Merge tag 'mm-nonmm-stable-2025-12-06-11-14' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Pull non-MM updates from Andrew Morton:

 - "panic: sys_info: Refactor and fix a potential issue" (Andy Shevchenko)
   fixes a build issue and does some cleanup in ib/sys_info.c

 - "Implement mul_u64_u64_div_u64_roundup()" (David Laight)
   enhances the 64-bit math code on behalf of a PWM driver and beefs up
   the test module for these library functions

 - "scripts/gdb/symbols: make BPF debug info available to GDB" (Ilya Leoshkevich)
   makes BPF symbol names, sizes, and line numbers available to the GDB
   debugger

 - "Enable hung_task and lockup cases to dump system info on demand" (Feng Tang)
   adds a sysctl which can be used to cause additional info dumping when
   the hung-task and lockup detectors fire

 - "lib/base64: add generic encoder/decoder, migrate users" (Kuan-Wei Chiu)
   adds a general base64 encoder/decoder to lib/ and migrates several
   users away from their private implementations

 - "rbree: inline rb_first() and rb_last()" (Eric Dumazet)
   makes TCP a little faster

 - "liveupdate: Rework KHO for in-kernel users" (Pasha Tatashin)
   reworks the KEXEC Handover interfaces in preparation for Live Update
   Orchestrator (LUO), and possibly for other future clients

 - "kho: simplify state machine and enable dynamic updates" (Pasha Tatashin)
   increases the flexibility of KEXEC Handover. Also preparation for LUO

 - "Live Update Orchestrator" (Pasha Tatashin)
   is a major new feature targeted at cloud environments. Quoting the
   cover letter:

      This series introduces the Live Update Orchestrator, a kernel
      subsystem designed to facilitate live kernel updates using a
      kexec-based reboot. This capability is critical for cloud
      environments, allowing hypervisors to be updated with minimal
      downtime for running virtual machines. LUO achieves this by
      preserving the state of selected resources, such as memory,
      devices and their dependencies, across the kernel transition.

      As a key feature, this series includes support for preserving
      memfd file descriptors, which allows critical in-memory data, such
      as guest RAM or any other large memory region, to be maintained in
      RAM across the kexec reboot.

   Mike Rappaport merits a mention here, for his extensive review and
   testing work.

 - "kexec: reorganize kexec and kdump sysfs" (Sourabh Jain)
   moves the kexec and kdump sysfs entries from /sys/kernel/ to
   /sys/kernel/kexec/ and adds back-compatibility symlinks which can
   hopefully be removed one day

 - "kho: fixes for vmalloc restoration" (Mike Rapoport)
   fixes a BUG which was being hit during KHO restoration of vmalloc()
   regions

* tag 'mm-nonmm-stable-2025-12-06-11-14' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm: (139 commits)
  calibrate: update header inclusion
  Reinstate "resource: avoid unnecessary lookups in find_next_iomem_res()"
  vmcoreinfo: track and log recoverable hardware errors
  kho: fix restoring of contiguous ranges of order-0 pages
  kho: kho_restore_vmalloc: fix initialization of pages array
  MAINTAINERS: TPM DEVICE DRIVER: update the W-tag
  init: replace simple_strtoul with kstrtoul to improve lpj_setup
  KHO: fix boot failure due to kmemleak access to non-PRESENT pages
  Documentation/ABI: new kexec and kdump sysfs interface
  Documentation/ABI: mark old kexec sysfs deprecated
  kexec: move sysfs entries to /sys/kernel/kexec
  test_kho: always print restore status
  kho: free chunks using free_page() instead of kfree()
  selftests/liveupdate: add kexec test for multiple and empty sessions
  selftests/liveupdate: add simple kexec-based selftest for LUO
  selftests/liveupdate: add userspace API selftests
  docs: add documentation for memfd preservation via LUO
  mm: memfd_luo: allow preserving memfd
  liveupdate: luo_file: add private argument to store runtime state
  mm: shmem: export some functions to internal.h
  ...

