powerpc/powernv/dump: Handle multiple writes to ack attribute

Even though we use self removing sysfs helper, we still need
to make sure we do the final kobject delete conditionally.
sysfs_remove_file_self() will handle parallel calls to remove
the sysfs attribute file and returns true only in the caller
that removed the attribute file. The other parallel callers
are returned false. Do the final kobject delete checking
the return value of sysfs_remove_file_self().
Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201017164236.264713-1-hegdevasant@linux.vnet.ibm.com

arch/powerpc/platforms/powernv/opal-dump.c

@@ -88,9 +88,14 @@ static ssize_t dump_ack_store(struct dump_obj *dump_obj,
 			      const char *buf,
 			      size_t count)
 {
+	/*
+	 * Try to self remove this attribute. If we are successful,
+	 * delete the kobject itself.
+	 */
+	if (sysfs_remove_file_self(&dump_obj->kobj, &attr->attr)) {
 		dump_send_ack(dump_obj->id);
-	sysfs_remove_file_self(&dump_obj->kobj, &attr->attr);
 		kobject_put(&dump_obj->kobj);
+	}
 	return count;
 }