Re: [PATCH] scsi: qla4xxx: fix potential deadlock on frwd/back lock

[Mike Christie]

On 9/28/23 9:54 AM, Chengfeng Ye wrote:
> iscsi_complete_pdu() is called by several rx callback, under
> softirq context. Thus the callsite of it inside qla4xxx_task_work()
> should better disable bottom half, as work queue is executed under
> process context, it needs to protect race with softirq context
> locking to avoid deadlock.
> 
> <potential deadlock #1>
> qla4xxx_task_work()
> --> iscsi_complete_pdu()
> --> spin_lock(&conn->session->back_lock);
> <interrupt>
>    --> iscsi_tcp_data_recv_done()
>    --> iscsi_complete_pdu()
>    --> spin_lock(&conn->session->back_lock) (deadlock)
> 

This will not happen, because qla4xxx does not use the libiscsi_tcp
code. If you had a iscsi session with qla4xxx and another
iscsi session with iscsi_tcp, then you could get an interrupt like
this, but the "conn->session->back_lock" that's taken above would
be for the iscsi_tcp session.

So you would need to make sure that the above can't happen for
iscsi_tcp an iscsi_tcp session. Same with the below analysis.


> <potential deadlock #2>
> qla4xxx_task_work()
> --> iscsi_complete_pdu()
> --> __iscsi_complete_pdu()
> --> spin_lock(&conn->session->frwd_lock)
> <interrupt>
>    --> iscsi_tcp_data_recv_done()
>    --> iscsi_complete_pdu()
>    --> __iscsi_complete_pdu()
>    --> spin_lock(&conn->session->frwd_lock) (deadlock)
> 
> This flaw was found by an experimental static analysis tool I am
> developing for irq-related deadlock.
> 
> To avoid the problem, disable bh inside qla4xxx_task_work() before
> calling iscsi_complete_pdu().
> 
> Signed-off-by: Chengfeng Ye <dg573847474@xxxxxxxxx>
> ---
>  drivers/scsi/qla4xxx/ql4_os.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/scsi/qla4xxx/ql4_os.c b/drivers/scsi/qla4xxx/ql4_os.c
> index 675332e49a7b..c60781148e6c 100644
> --- a/drivers/scsi/qla4xxx/ql4_os.c
> +++ b/drivers/scsi/qla4xxx/ql4_os.c
> @@ -3382,7 +3382,9 @@ static void qla4xxx_task_work(struct work_struct *wdata)
>  		hdr->itt = itt;
>  		data = task_data->resp_buffer + hdr_len;
>  		data_len = task_data->resp_len - hdr_len;
> +		local_bh_disable();
>  		iscsi_complete_pdu(conn, hdr, data, data_len);
> +		local_bh_enable();
>  		break;
>  	default:
>  		ql4_printk(KERN_ERR, ha, "Passthru failed status = 0x%x\n",