--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -248,6 +248,7 @@ config X86
 	select HAVE_RSEQ
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_UNSTABLE_SCHED_CLOCK
+	select HAVE_UMCG			if X86_64
 	select HAVE_USER_RETURN_NOTIFIER
 	select HAVE_GENERIC_VDSO
 	select HOTPLUG_SMT			if SMP
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -371,6 +371,9 @@
 447	common	memfd_secret		sys_memfd_secret
 448	common	process_mrelease	sys_process_mrelease
 449	common	futex_waitv		sys_futex_waitv
+450	common	umcg_ctl		sys_umcg_ctl
+451	common	umcg_wait		sys_umcg_wait
+452	common	umcg_kick		sys_umcg_kick
 
 #
 # Due to a historical design error, certain syscalls are numbered differently
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -83,6 +83,7 @@ struct thread_info {
 #define TIF_NEED_RESCHED	3	/* rescheduling necessary */
 #define TIF_SINGLESTEP		4	/* reenable singlestep on user return*/
 #define TIF_SSBD		5	/* Speculative store bypass disable */
+#define TIF_UMCG		6	/* UMCG return to user hook */
 #define TIF_SPEC_IB		9	/* Indirect branch speculation mitigation */
 #define TIF_SPEC_L1D_FLUSH	10	/* Flush L1D on mm switches (processes) */
 #define TIF_USER_RETURN_NOTIFY	11	/* notify kernel of userspace return */
@@ -107,6 +108,7 @@ struct thread_info {
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
 #define _TIF_SINGLESTEP		(1 << TIF_SINGLESTEP)
 #define _TIF_SSBD		(1 << TIF_SSBD)
+#define _TIF_UMCG		(1 << TIF_UMCG)
 #define _TIF_SPEC_IB		(1 << TIF_SPEC_IB)
 #define _TIF_SPEC_L1D_FLUSH	(1 << TIF_SPEC_L1D_FLUSH)
 #define _TIF_USER_RETURN_NOTIFY	(1 << TIF_USER_RETURN_NOTIFY)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1838,6 +1838,7 @@ static int bprm_execve(struct linux_binp
 	current->fs->in_exec = 0;
 	current->in_execve = 0;
 	rseq_execve(current);
+	umcg_execve(current);
 	acct_update_integrals(current);
 	task_numa_free(current, false);
 	return retval;
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -23,6 +23,10 @@
 # define _TIF_UPROBE			(0)
 #endif
 
+#ifndef _TIF_UMCG
+# define _TIF_UMCG			(0)
+#endif
+
 /*
  * SYSCALL_WORK flags handled in syscall_enter_from_user_mode()
  */
@@ -43,11 +47,13 @@
 				 SYSCALL_WORK_SYSCALL_EMU |		\
 				 SYSCALL_WORK_SYSCALL_AUDIT |		\
 				 SYSCALL_WORK_SYSCALL_USER_DISPATCH |	\
+				 SYSCALL_WORK_SYSCALL_UMCG |		\
 				 ARCH_SYSCALL_WORK_ENTER)
 #define SYSCALL_WORK_EXIT	(SYSCALL_WORK_SYSCALL_TRACEPOINT |	\
 				 SYSCALL_WORK_SYSCALL_TRACE |		\
 				 SYSCALL_WORK_SYSCALL_AUDIT |		\
 				 SYSCALL_WORK_SYSCALL_USER_DISPATCH |	\
+				 SYSCALL_WORK_SYSCALL_UMCG |		\
 				 SYSCALL_WORK_SYSCALL_EXIT_TRAP	|	\
 				 ARCH_SYSCALL_WORK_EXIT)
 
@@ -221,8 +227,11 @@ static inline void local_irq_disable_exi
  */
 static inline void irqentry_irq_enable(struct pt_regs *regs)
 {
-	if (!regs_irqs_disabled(regs))
+	if (!regs_irqs_disabled(regs)) {
 		local_irq_enable();
+		if (user_mode(regs) && (current->flags & PF_UMCG_WORKER))
+			umcg_sys_enter(regs, -1);
+	}
 }
 
 /**
@@ -232,8 +241,11 @@ static inline void irqentry_irq_enable(s
  */
 static inline void irqentry_irq_disable(struct pt_regs *regs)
 {
-	if (!regs_irqs_disabled(regs))
+	if (!regs_irqs_disabled(regs)) {
+		if (user_mode(regs) && (current->flags & PF_UMCG_WORKER))
+			umcg_sys_exit(regs);
 		local_irq_disable();
+	}
 }
 
