lustre: hold lock while walking changelog dev list

This patch came from the 2.10.5 branch of AmazonFSxLustreClient repo.

The patch from that repo is:

    mdc: hold lock while walking changelog dev list (LU-12566)

    Prevent the following GPF which is causing stuck processes,
    when running mount and umount concurrently on the same host.

        general protection fault: 0000 [#1] SMP PTI
        ...
        RIP: 0010:mdc_changelog_cdev_finish+0x3f/0x1b9 [mdc]
        ...
        Call Trace:
           mdc_precleanup+0x2a/0x3c0 [mdc]

    Original patch was:

        LU-12566 mdc: hold lock while walking changelog dev list

        In mdc_changelog_cdev_finish() we need chlg_registered_dev_lock
        while walking and changing entries on the chlog_registered_devs
        and ced_obds lists in chlg_registered_dev_find_by_obd().

        Move the calling of chlg_registered_dev_find_by_obd() under the
        mutex, and add assertions to the places where the lists are walked
        and changed that the mutex is held.

        Lustre-change: https://review.whamcloud.com/35668
        Lustre-commit: a260c530801db7f58efa93b774f06b0ce72649a3

        Fixes: 1d40214d96dd ("LU-7659 mdc: expose changelog through char devices")
        Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
        Change-Id: Ib62fdff87cde6a4bcfb9bea24a2ea72a933ebbe5
        Signed-off-by: Minh Diep <mdiep@whamcloud.com>
        Reviewed-on: https://review.whamcloud.com/35835

Signed-off-by: Andy Strohman <astroh@amazon.com>

Author: Andy Strohman

drivers/staging/lustrefsx/lustre/mdc/mdc_changelog.c

@@ -612,6 +612,7 @@ chlg_registered_dev_find_by_name(const char *name)
 {
 	struct chlg_registered_dev *dit;
 
+	LASSERT(mutex_is_locked(&chlg_registered_dev_lock));
 	list_for_each_entry(dit, &chlg_registered_devices, ced_link)
 		if (strcmp(name, dit->ced_name) == 0)
 			return dit;
@@ -630,6 +631,7 @@ chlg_registered_dev_find_by_obd(const struct obd_device *obd)
 	struct chlg_registered_dev *dit;
 	struct obd_device *oit;
 
+	LASSERT(mutex_is_locked(&chlg_registered_dev_lock));
 	list_for_each_entry(dit, &chlg_registered_devices, ced_link)
 		list_for_each_entry(oit, &dit->ced_obds,
 				    u.cli.cl_chg_dev_linkage)
@@ -702,6 +704,7 @@ static void chlg_dev_clear(struct kref *kref)
 						      ced_refs);
 	ENTRY;
 
+	LASSERT(mutex_is_locked(&chlg_registered_dev_lock));
 	list_del(&entry->ced_link);
 	misc_deregister(&entry->ced_misc);
 	OBD_FREE_PTR(entry);
@@ -713,10 +716,11 @@ static void chlg_dev_clear(struct kref *kref)
  */
 void mdc_changelog_cdev_finish(struct obd_device *obd)
 {
-	struct chlg_registered_dev *dev = chlg_registered_dev_find_by_obd(obd);
-	ENTRY;
+	struct chlg_registered_dev *dev;
 
+	ENTRY;
 	mutex_lock(&chlg_registered_dev_lock);
+	dev = chlg_registered_dev_find_by_obd(obd);
 	list_del_init(&obd->u.cli.cl_chg_dev_linkage);
 	kref_put(&dev->ced_refs, chlg_dev_clear);
 	mutex_unlock(&chlg_registered_dev_lock);