1 files changed, 646 insertions, 0 deletions

diff --git a/kernel/liveupdate/luo_session.c b/kernel/liveupdate/luo_session.c
new file mode 100644
index 000000000000..dbdbc3bd7929
--- /dev/null
+++ b/kernel/liveupdate/luo_session.c
@@ -0,0 +1,646 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright (c) 2025, Google LLC.
+ * Pasha Tatashin <pasha.tatashin@soleen.com>
+ */
+
+/**
+ * DOC: LUO Sessions
+ *
+ * LUO Sessions provide the core mechanism for grouping and managing `struct
+ * file *` instances that need to be preserved across a kexec-based live
+ * update. Each session acts as a named container for a set of file objects,
+ * allowing a userspace agent to manage the lifecycle of resources critical to a
+ * workload.
+ *
+ * Core Concepts:
+ *
+ * - Named Containers: Sessions are identified by a unique, user-provided name,
+ *   which is used for both creation in the current kernel and retrieval in the
+ *   next kernel.
+ *
+ * - Userspace Interface: Session management is driven from userspace via
+ *   ioctls on /dev/liveupdate.
+ *
+ * - Serialization: Session metadata is preserved using the KHO framework. When
+ *   a live update is triggered via kexec, an array of `struct luo_session_ser`
+ *   is populated and placed in a preserved memory region. An FDT node is also
+ *   created, containing the count of sessions and the physical address of this
+ *   array.
+ *
+ * Session Lifecycle:
+ *
+ * 1.  Creation: A userspace agent calls `luo_session_create()` to create a
+ *     new, empty session and receives a file descriptor for it.
+ *
+ * 2.  Serialization: When the `reboot(LINUX_REBOOT_CMD_KEXEC)` syscall is
+ *     made, `luo_session_serialize()` is called. It iterates through all
+ *     active sessions and writes their metadata into a memory area preserved
+ *     by KHO.
+ *
+ * 3.  Deserialization (in new kernel): After kexec, `luo_session_deserialize()`
+ *     runs, reading the serialized data and creating a list of `struct
+ *     luo_session` objects representing the preserved sessions.
+ *
+ * 4.  Retrieval: A userspace agent in the new kernel can then call
+ *     `luo_session_retrieve()` with a session name to get a new file
+ *     descriptor and access the preserved state.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/anon_inodes.h>
+#include <linux/cleanup.h>
+#include <linux/err.h>
+#include <linux/errno.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/io.h>
+#include <linux/kexec_handover.h>
+#include <linux/kho/abi/luo.h>
+#include <linux/libfdt.h>
+#include <linux/list.h>
+#include <linux/liveupdate.h>
+#include <linux/mutex.h>
+#include <linux/rwsem.h>
+#include <linux/slab.h>
+#include <linux/unaligned.h>
+#include <uapi/linux/liveupdate.h>
+#include "luo_internal.h"
+
+/* 16 4K pages, give space for 744 sessions */
+#define LUO_SESSION_PGCNT	16ul
+#define LUO_SESSION_MAX		(((LUO_SESSION_PGCNT << PAGE_SHIFT) -	\
+		sizeof(struct luo_session_header_ser)) /		\
+		sizeof(struct luo_session_ser))
+
+/**
+ * struct luo_session_header - Header struct for managing LUO sessions.
+ * @count:      The number of sessions currently tracked in the @list.
+ * @list:       The head of the linked list of `struct luo_session` instances.
+ * @rwsem:      A read-write semaphore providing synchronized access to the
+ *              session list and other fields in this structure.
+ * @header_ser: The header data of serialization array.
+ * @ser:        The serialized session data (an array of
+ *              `struct luo_session_ser`).
+ * @active:     Set to true when first initialized. If previous kernel did not
+ *              send session data, active stays false for incoming.
+ */
+struct luo_session_header {
+	long count;