 /**
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -67,6 +67,7 @@ struct sighand_struct;
 struct signal_struct;
 struct task_delay_info;
 struct task_group;
+struct umcg_task;
 
 /*
  * Task state bitmask. NOTE! These bits are also
@@ -1294,6 +1295,19 @@ struct task_struct {
 	unsigned long rseq_event_mask;
 #endif
 
+#ifdef CONFIG_UMCG
+	/* setup by sys_umcg_ctrl() */
+	clockid_t		umcg_clock;
+	struct umcg_task __user	*umcg_task;
+
+	/* setup by umcg_pin_enter() */
+	struct page		*umcg_page;
+
+	struct task_struct	*umcg_server;
+	struct umcg_task __user *umcg_server_task;
+	struct page		*umcg_server_page;
+#endif
+
 	struct tlbflush_unmap_batch	tlb_ubc;
 
 	union {
@@ -1687,6 +1701,13 @@ extern struct pid *cad_pid;
 #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
 #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
 #define PF_SWAPWRITE		0x00800000	/* Allowed to write to swap */
+
+#ifdef CONFIG_UMCG
+#define PF_UMCG_WORKER		0x01000000	/* UMCG worker */
+#else
+#define PF_UMCG_WORKER		0x00000000
+#endif
+
 #define PF_NO_SETAFFINITY	0x04000000	/* Userland is not allowed to meddle with cpus_mask */
 #define PF_MCE_EARLY		0x08000000      /* Early kill for mce process policy */
 #define PF_MEMALLOC_PIN		0x10000000	/* Allocation context constrained to zones which allow long term pinning. */
@@ -2294,6 +2315,67 @@ static inline void rseq_execve(struct ta
 {
 }
 
+#endif
+
+#ifdef CONFIG_UMCG
+
+extern void umcg_sys_enter(struct pt_regs *regs, long syscall);
+extern void umcg_sys_exit(struct pt_regs *regs);
+extern void umcg_notify_resume(struct pt_regs *regs);
+extern void umcg_worker_exit(void);
+extern void umcg_clear_child(struct task_struct *tsk);
+
+/* Called by bprm_execve() in fs/exec.c. */
+static inline void umcg_execve(struct task_struct *tsk)
+{
+	if (tsk->umcg_task)
+		umcg_clear_child(tsk);
+}
+
+/* Called by do_exit() in kernel/exit.c. */
+static inline void umcg_handle_exit(void)
+{
+	if (current->flags & PF_UMCG_WORKER)
+		umcg_worker_exit();
+}
+
+/*
+ * umcg_wq_worker_[sleeping|running] are called in core.c by
+ * sched_submit_work() and sched_update_worker().
+ */
+extern void umcg_wq_worker_sleeping(struct task_struct *tsk);
+extern void umcg_wq_worker_running(struct task_struct *tsk);
+
+#else  /* CONFIG_UMCG */
+
+static inline void umcg_sys_enter(struct pt_regs *regs, long syscall)
+{
+}
+
+static inline void umcg_sys_exit(struct pt_regs *regs)
+{
+}
+
+static inline void umcg_notify_resume(struct pt_regs *regs)
+{
+}
+
+static inline void umcg_clear_child(struct task_struct *tsk)
+{
+}
+static inline void umcg_execve(struct task_struct *tsk)
+{
+}
+static inline void umcg_handle_exit(void)
+{
+}
+static inline void umcg_wq_worker_sleeping(struct task_struct *tsk)
+{
+}
+static inline void umcg_wq_worker_running(struct task_struct *tsk)
+{
+}
+
 #endif
 
 #ifdef CONFIG_DEBUG_RSEQ
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -72,6 +72,7 @@ struct open_how;
 struct mount_attr;
 struct landlock_ruleset_attr;
 enum landlock_rule_type;
+struct umcg_task;
 
 #include <linux/types.h>
 #include <linux/aio_abi.h>
@@ -1057,6 +1058,9 @@ asmlinkage long sys_landlock_add_rule(in
 		const void __user *rule_attr, __u32 flags);
 asmlinkage long sys_landlock_restrict_self(int ruleset_fd, __u32 flags);
 asmlinkage long sys_memfd_secret(unsigned int flags);
+asmlinkage long sys_umcg_ctl(u32 flags, struct umcg_task __user *self, clockid_t which_clock);
+asmlinkage long sys_umcg_wait(u32 flags, u64 abs_timeout);
+asmlinkage long sys_umcg_kick(u32 flags, pid_t tid);
 
