scsi: ibmvscsi: fix tripping of blk_mq_run_hw_queue WARN_ON

After a successful SRP login response we call scsi_unblock_requests() to
kick any pending IOs. The callback to process this SRP response happens in
a tasklet and therefore is in softirq context. The result of such is that
when blk-mq is enabled, it is no longer safe to call scsi_unblock_requests()
from this context. The result of duing so triggers the following WARN_ON
splat in dmesg after a host reset or CRQ reenablement.

WARNING: CPU: 0 PID: 0 at block/blk-mq.c:1375 __blk_mq_run_hw_queue+0x120/0x180
Modules linked in:
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.0.0-rc8 #4
NIP [c0000000009771e0] __blk_mq_run_hw_queue+0x120/0x180
LR [c000000000977484] __blk_mq_delay_run_hw_queue+0x244/0x250
Call Trace:

__blk_mq_delay_run_hw_queue+0x244/0x250
blk_mq_run_hw_queue+0x8c/0x1c0
blk_mq_run_hw_queues+0x60/0x90
scsi_run_queue+0x1e4/0x3b0
scsi_run_host_queues+0x48/0x80
login_rsp+0xb0/0x100
ibmvscsi_handle_crq+0x30c/0x3e0
ibmvscsi_task+0x54/0xe0
tasklet_action_common.isra.3+0xc4/0x1a0
__do_softirq+0x174/0x3f4
irq_exit+0xf0/0x120
__do_irq+0xb0/0x210
call_do_irq+0x14/0x24
do_IRQ+0x9c/0x130
hardware_interrupt_common+0x14c/0x150

This patch fixes the issue by introducing a new host action for unblocking
the scsi requests in our seperate work thread.
Signed-off-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/ibmvscsi/ibmvscsi.c

@@ -1179,7 +1179,8 @@ static void login_rsp(struct srp_event_struct *evt_struct)
 		   be32_to_cpu(evt_struct->xfer_iu->srp.login_rsp.req_lim_delta));
 
 	/* If we had any pending I/Os, kick them */
-	scsi_unblock_requests(hostdata->host);
+	hostdata->action = IBMVSCSI_HOST_ACTION_UNBLOCK;
+	wake_up(&hostdata->work_wait_q);
 }
 
 /**
@@ -2123,6 +2124,7 @@ static void ibmvscsi_do_work(struct ibmvscsi_host_data *hostdata)
 	spin_lock_irqsave(hostdata->host->host_lock, flags);
 	switch (hostdata->action) {
 	case IBMVSCSI_HOST_ACTION_NONE:
+	case IBMVSCSI_HOST_ACTION_UNBLOCK:
 		break;
 	case IBMVSCSI_HOST_ACTION_RESET:
 		spin_unlock_irqrestore(hostdata->host->host_lock, flags);
@@ -2164,6 +2166,7 @@ static int __ibmvscsi_work_to_do(struct ibmvscsi_host_data *hostdata)
 		return 0;
 	case IBMVSCSI_HOST_ACTION_RESET:
 	case IBMVSCSI_HOST_ACTION_REENABLE:
+	case IBMVSCSI_HOST_ACTION_UNBLOCK:
 	default:
 		break;
 	}

drivers/scsi/ibmvscsi/ibmvscsi.h

@@ -92,6 +92,7 @@ enum ibmvscsi_host_action {
 	IBMVSCSI_HOST_ACTION_NONE = 0,
 	IBMVSCSI_HOST_ACTION_RESET,
 	IBMVSCSI_HOST_ACTION_REENABLE,
+	IBMVSCSI_HOST_ACTION_UNBLOCK,
 };
 
 /* all driver data associated with a host adapter */