+	struct list_head list;
+	struct rw_semaphore rwsem;
+	struct luo_session_header_ser *header_ser;
+	struct luo_session_ser *ser;
+	bool active;
+};
+
+/**
+ * struct luo_session_global - Global container for managing LUO sessions.
+ * @incoming:     The sessions passed from the previous kernel.
+ * @outgoing:     The sessions that are going to be passed to the next kernel.
+ */
+struct luo_session_global {
+	struct luo_session_header incoming;
+	struct luo_session_header outgoing;
+};
+
+static struct luo_session_global luo_session_global = {
+	.incoming = {
+		.list = LIST_HEAD_INIT(luo_session_global.incoming.list),
+		.rwsem = __RWSEM_INITIALIZER(luo_session_global.incoming.rwsem),
+	},
+	.outgoing = {
+		.list = LIST_HEAD_INIT(luo_session_global.outgoing.list),
+		.rwsem = __RWSEM_INITIALIZER(luo_session_global.outgoing.rwsem),
+	},
+};
+
+static struct luo_session *luo_session_alloc(const char *name)
+{
+	struct luo_session *session = kzalloc(sizeof(*session), GFP_KERNEL);
+
+	if (!session)
+		return ERR_PTR(-ENOMEM);
+
+	strscpy(session->name, name, sizeof(session->name));
+	INIT_LIST_HEAD(&session->file_set.files_list);
+	luo_file_set_init(&session->file_set);
+	INIT_LIST_HEAD(&session->list);
+	mutex_init(&session->mutex);
+
+	return session;
+}
+
+static void luo_session_free(struct luo_session *session)
+{
+	luo_file_set_destroy(&session->file_set);
+	mutex_destroy(&session->mutex);
+	kfree(session);
+}
+
+static int luo_session_insert(struct luo_session_header *sh,
+			      struct luo_session *session)
+{
+	struct luo_session *it;
+
+	guard(rwsem_write)(&sh->rwsem);
+
+	/*
+	 * For outgoing we should make sure there is room in serialization array
+	 * for new session.
+	 */
+	if (sh == &luo_session_global.outgoing) {
+		if (sh->count == LUO_SESSION_MAX)
+			return -ENOMEM;
+	}
+
+	/*
+	 * For small number of sessions this loop won't hurt performance
+	 * but if we ever start using a lot of sessions, this might
+	 * become a bottle neck during deserialization time, as it would
+	 * cause O(n*n) complexity.
+	 */
+	list_for_each_entry(it, &sh->list, list) {
+		if (!strncmp(it->name, session->name, sizeof(it->name)))
+			return -EEXIST;
+	}
+	list_add_tail(&session->list, &sh->list);
+	sh->count++;
+
+	return 0;
+}
+
+static void luo_session_remove(struct luo_session_header *sh,
+			       struct luo_session *session)
+{
+	guard(rwsem_write)(&sh->rwsem);
+	list_del(&session->list);
+	sh->count--;
+}
+
+static int luo_session_finish_one(struct luo_session *session)
+{
+	guard(mutex)(&session->mutex);
+	return luo_file_finish(&session->file_set);
+}
+
+static void luo_session_unfreeze_one(struct luo_session *session,
+				     struct luo_session_ser *ser)
+{
+	guard(mutex)(&session->mutex);
+	luo_file_unfreeze(&session->file_set, &ser->file_set_ser);
+}
+
+static int luo_session_freeze_one(struct luo_session *session,
+				  struct luo_session_ser *ser)
+{
+	guard(mutex)(&session->mutex);
+	return luo_file_freeze(&session->file_set, &ser->file_set_ser);
+}
+
+static int luo_session_release(struct inode *inodep, struct file *filep)
+{
+	struct luo_session *session = filep->private_data;
+	struct luo_session_header *sh;
+
+	/* If retrieved is set, it means this session is from incoming list */
+	if (session->retrieved) {
+		int err = luo_session_finish_one(session);
+
+		if (err) {
+			pr_warn("Unable to finish session [%s] on release\n",
+				session->name);
+			return err;
+		}
+		sh = &luo_session_global.incoming;
+	} else {
+		scoped_guard(mutex, &session->mutex)
+			luo_file_unpreserve_files(&session->file_set);
+		sh = &luo_session_global.outgoing;
+	}
+
+	luo_session_remove(sh, session);
+	luo_session_free(session);
+
+	return 0;
+}
+
