cxl/pmem: Fix reference counting for delayed work

There is a potential race between queue_work() returning and the
queued-work running that could result in put_device() running before
get_device(). Introduce the cxl_nvdimm_bridge_state_work() helper that
takes the reference unconditionally, but drops it if no new work was
queued, to keep the references balanced.

Fixes: 8fdcb170 ("cxl/pmem: Add initial infrastructure for pmem support")
Cc: <stable@vger.kernel.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Link: https://lore.kernel.org/r/163553734757.2509761.3305231863616785470.stgit@dwillia2-desk3.amr.corp.intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/pmem.c

@@ -266,14 +266,24 @@ static void cxl_nvb_update_state(struct work_struct *work)
 	put_device(&cxl_nvb->dev);
 }
 
+static void cxl_nvdimm_bridge_state_work(struct cxl_nvdimm_bridge *cxl_nvb)
+{
+	/*
+	 * Take a reference that the workqueue will drop if new work
+	 * gets queued.
+	 */
+	get_device(&cxl_nvb->dev);
+	if (!queue_work(cxl_pmem_wq, &cxl_nvb->state_work))
+		put_device(&cxl_nvb->dev);
+}
+
 static void cxl_nvdimm_bridge_remove(struct device *dev)
 {
 	struct cxl_nvdimm_bridge *cxl_nvb = to_cxl_nvdimm_bridge(dev);
 
 	if (cxl_nvb->state == CXL_NVB_ONLINE)
 		cxl_nvb->state = CXL_NVB_OFFLINE;
-	if (queue_work(cxl_pmem_wq, &cxl_nvb->state_work))
-		get_device(&cxl_nvb->dev);
+	cxl_nvdimm_bridge_state_work(cxl_nvb);
 }
 
 static int cxl_nvdimm_bridge_probe(struct device *dev)
@@ -294,8 +304,7 @@ static int cxl_nvdimm_bridge_probe(struct device *dev)
 	}
 
 	cxl_nvb->state = CXL_NVB_ONLINE;
-	if (queue_work(cxl_pmem_wq, &cxl_nvb->state_work))
-		get_device(&cxl_nvb->dev);
+	cxl_nvdimm_bridge_state_work(cxl_nvb);
 
 	return 0;
 }