 /*
  * Architecture-specific system calls
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -46,6 +46,7 @@ enum syscall_work_bit {
 	SYSCALL_WORK_BIT_SYSCALL_AUDIT,
 	SYSCALL_WORK_BIT_SYSCALL_USER_DISPATCH,
 	SYSCALL_WORK_BIT_SYSCALL_EXIT_TRAP,
+	SYSCALL_WORK_BIT_SYSCALL_UMCG,
 };
 
 #define SYSCALL_WORK_SECCOMP		BIT(SYSCALL_WORK_BIT_SECCOMP)
@@ -55,6 +56,7 @@ enum syscall_work_bit {
 #define SYSCALL_WORK_SYSCALL_AUDIT	BIT(SYSCALL_WORK_BIT_SYSCALL_AUDIT)
 #define SYSCALL_WORK_SYSCALL_USER_DISPATCH BIT(SYSCALL_WORK_BIT_SYSCALL_USER_DISPATCH)
 #define SYSCALL_WORK_SYSCALL_EXIT_TRAP	BIT(SYSCALL_WORK_BIT_SYSCALL_EXIT_TRAP)
+#define SYSCALL_WORK_SYSCALL_UMCG	BIT(SYSCALL_WORK_BIT_SYSCALL_UMCG)
 #endif
 
 #include <asm/thread_info.h>
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -883,8 +883,15 @@ __SYSCALL(__NR_process_mrelease, sys_pro
 #define __NR_futex_waitv 449
 __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
 
+#define __NR_umcg_ctl 450
+__SYSCALL(__NR_umcg_ctl, sys_umcg_ctl)
+#define __NR_umcg_wait 451
+__SYSCALL(__NR_umcg_wait, sys_umcg_wait)
+#define __NR_umcg_kick 452
+__SYSCALL(__NR_umcg_kick, sys_umcg_kick)
+
 #undef __NR_syscalls
-#define __NR_syscalls 450
+#define __NR_syscalls 453
 
