cxl/pmem: Fix nvdimm registration races

A loop of the form:

while true; do modprobe cxl_pci; modprobe -r cxl_pci; done

...fails with the following crash signature:

BUG: kernel NULL pointer dereference, address: 0000000000000040
[..]
RIP: 0010:cxl_internal_send_cmd+0x5/0xb0 [cxl_core]
[..]
Call Trace:
<TASK>
cxl_pmem_ctl+0x121/0x240 [cxl_pmem]
nvdimm_get_config_data+0xd6/0x1a0 [libnvdimm]
nd_label_data_init+0x135/0x7e0 [libnvdimm]
nvdimm_probe+0xd6/0x1c0 [libnvdimm]
nvdimm_bus_probe+0x7a/0x1e0 [libnvdimm]
really_probe+0xde/0x380
__driver_probe_device+0x78/0x170
driver_probe_device+0x1f/0x90
__device_attach_driver+0x85/0x110
bus_for_each_drv+0x7d/0xc0
__device_attach+0xb4/0x1e0
bus_probe_device+0x9f/0xc0
device_add+0x445/0x9c0
nd_async_device_register+0xe/0x40 [libnvdimm]
async_run_entry_fn+0x30/0x130

...namely that the bottom half of async nvdimm device registration runs
after the CXL has already torn down the context that cxl_pmem_ctl()
needs. Unlike the ACPI NFIT case that benefits from launching multiple
nvdimm device registrations in parallel from those listed in the table,
CXL is already marked PROBE_PREFER_ASYNCHRONOUS. So provide for a
synchronous registration path to preclude this scenario.

Fixes: 21083f51521f ("cxl/pmem: Register 'pmem' / cxl_nvdimm devices")
Cc: <[e-mail]>
Reported-by: Dave Jiang <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>

drivers/cxl/pmem.c

···
 		return rc;
 
 	set_bit(NDD_LABELING, &flags);
+	set_bit(NDD_REGISTER_SYNC, &flags);
 	set_bit(ND_CMD_GET_CONFIG_SIZE, &cmd_mask);
 	set_bit(ND_CMD_GET_CONFIG_DATA, &cmd_mask);
 	set_bit(ND_CMD_SET_CONFIG_DATA, &cmd_mask);

drivers/nvdimm/bus.c

···
 	put_device(dev);
 }
 
-void nd_device_register(struct device *dev)
+static void __nd_device_register(struct device *dev, bool sync)
 {
 	if (!dev)
 		return;
···
 	}
 	get_device(dev);
 
-	async_schedule_dev_domain(nd_async_device_register, dev,
-				  &nd_async_domain);
+	if (sync)
+		nd_async_device_register(dev, 0);
+	else
+		async_schedule_dev_domain(nd_async_device_register, dev,
+					  &nd_async_domain);
+}
+
+void nd_device_register(struct device *dev)
+{
+	__nd_device_register(dev, false);
 }
 EXPORT_SYMBOL(nd_device_register);
+
+void nd_device_register_sync(struct device *dev)
+{
+	__nd_device_register(dev, true);
+}
 
 void nd_device_unregister(struct device *dev, enum nd_async_mode mode)
 {

drivers/nvdimm/dimm_devs.c

···
 	nvdimm->sec.ext_flags = nvdimm_security_flags(nvdimm, NVDIMM_MASTER);
 	device_initialize(dev);
 	lockdep_set_class(&dev->mutex, &nvdimm_key);
-	nd_device_register(dev);
+	if (test_bit(NDD_REGISTER_SYNC, &flags))
+		nd_device_register_sync(dev);
+	else
+		nd_device_register(dev);
 
 	return nvdimm;
 }

drivers/nvdimm/nd-core.h

···
 void nvdimm_bus_destroy_ndctl(struct nvdimm_bus *nvdimm_bus);
 void nd_synchronize(void);
 void nd_device_register(struct device *dev);
+void nd_device_register_sync(struct device *dev);
 struct nd_label_id;
 char *nd_label_gen_id(struct nd_label_id *label_id, const uuid_t *uuid,
 		      u32 flags);

include/linux/libnvdimm.h

···
 	 */
 	NDD_INCOHERENT = 7,
 
+	/* dimm provider wants synchronous registration by __nvdimm_create() */
+	NDD_REGISTER_SYNC = 8,
+
 	/* need to set a limit somewhere, but yes, this is likely overkill */
 	ND_IOCTL_MAX_BUFLEN = SZ_4M,
 	ND_CMD_MAX_ELEM = 5,