scsi: zfcp: Avoid benign overflow of the Request Queue's free-level

zfcp_qdio_send() and zfcp_qdio_int_req() run concurrently, adding and
completing SBALs on the Request Queue. There's a theoretical race where
zfcp_qdio_int_req() completes a number of SBALs & increments the queue's
free-level _before_ zfcp_qdio_send() was able to decrement it.

This can cause ->req_q_free to momentarily hold a value larger than
QDIO_MAX_BUFFERS_PER_Q. Luckily zfcp_qdio_send() is always called under
->req_q_lock, and all readers of the free-level also take this lock. So we
can trust that zfcp_qdio_send() will clean up such a temporary overflow
before anyone can actually observe it.

But it's still confusing and annoying to worry about. So adjust the code to
avoid this race.

Link: https://lore.kernel.org/r/7f61f59a1f8db270312e64644f9173b8f1ac895f.1593780621.git.bblock@linux.ibm.comReviewed-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/s390/scsi/zfcp_qdio.c

@@ -260,17 +260,20 @@ int zfcp_qdio_send(struct zfcp_qdio *qdio, struct zfcp_qdio_req *q_req)
 	zfcp_qdio_account(qdio);
 	spin_unlock(&qdio->stat_lock);
 
+	atomic_sub(sbal_number, &qdio->req_q_free);
+
 	retval = do_QDIO(qdio->adapter->ccw_device, QDIO_FLAG_SYNC_OUTPUT, 0,
 			 q_req->sbal_first, sbal_number);
 
 	if (unlikely(retval)) {
+		/* Failed to submit the IO, roll back our modifications. */
+		atomic_add(sbal_number, &qdio->req_q_free);
 		zfcp_qdio_zero_sbals(qdio->req_q, q_req->sbal_first,
 				     sbal_number);
 		return retval;
 	}
 
 	/* account for transferred buffers */
-	atomic_sub(sbal_number, &qdio->req_q_free);
 	qdio->req_q_idx += sbal_number;
 	qdio->req_q_idx %= QDIO_MAX_BUFFERS_PER_Q;