. iscsi_tcp_hdr_dissect --> iscsi_tcp_r2t_rsp --> iscsi_requeue_task ::
put the task on the requeue list -- the call to iscsi_tcp_r2t_rsp is
under the fwd lock
If the target were to send a CC at this point, we are adding the task
under the frwd lock, but the completion path would only have the back
lock. The list_empty, list_add and list_del calls then race and we could
end up in a bad state right?
t4. iscsi_data_xmit :: we remove the task from the requeue list
(libiscsi.c: 1578) - under fwd lock
We could also get the bad CC at this time and end up doing 2 list_dels
at the same time. The t4 one under the frwd lock and the cc handling
completion one under the back lock like in t3.
Do you agree to t1...t4 being what's possible for a given task? or I
missed something?
The target shouldn't
send a Check Condition at this time, but some do. If that happens, then
iscsi_queuecommand could be adding a new task to the cmdqueue, while the
recv path is handling the CC for the task with the outsanding R2T. The
recv path iscsi_complete_task call sees that task it on the cmdqueue and
deletes it from the list at the same time iscsi_queuecommand is adding a new
task.
This should not happen per the iscsi spec. There is some wording about
waiting to finish the sequence in progress, but targets goof this up.
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