+static int luo_session_preserve_fd(struct luo_session *session,
+				   struct luo_ucmd *ucmd)
+{
+	struct liveupdate_session_preserve_fd *argp = ucmd->cmd;
+	int err;
+
+	guard(mutex)(&session->mutex);
+	err = luo_preserve_file(&session->file_set, argp->token, argp->fd);
+	if (err)
+		return err;
+
+	err = luo_ucmd_respond(ucmd, sizeof(*argp));
+	if (err)
+		pr_warn("The file was successfully preserved, but response to user failed\n");
+
+	return err;
+}
+
+static int luo_session_retrieve_fd(struct luo_session *session,
+				   struct luo_ucmd *ucmd)
+{
+	struct liveupdate_session_retrieve_fd *argp = ucmd->cmd;
+	struct file *file;
+	int err;
+
+	argp->fd = get_unused_fd_flags(O_CLOEXEC);
+	if (argp->fd < 0)
+		return argp->fd;
+
+	guard(mutex)(&session->mutex);
+	err = luo_retrieve_file(&session->file_set, argp->token, &file);
+	if (err < 0)
+		goto  err_put_fd;
+
+	err = luo_ucmd_respond(ucmd, sizeof(*argp));
+	if (err)
+		goto err_put_file;
+
+	fd_install(argp->fd, file);
+
+	return 0;
+
+err_put_file:
+	fput(file);
+err_put_fd:
+	put_unused_fd(argp->fd);
+
+	return err;
+}
+
+static int luo_session_finish(struct luo_session *session,
+			      struct luo_ucmd *ucmd)
+{
+	struct liveupdate_session_finish *argp = ucmd->cmd;
+	int err = luo_session_finish_one(session);
+
+	if (err)
+		return err;
+
+	return luo_ucmd_respond(ucmd, sizeof(*argp));
+}
+
+union ucmd_buffer {
+	struct liveupdate_session_finish finish;
+	struct liveupdate_session_preserve_fd preserve;
+	struct liveupdate_session_retrieve_fd retrieve;
+};
+
+struct luo_ioctl_op {
+	unsigned int size;
+	unsigned int min_size;
+	unsigned int ioctl_num;
+	int (*execute)(struct luo_session *session, struct luo_ucmd *ucmd);
+};
+
+#define IOCTL_OP(_ioctl, _fn, _struct, _last)                                  \
+	[_IOC_NR(_ioctl) - LIVEUPDATE_CMD_SESSION_BASE] = {                    \
+		.size = sizeof(_struct) +                                      \
+			BUILD_BUG_ON_ZERO(sizeof(union ucmd_buffer) <          \
+					  sizeof(_struct)),                    \
+		.min_size = offsetofend(_struct, _last),                       \
+		.ioctl_num = _ioctl,                                           \
+		.execute = _fn,                                                \
+	}
+
+static const struct luo_ioctl_op luo_session_ioctl_ops[] = {
+	IOCTL_OP(LIVEUPDATE_SESSION_FINISH, luo_session_finish,
+		 struct liveupdate_session_finish, reserved),
+	IOCTL_OP(LIVEUPDATE_SESSION_PRESERVE_FD, luo_session_preserve_fd,
+		 struct liveupdate_session_preserve_fd, token),
+	IOCTL_OP(LIVEUPDATE_SESSION_RETRIEVE_FD, luo_session_retrieve_fd,
+		 struct liveupdate_session_retrieve_fd, token),
+};
+
+static long luo_session_ioctl(struct file *filep, unsigned int cmd,
+			      unsigned long arg)
+{
+	struct luo_session *session = filep->private_data;
+	const struct luo_ioctl_op *op;
+	struct luo_ucmd ucmd = {};
+	union ucmd_buffer buf;
+	unsigned int nr;
+	int ret;
+
+	nr = _IOC_NR(cmd);
+	if (nr < LIVEUPDATE_CMD_SESSION_BASE || (nr - LIVEUPDATE_CMD_SESSION_BASE) >=
+	    ARRAY_SIZE(luo_session_ioctl_ops)) {
+		return -EINVAL;
+	}
+
+	ucmd.ubuffer = (void __user *)arg;
+	ret = get_user(ucmd.user_size, (u32 __user *)ucmd.ubuffer);
+	if (ret)
+		return ret;
+
+	op = &luo_session_ioctl_ops[nr - LIVEUPDATE_CMD_SESSION_BASE];
+	if (op->ioctl_num != cmd)
+		return -ENOIOCTLCMD;
+	if (ucmd.user_size < op->min_size)
+		return -EINVAL;
+
+	ucmd.cmd = &buf;
+	ret = copy_struct_from_user(ucmd.cmd, op->size, ucmd.ubuffer,
+				    ucmd.user_size);
+	if (ret)
+		return ret;
+
+	return op->execute(session, &ucmd);
+}
+
+static const struct file_operations luo_session_fops = {
+	.owner = THIS_MODULE,
+	.release = luo_session_release,