 /*
  * 32 bit systems traditionally used different
--- /dev/null
+++ b/include/uapi/linux/umcg.h
@@ -0,0 +1,141 @@
+/* SPDX-License-Identifier: GPL-2.0+ WITH Linux-syscall-note */
+#ifndef _UAPI_LINUX_UMCG_H
+#define _UAPI_LINUX_UMCG_H
+
+#include <linux/types.h>
+
+/*
+ * UMCG: User Managed Concurrency Groups.
+ *
+ * Syscalls (see kernel/sched/umcg.c):
+ *      sys_umcg_ctl()  - register/unregister UMCG tasks;
+ *      sys_umcg_wait() - wait/wake/context-switch.
+ *      sys_umcg_kick() - prod a UMCG task
+ *
+ * struct umcg_task (below): controls the state of UMCG tasks.
+ */
+
+/*
+ * UMCG task states, the first 8 bits of struct umcg_task.state.
+ *
+ *   ,--------(TF_PREEMPT + notify_resume)-------.   ,----------.
+ *   |                                           v   |          |
+ * RUNNING -(schedule)-> BLOCKED -(sys_exit)-> RUNNABLE  (signal + notify_resume)
+ *   ^                      ^                    | | ^          |
+ *   |                      `-----(schedule)-----' | |          |
+ *   `--------------(sys_umcg_wait)----------------' `----------'
+ *
+ */
+#define UMCG_TASK_NONE			0x0000U
+#define UMCG_TASK_RUNNING		0x0001U
+#define UMCG_TASK_RUNNABLE		0x0002U
+#define UMCG_TASK_BLOCKED		0x0003U
+
+#define UMCG_TASK_MASK			0x00ffU
+
+/*
+ * UMCG_TF_PREEMPT: userspace indicates the worker should be preempted.
+ *
+ * Must only be set on UMCG_TASK_RUNNING; once set, any subsequent
+ * return-to-user (eg sys_umcg_kick()) will perform the equivalent of
+ * sys_umcg_wait() on it. That is, it will wake next_tid/server_tid, transfer
+ * to RUNNABLE and enqueue on the server's runnable list.
+ */
+#define UMCG_TF_PREEMPT			0x0100U
+/*
+ * UMCG_TF_COND_WAIT: indicate the task *will* call sys_umcg_wait()
+ *
+ * Enables server loops like (vs umcg_sys_exit()):
+ *
+ *   for(;;) {
+ *	self->state = UMCG_TASK_RUNNABLE | UMCG_TF_COND_WAIT;
+ *
+ *	// smp_mb() implied by xchg()
+ *	runnable_ptr = (void *)xchg(self->runnable_workers_ptr, NULL);
+ *	while (runnable_ptr) {
+ *		next = (void *)runnable_ptr->runnable_workers_ptr;
+ *		umcg_server_add_runnable(self, runnable_ptr);
+ *		runnable_ptr = next;
+ *	}
+ *
+ *	self->next_tid = umcg_server_pick_next(self);
+ *	sys_umcg_wait(0, 0);
+ *   }
+ *
+ * without a signal or interrupt in between setting umcg_task::state and
+ * sys_umcg_wait() resulting in an infinite wait in umcg_notify_resume().
+ */
+#define UMCG_TF_COND_WAIT		0x0200U
+
+#define UMCG_TF_MASK			0xff00U
+
+#define UMCG_TASK_ALIGN			64
+
+/**
+ * struct umcg_task - controls the state of UMCG tasks.
+ *
+ * The struct is aligned at 64 bytes to ensure that it fits into
+ * a single cache line.
+ */
+struct umcg_task {
+	/**
+	 * @state: the current state of the UMCG task described by
+	 *         this struct.
+	 *
+	 * Readable/writable by both the kernel and the userspace.
+	 *
+	 * UMCG task state:
+	 *   bits  0 -  7: task state;
+	 *   bits  8 - 15: state flags;
+	 *   bits 16 - 31: for userspace use;
+	 */
+	__u32	state;				/* r/w */
+
+#define UMCG_TID_RUNNING	0x80000000U
+#define UMCG_TID_MASK		0x3fffffffU
+	/**
+	 * @next_tid: the TID of the UMCG task that should be context-switched
+	 *            into in sys_umcg_wait(). Can be zero.
+	 *
+	 * @server_tid: the TID of the UMCG server that hosts this task,
+	 *		when RUNNABLE this task will get added to it's
+	 *		runnable_workers_ptr list.
+	 *
+	 * Read-only for the kernel, read/write for the userspace.
+	 */
+	__u32	next_tid;			/* r   */
+	__u32	server_tid;			/* r   */
+
+	__u32	__hole[1];
+
+	/*
+	 * Timestamps for when last we became BLOCKED, RUNNABLE.
+	 */
+	__u64	blocked_ts;			/*   w */
+	__u64   runnable_ts;			/*   w */
+
+	/**
+	 * @runnable_workers_ptr: a single-linked list of runnable workers.
+	 *
+	 * Readable/writable by both the kernel and the userspace: the
+	 * kernel adds items to the list, userspace removes them.
+	 */
+	__u64	runnable_workers_ptr;		/* r/w */
+
+	__u64	__zero[3];
+
+} __attribute__((packed, aligned(UMCG_TASK_ALIGN)));
+
+/**
+ * enum umcg_ctl_flag - flags to pass to sys_umcg_ctl
+ * @UMCG_CTL_REGISTER:   register the current task as a UMCG task
+ * @UMCG_CTL_UNREGISTER: unregister the current task as a UMCG task
+ * @UMCG_CTL_WORKER:     register the current task as a UMCG worker
+ */
+enum umcg_ctl_flag {
+	UMCG_CTL_REGISTER	= 0x00001,
+	UMCG_CTL_UNREGISTER	= 0x00002,
+	UMCG_CTL_WORKER		= 0x10000,
+};
+
+#endif /* _UAPI_LINUX_UMCG_H */
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1686,6 +1686,21 @@ config MEMBARRIER
 
 	  If unsure, say Y.
 
+config HAVE_UMCG
+	bool
+
+config UMCG
+	bool "Enable User Managed Concurrency Groups API"
+	depends on 64BIT
+	depends on GENERIC_ENTRY
+	depends on HAVE_UMCG
+	default n
+	help
+	  Enable User Managed Concurrency Groups API, which form the basis
+	  for an in-process M:N userspace scheduling framework.
+	  At the moment this is an experimental/RFC feature that is not
+	  guaranteed to be backward-compatible.
+
 config KALLSYMS
 	bool "Load all symbols for debugging/ksymoops" if EXPERT
 	default y
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -6,6 +6,7 @@
 #include <linux/livepatch.h>
 #include <linux/audit.h>
 #include <linux/tick.h>
+#include <linux/sched.h>
 
