Re: [PATCH v2 1/1] scsi: libiscsi: fix NOP race condition

[Mike Christie <michael.christie@oracle.com>]

On 9/25/20 1:41 PM, lduncan@suse.com wrote:
> From: Lee Duncan <lduncan@suse.com>
> 
> iSCSI NOPs are sometimes "lost", mistakenly sent to the
> user-land iscsid daemon instead of handled in the kernel,
> as they should be, resulting in a message from the daemon like:
> 
>> iscsid: Got nop in, but kernel supports nop handling.
> 
> This can occur because of the forward- and back-locks
> in the kernel iSCSI code, and the fact that an iSCSI NOP
> response can be processed before processing of the NOP send
> is complete. This can result in "conn->ping_task" being NULL
> in iscsi_nop_out_rsp(), when the pointer is actually in
> the process of being set.
> 
> To work around this, we add a new state to the "ping_task"
> pointer. In addition to NULL (not assigned) and a pointer
> (assigned), we add the state "being set", which is signaled
> with an INVALID pointer (using "-1").
> 
> Signed-off-by: Lee Duncan <lduncan@suse.com>
> ---
>  drivers/scsi/libiscsi.c | 13 ++++++++++---
>  include/scsi/libiscsi.h |  3 +++
>  2 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/scsi/libiscsi.c b/drivers/scsi/libiscsi.c
> index 1e9c3171fa9f..cade108c33b6 100644
> --- a/drivers/scsi/libiscsi.c
> +++ b/drivers/scsi/libiscsi.c
> @@ -738,6 +738,9 @@ __iscsi_conn_send_pdu(struct iscsi_conn *conn, struct iscsi_hdr *hdr,
>  						   task->conn->session->age);
>  	}
>  
> +	if (unlikely(READ_ONCE(conn->ping_task) == INVALID_SCSI_TASK))
> +		WRITE_ONCE(conn->ping_task, task);
> +
>  	if (!ihost->workq) {
>  		if (iscsi_prep_mgmt_task(conn, task))
>  			goto free_task;

I think the API gets a little weird now where in some cases
__iscsi_conn_send_pdu checks the opcode to see what type of request
it is but above we the caller sets the ping_task.

For login, tmfs and passthrough, we assume the __iscsi_conn_send_pdu
has sent or cleaned up everything. I think it might be nicer to just
have __iscsi_conn_send_pdu set the ping_task field before doing the
xmit/queue call. It would then work similar to the conn->login_task
case where that function knows about that special task too.

So in __iscsi_conn_send_pdu add a "if (opcode == ISCSI_OP_NOOP_OUT)",
and check if it's a nop we need to track. If so set conn->ping_task.