+	.unlocked_ioctl = luo_session_ioctl,
+};
+
+/* Create a "struct file" for session */
+static int luo_session_getfile(struct luo_session *session, struct file **filep)
+{
+	char name_buf[128];
+	struct file *file;
+
+	lockdep_assert_held(&session->mutex);
+	snprintf(name_buf, sizeof(name_buf), "[luo_session] %s", session->name);
+	file = anon_inode_getfile(name_buf, &luo_session_fops, session, O_RDWR);
+	if (IS_ERR(file))
+		return PTR_ERR(file);
+
+	*filep = file;
+
+	return 0;
+}
+
+int luo_session_create(const char *name, struct file **filep)
+{
+	struct luo_session *session;
+	int err;
+
+	session = luo_session_alloc(name);
+	if (IS_ERR(session))
+		return PTR_ERR(session);
+
+	err = luo_session_insert(&luo_session_global.outgoing, session);
+	if (err)
+		goto err_free;
+
+	scoped_guard(mutex, &session->mutex)
+		err = luo_session_getfile(session, filep);
+	if (err)
+		goto err_remove;
+
+	return 0;
+
+err_remove:
+	luo_session_remove(&luo_session_global.outgoing, session);
+err_free:
+	luo_session_free(session);
+
+	return err;
+}
+
+int luo_session_retrieve(const char *name, struct file **filep)
+{
+	struct luo_session_header *sh = &luo_session_global.incoming;
+	struct luo_session *session = NULL;
+	struct luo_session *it;
+	int err;
+
+	scoped_guard(rwsem_read, &sh->rwsem) {
+		list_for_each_entry(it, &sh->list, list) {
+			if (!strncmp(it->name, name, sizeof(it->name))) {
+				session = it;
+				break;
+			}
+		}
+	}
+
+	if (!session)
+		return -ENOENT;
+
+	guard(mutex)(&session->mutex);
+	if (session->retrieved)
+		return -EINVAL;
+
+	err = luo_session_getfile(session, filep);
+	if (!err)
+		session->retrieved = true;
+
+	return err;
+}
+
+int __init luo_session_setup_outgoing(void *fdt_out)
+{
+	struct luo_session_header_ser *header_ser;
+	u64 header_ser_pa;
+	int err;
+
+	header_ser = kho_alloc_preserve(LUO_SESSION_PGCNT << PAGE_SHIFT);
+	if (IS_ERR(header_ser))
+		return PTR_ERR(header_ser);
+	header_ser_pa = virt_to_phys(header_ser);
+
+	err = fdt_begin_node(fdt_out, LUO_FDT_SESSION_NODE_NAME);
+	err |= fdt_property_string(fdt_out, "compatible",
+				   LUO_FDT_SESSION_COMPATIBLE);
+	err |= fdt_property(fdt_out, LUO_FDT_SESSION_HEADER, &header_ser_pa,
+			    sizeof(header_ser_pa));
+	err |= fdt_end_node(fdt_out);
+
+	if (err)
+		goto err_unpreserve;
+
+	luo_session_global.outgoing.header_ser = header_ser;
+	luo_session_global.outgoing.ser = (void *)(header_ser + 1);
+	luo_session_global.outgoing.active = true;
+
+	return 0;
+
+err_unpreserve:
+	kho_unpreserve_free(header_ser);
+	return err;
+}
+
+int __init luo_session_setup_incoming(void *fdt_in)
+{
+	struct luo_session_header_ser *header_ser;
+	int err, header_size, offset;
+	u64 header_ser_pa;
+	const void *ptr;
+
+	offset = fdt_subnode_offset(fdt_in, 0, LUO_FDT_SESSION_NODE_NAME);
+	if (offset < 0) {
+		pr_err("Unable to get session node: [%s]\n",
+		       LUO_FDT_SESSION_NODE_NAME);
+		return -EINVAL;
+	}
+
+	err = fdt_node_check_compatible(fdt_in, offset,
+					LUO_FDT_SESSION_COMPATIBLE);
+	if (err) {
+		pr_err("Session node incompatible [%s]\n",
+		       LUO_FDT_SESSION_COMPATIBLE);
+		return -EINVAL;
+	}
+
+	header_size = 0;
+	ptr = fdt_getprop(fdt_in, offset, LUO_FDT_SESSION_HEADER, &header_size);
+	if (!ptr || header_size != sizeof(u64)) {
+		pr_err("Unable to get session header '%s' [%d]\n",
+		       LUO_FDT_SESSION_HEADER, header_size);
+		return -EINVAL;
+	}
+
+	header_ser_pa = get_unaligned((u64 *)ptr);
+	header_ser = phys_to_virt(header_ser_pa);
+
+	luo_session_global.incoming.header_ser = header_ser;
+	luo_session_global.incoming.ser = (void *)(header_ser + 1);
+	luo_session_global.incoming.active = true;
+
+	return 0;
+}
+
+int luo_session_deserialize(void)
+{
