tools/testing/nvdimm: fix SIGTERM vs hotplug crash

commit d8d378fa upstream.

The unit tests crash when hotplug races the previous probe. This race
requires that the loading of the nfit_test module be terminated with
SIGTERM, and the module to be unloaded while the ars scan is still
running.

In contrast to the normal nfit driver, the unit test calls
acpi_nfit_init() twice to simulate hotplug, whereas the nominal case
goes through the acpi_nfit_notify() event handler.  The
acpi_nfit_notify() path is careful to flush the previous region
registration before servicing the hotplug event. The unit test was
missing this guarantee.

 BUG: unable to handle kernel NULL pointer dereference at           (null)
 IP: [<ffffffff810cdce7>] pwq_activate_delayed_work+0x47/0x170
 [..]
 Call Trace:
  [<ffffffff810ce186>] pwq_dec_nr_in_flight+0x66/0xa0
  [<ffffffff810ce490>] process_one_work+0x2d0/0x680
  [<ffffffff810ce331>] ? process_one_work+0x171/0x680
  [<ffffffff810ce88e>] worker_thread+0x4e/0x480
  [<ffffffff810ce840>] ? process_one_work+0x680/0x680
  [<ffffffff810ce840>] ? process_one_work+0x680/0x680
  [<ffffffff810d5343>] kthread+0xf3/0x110
  [<ffffffff8199846f>] ret_from_fork+0x1f/0x40
  [<ffffffff810d5250>] ? kthread_create_on_node+0x230/0x230
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/testing/nvdimm/test/nfit.c

@@ -13,6 +13,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 #include <linux/platform_device.h>
 #include <linux/dma-mapping.h>
+#include <linux/workqueue.h>
 #include <linux/libnvdimm.h>
 #include <linux/vmalloc.h>
 #include <linux/device.h>
@@ -1246,6 +1247,7 @@ static int nfit_test_probe(struct platform_device *pdev)
 	if (nfit_test->setup != nfit_test0_setup)
 		return 0;
 
+	flush_work(&acpi_desc->work);
 	nfit_test->setup_hotplug = 1;
 	nfit_test->setup(nfit_test);