IB/hfi1: Don't cancel unused work item

In the iowait structure, two iowait_work entries were included to queue a
given object: one for normal IB operations, and the other for TID RDMA
operations. For non-TID RDMA operations, the iowait_work structure for TID
RDMA is initialized to contain a NULL function (not used). When the QP is
reset, the function iowait_cancel_work will be called to cancel any
pending work. The problem is that this function will call
cancel_work_sync() for both iowait_work entries, even though the one for
TID RDMA is not used at all. Eventually, the call cascades to
__flush_work(), wherein a WARN_ON will be triggered due to the fact that
work->func is NULL.

The WARN_ON was introduced in commit 4d43d395 ("workqueue: Try to
catch flush_work() without INIT_WORK().")

This patch fixes the issue by making sure that a work function is present
for TID RDMA before calling cancel_work_sync in iowait_cancel_work.

Fixes: 4d43d395 ("workqueue: Try to catch flush_work() without INIT_WORK().")
Fixes: 5da0fc9d ("IB/hfi1: Prepare resource waits for dual leg")
Link: https://lore.kernel.org/r/20191219211941.58387.39883.stgit@awfm-01.aw.intel.comReviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Kaike Wan <kaike.wan@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/hfi1/iowait.c

@@ -81,7 +81,9 @@ void iowait_init(struct iowait *wait, u32 tx_limit,
 void iowait_cancel_work(struct iowait *w)
 {
 	cancel_work_sync(&iowait_get_ib_work(w)->iowork);
-	cancel_work_sync(&iowait_get_tid_work(w)->iowork);
+	/* Make sure that the iowork for TID RDMA is used */
+	if (iowait_get_tid_work(w)->iowork.func)
+		cancel_work_sync(&iowait_get_tid_work(w)->iowork);
 }
 
 /**