cxl/pmem: Fix nvdimm unregistration when cxl_pmem driver is absent

The cxl_pmem.ko module houses the driver for both cxl_nvdimm_bridge
objects and cxl_nvdimm objects. When the core creates a cxl_nvdimm it
arranges for it to be autoremoved when the bridge goes down. However, if
the bridge never initialized because the cxl_pmem.ko module never
loaded, it sets up a the following crash scenario:

    BUG: kernel NULL pointer dereference, address: 0000000000000478
    [..]
    RIP: 0010:cxl_nvdimm_probe+0x99/0x140 [cxl_pmem]
    [..]
    Call Trace:
     <TASK>
     cxl_bus_probe+0x17/0x50 [cxl_core]
     really_probe+0xde/0x380
     __driver_probe_device+0x78/0x170
     driver_probe_device+0x1f/0x90
     __driver_attach+0xd2/0x1c0
     bus_for_each_dev+0x79/0xc0
     bus_add_driver+0x1b1/0x200
     driver_register+0x89/0xe0
     cxl_pmem_init+0x50/0xff0 [cxl_pmem]

It turns out the recent rework to simplify nvdimm probing obviated the
need to unregister cxl_nvdimm objects at cxl_nvdimm_bridge ->remove()
time. Leave the cxl_nvdimm device registered until the hosting
cxl_memdev departs. The alternative is that the cxl_memdev needs to be
reattached whenever the cxl_nvdimm_bridge attach state cycles, which is
awkward and unnecessary.

The only requirement is to make sure that when the cxl_nvdimm_bridge
goes away any dependent cxl_nvdimm objects are shutdown. Handle that in
unregister_nvdimm_bus().

With these registration entanglements removed there is no longer a need
to pre-load the cxl_pmem module in cxl_acpi.

Fixes: cb9cfff8 ("cxl/acpi: Simplify cxl_nvdimm_bridge probing")
Reported-by: Gregory Price <gregory.price@memverge.com>
Debugged-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Tested-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/167426077263.3955046.9695309346988027311.stgit@dwillia2-xfh.jf.intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/acpi.c

@@ -736,4 +736,3 @@ module_exit(cxl_acpi_exit);
 MODULE_LICENSE("GPL v2");
 MODULE_IMPORT_NS(CXL);
 MODULE_IMPORT_NS(ACPI);
-MODULE_SOFTDEP("pre: cxl_pmem");

drivers/cxl/core/pmem.c

@@ -227,34 +227,16 @@ static struct cxl_nvdimm *cxl_nvdimm_alloc(struct cxl_nvdimm_bridge *cxl_nvb,
 	return cxl_nvd;
 }
 
-static void cxl_nvd_unregister(void *_cxl_nvd)
+static void cxlmd_release_nvdimm(void *_cxlmd)
 {
-	struct cxl_nvdimm *cxl_nvd = _cxl_nvd;
-	struct cxl_memdev *cxlmd = cxl_nvd->cxlmd;
+	struct cxl_memdev *cxlmd = _cxlmd;
+	struct cxl_nvdimm *cxl_nvd = cxlmd->cxl_nvd;
 	struct cxl_nvdimm_bridge *cxl_nvb = cxlmd->cxl_nvb;
 
-	/*
-	 * Either the bridge is in ->remove() context under the device_lock(),
-	 * or cxlmd_release_nvdimm() is cancelling the bridge's release action
-	 * for @cxl_nvd and doing it itself (while manually holding the bridge
-	 * lock).
-	 */
-	device_lock_assert(&cxl_nvb->dev);
 	cxl_nvd->cxlmd = NULL;
 	cxlmd->cxl_nvd = NULL;
+	cxlmd->cxl_nvb = NULL;
 	device_unregister(&cxl_nvd->dev);
-}
-
-static void cxlmd_release_nvdimm(void *_cxlmd)
-{
-	struct cxl_memdev *cxlmd = _cxlmd;
-	struct cxl_nvdimm_bridge *cxl_nvb = cxlmd->cxl_nvb;
-
-	device_lock(&cxl_nvb->dev);
-	if (cxlmd->cxl_nvd)
-		devm_release_action(&cxl_nvb->dev, cxl_nvd_unregister,
-				    cxlmd->cxl_nvd);
-	device_unlock(&cxl_nvb->dev);
 	put_device(&cxl_nvb->dev);
 }
 
@@ -293,22 +275,6 @@ int devm_cxl_add_nvdimm(struct cxl_memdev *cxlmd)
 
 	dev_dbg(&cxlmd->dev, "register %s\n", dev_name(dev));
 
-	/*
-	 * The two actions below arrange for @cxl_nvd to be deleted when either
-	 * the top-level PMEM bridge goes down, or the endpoint device goes
-	 * through ->remove().
-	 */
-	device_lock(&cxl_nvb->dev);
-	if (cxl_nvb->dev.driver)
-		rc = devm_add_action_or_reset(&cxl_nvb->dev, cxl_nvd_unregister,
-					      cxl_nvd);
-	else
-		rc = -ENXIO;
-	device_unlock(&cxl_nvb->dev);
-
-	if (rc)
-		goto err_alloc;
-
 	/* @cxlmd carries a reference on @cxl_nvb until cxlmd_release_nvdimm */
 	return devm_add_action_or_reset(&cxlmd->dev, cxlmd_release_nvdimm, cxlmd);
 

drivers/cxl/pmem.c

@@ -225,11 +225,35 @@ static int cxl_pmem_ctl(struct nvdimm_bus_descriptor *nd_desc,
 	return cxl_pmem_nvdimm_ctl(nvdimm, cmd, buf, buf_len);
 }
 
+static int detach_nvdimm(struct device *dev, void *data)
+{
+	struct cxl_nvdimm *cxl_nvd;
+	bool release = false;
+
+	if (!is_cxl_nvdimm(dev))
+		return 0;
+
+	device_lock(dev);
+	if (!dev->driver)
+		goto out;
+
+	cxl_nvd = to_cxl_nvdimm(dev);
+	if (cxl_nvd->cxlmd && cxl_nvd->cxlmd->cxl_nvb == data)
+		release = true;
+out:
+	device_unlock(dev);
+	if (release)
+		device_release_driver(dev);
+	return 0;
+}
+
 static void unregister_nvdimm_bus(void *_cxl_nvb)
 {
 	struct cxl_nvdimm_bridge *cxl_nvb = _cxl_nvb;
 	struct nvdimm_bus *nvdimm_bus = cxl_nvb->nvdimm_bus;
 
+	bus_for_each_dev(&cxl_bus_type, NULL, cxl_nvb, detach_nvdimm);
+
 	cxl_nvb->nvdimm_bus = NULL;
 	nvdimm_bus_unregister(nvdimm_bus);
 }