+	struct luo_session_header *sh = &luo_session_global.incoming;
+	static bool is_deserialized;
+	static int err;
+
+	/* If has been deserialized, always return the same error code */
+	if (is_deserialized)
+		return err;
+
+	is_deserialized = true;
+	if (!sh->active)
+		return 0;
+
+	/*
+	 * Note on error handling:
+	 *
+	 * If deserialization fails (e.g., allocation failure or corrupt data),
+	 * we intentionally skip cleanup of sessions that were already restored.
+	 *
+	 * A partial failure leaves the preserved state inconsistent.
+	 * Implementing a safe "undo" to unwind complex dependencies (sessions,
+	 * files, hardware state) is error-prone and provides little value, as
+	 * the system is effectively in a broken state.
+	 *
+	 * We treat these resources as leaked. The expected recovery path is for
+	 * userspace to detect the failure and trigger a reboot, which will
+	 * reliably reset devices and reclaim memory.
+	 */
+	for (int i = 0; i < sh->header_ser->count; i++) {
+		struct luo_session *session;
+
+		session = luo_session_alloc(sh->ser[i].name);
+		if (IS_ERR(session)) {
+			pr_warn("Failed to allocate session [%s] during deserialization %pe\n",
+				sh->ser[i].name, session);
+			return PTR_ERR(session);
+		}
+
+		err = luo_session_insert(sh, session);
+		if (err) {
+			pr_warn("Failed to insert session [%s] %pe\n",
+				session->name, ERR_PTR(err));
+			luo_session_free(session);
+			return err;
+		}
+
+		scoped_guard(mutex, &session->mutex) {
+			luo_file_deserialize(&session->file_set,
+					     &sh->ser[i].file_set_ser);
+		}
+	}
+
+	kho_restore_free(sh->header_ser);
+	sh->header_ser = NULL;
+	sh->ser = NULL;
+
+	return 0;
+}
+
+int luo_session_serialize(void)
+{
+	struct luo_session_header *sh = &luo_session_global.outgoing;
+	struct luo_session *session;
+	int i = 0;
+	int err;
+
+	guard(rwsem_write)(&sh->rwsem);
+	list_for_each_entry(session, &sh->list, list) {
+		err = luo_session_freeze_one(session, &sh->ser[i]);
+		if (err)
+			goto err_undo;
+
+		strscpy(sh->ser[i].name, session->name,
+			sizeof(sh->ser[i].name));
+		i++;
+	}
+	sh->header_ser->count = sh->count;
+
+	return 0;
+
+err_undo:
+	list_for_each_entry_continue_reverse(session, &sh->list, list) {
+		i--;
+		luo_session_unfreeze_one(session, &sh->ser[i]);
+		memset(sh->ser[i].name, 0, sizeof(sh->ser[i].name));
+	}
+
+	return err;
+}
+
+/**
+ * luo_session_quiesce - Ensure no active sessions exist and lock session lists.
+ *
+ * Acquires exclusive write locks on both incoming and outgoing session lists.
+ * It then validates no sessions exist in either list.
+ *
+ * This mechanism is used during file handler un/registration to ensure that no
+ * sessions are currently using the handler, and no new sessions can be created
+ * while un/registration is in progress.
+ *
+ * This prevents registering new handlers while sessions are active or
+ * while deserialization is in progress.
+ *
+ * Return:
+ * true  - System is quiescent (0 sessions) and locked.
+ * false - Active sessions exist. The locks are released internally.
+ */
+bool luo_session_quiesce(void)
+{
+	down_write(&luo_session_global.incoming.rwsem);
+	down_write(&luo_session_global.outgoing.rwsem);
+
+	if (luo_session_global.incoming.count ||
+	    luo_session_global.outgoing.count) {
+		up_write(&luo_session_global.outgoing.rwsem);
+		up_write(&luo_session_global.incoming.rwsem);
+		return false;
+	}
+
+	return true;
+}
+
+/**
+ * luo_session_resume - Unlock session lists and resume normal activity.
+ *
+ * Releases the exclusive locks acquired by a successful call to
+ * luo_session_quiesce().
+ */
+void luo_session_resume(void)
+{
+	up_write(&luo_session_global.outgoing.rwsem);
+	up_write(&luo_session_global.incoming.rwsem);
+}