 #include "common.h"
 
@@ -76,6 +77,9 @@ static long syscall_trace_enter(struct p
 	if (unlikely(work & SYSCALL_WORK_SYSCALL_TRACEPOINT))
 		trace_sys_enter(regs, syscall);
 
+	if (work & SYSCALL_WORK_SYSCALL_UMCG)
+		umcg_sys_enter(regs, syscall);
+
 	syscall_enter_audit(regs, syscall);
 
 	return ret ? : syscall;
@@ -155,8 +159,7 @@ static unsigned long exit_to_user_mode_l
 	 * Before returning to user space ensure that all pending work
 	 * items have been completed.
 	 */
-	while (ti_work & EXIT_TO_USER_MODE_WORK) {
-
+	do {
 		local_irq_enable_exit_to_user(ti_work);
 
 		if (ti_work & _TIF_NEED_RESCHED)
@@ -168,6 +171,10 @@ static unsigned long exit_to_user_mode_l
 		if (ti_work & _TIF_PATCH_PENDING)
 			klp_update_patch_state(current);
 
+		/* must be before handle_signal_work(); terminates on sigpending */
+		if (ti_work & _TIF_UMCG)
+			umcg_notify_resume(regs);
+
 		if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL))
 			handle_signal_work(regs, ti_work);
 
@@ -188,7 +195,7 @@ static unsigned long exit_to_user_mode_l
 		tick_nohz_user_enter_prepare();
 
 		ti_work = read_thread_flags();
-	}
+	} while (ti_work & EXIT_TO_USER_MODE_WORK);
 
 	/* Return the latest work state for arch_exit_to_user_mode() */
 	return ti_work;
@@ -203,7 +210,7 @@ static void exit_to_user_mode_prepare(st
 	/* Flush pending rcuog wakeup before the last need_resched() check */
 	tick_nohz_user_enter_prepare();
 
-	if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK))
+	if (unlikely(ti_work & (EXIT_TO_USER_MODE_WORK | _TIF_UMCG)))
 		ti_work = exit_to_user_mode_loop(regs, ti_work);
 
 	arch_exit_to_user_mode_prepare(regs, ti_work);
@@ -253,6 +260,9 @@ static void syscall_exit_work(struct pt_
 	step = report_single_step(work);
 	if (step || work & SYSCALL_WORK_SYSCALL_TRACE)
 		arch_syscall_exit_tracehook(regs, step);
+
+	if (work & SYSCALL_WORK_SYSCALL_UMCG)
+		umcg_sys_exit(regs);
 }
 
 /*
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -749,6 +749,10 @@ void __noreturn do_exit(long code)
 	if (unlikely(!tsk->pid))
 		panic("Attempted to kill the idle task!");
 
+	/* Turn off UMCG sched hooks. */
+	if (unlikely(tsk->flags & PF_UMCG_WORKER))
+		tsk->flags &= ~PF_UMCG_WORKER;
+
 	/*
 	 * If do_exit is called because this processes oopsed, it's possible
 	 * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before
@@ -786,6 +790,7 @@ void __noreturn do_exit(long code)
 
 	io_uring_files_cancel();
 	exit_signals(tsk);  /* sets PF_EXITING */
+	umcg_handle_exit();
 
 	/* sync mm's RSS info before statistics gathering */
 	if (tsk->mm)
--- a/kernel/sched/Makefile
+++ b/kernel/sched/Makefile
@@ -41,3 +41,4 @@ obj-$(CONFIG_MEMBARRIER) += membarrier.o
 obj-$(CONFIG_CPU_ISOLATION) += isolation.o
 obj-$(CONFIG_PSI) += psi.o
 obj-$(CONFIG_SCHED_CORE) += core_sched.o
+obj-$(CONFIG_UMCG) += umcg.o
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4272,6 +4272,7 @@ static void __sched_fork(unsigned long c
 	p->wake_entry.u_flags = CSD_TYPE_TTWU;
 	p->migration_pending = NULL;
 #endif
+	umcg_clear_child(p);
 }
 
 DEFINE_STATIC_KEY_FALSE(sched_numa_balancing);
@@ -6330,9 +6331,11 @@ static inline void sched_submit_work(str
 	 * If a worker goes to sleep, notify and ask workqueue whether it
 	 * wants to wake up a task to maintain concurrency.
 	 */
-	if (task_flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
+	if (task_flags & (PF_WQ_WORKER | PF_IO_WORKER | PF_UMCG_WORKER)) {
 		if (task_flags & PF_WQ_WORKER)
 			wq_worker_sleeping(tsk);
+		else if (task_flags & PF_UMCG_WORKER)
+			umcg_wq_worker_sleeping(tsk);
 		else
 			io_wq_worker_sleeping(tsk);
 	}
@@ -6350,9 +6353,11 @@ static inline void sched_submit_work(str
 
 static void sched_update_worker(struct task_struct *tsk)
 {
-	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
+	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER | PF_UMCG_WORKER)) {
 		if (tsk->flags & PF_WQ_WORKER)
 			wq_worker_running(tsk);
+		else if (tsk->flags & PF_UMCG_WORKER)
+			umcg_wq_worker_running(tsk);
 		else
 			io_wq_worker_running(tsk);
 	}
--- /dev/null
+++ b/kernel/sched/umcg.c
@@ -0,0 +1,954 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+/*
+ * User Managed Concurrency Groups (UMCG).
+ *
+ */
+
+#include <linux/syscalls.h>
+#include <linux/types.h>
+#include <linux/uaccess.h>
+#include <linux/umcg.h>
+#include <linux/mm.h>
+
+#include <asm/syscall.h>
+#include <asm/ptrace.h>
+
+#include "sched.h"
+
+static struct task_struct *umcg_get_task(u32 tid)
+{
+	struct task_struct *tsk = NULL;
+
+	if (tid) {
+		rcu_read_lock();
+		tsk = find_task_by_vpid(tid & UMCG_TID_MASK);
+		if (tsk && current->mm == tsk->mm && tsk->umcg_task)
+			get_task_struct(tsk);
+		else
+			tsk = NULL;
+		rcu_read_unlock();
+	}
+
+	return tsk;
+}
+
+/*
+ * Pinning a page inhibits rmap based unmap for Anon pages. Doing a load
+ * through the user mapping ensures the user mapping exists.
+ */
+#define umcg_pin_and_load(_self, _pagep, _member)				\
+({										\
+	__label__ __out;							\
+	int __ret = -EFAULT;							\
+										\
+	if (pin_user_pages_fast((unsigned long)(_self), 1, 0, &(_pagep)) != 1)	\
+		goto __out;							\
+										\
+	if (!PageAnon(_pagep) ||						\
+	    get_user(_member, &(_self)->_member)) {				\
+		unpin_user_page(_pagep);					\
+		goto __out;							\
+	}									\
+	__ret = 0;								\
+__out:	__ret;									\
+})
+
+/**
+ * umcg_pin_pages: pin pages containing struct umcg_task of
+ *		   this task and its server (possibly this task again).
+ */
+static int umcg_pin_pages(void)
+{
+	struct task_struct *server = NULL, *tsk = current;
+	struct umcg_task __user *self = READ_ONCE(tsk->umcg_task);
+	int server_tid;
+	int ret;
+
+	/* must not have stale state */
+	if (WARN_ON_ONCE(tsk->umcg_page ||
+			 tsk->umcg_server_page ||
+			 tsk->umcg_server_task ||
+			 tsk->umcg_server))
+		return -EBUSY;
+
+	ret = umcg_pin_and_load(self, tsk->umcg_page, server_tid);
+	if (ret)
+		goto clear_self;
+
+	ret = -ESRCH;
+	server = umcg_get_task(server_tid);
+	if (!server)
+		goto unpin_self;
+
+	/* must cache due to possible concurrent change */
+	tsk->umcg_server_task = READ_ONCE(server->umcg_task);
+	ret = umcg_pin_and_load(tsk->umcg_server_task, tsk->umcg_server_page, server_tid);
+	if (ret)
+		goto clear_server;
+
+	tsk->umcg_server = server;
+
+	return 0;
+
+clear_server:
+	tsk->umcg_server_task = NULL;
+	tsk->umcg_server_page = NULL;
+
+unpin_self:
+	unpin_user_page(tsk->umcg_page);
+clear_self:
+	tsk->umcg_page = NULL;
+
+	return ret;
+}
+
+static void umcg_unpin_pages(void)
+{
+	struct task_struct *tsk = current;
+
+	if (tsk->umcg_server) {
+		unpin_user_page(tsk->umcg_page);
+		tsk->umcg_page = NULL;
+
+		unpin_user_page(tsk->umcg_server_page);
+		tsk->umcg_server_page = NULL;
+		tsk->umcg_server_task = NULL;
+
+		put_task_struct(tsk->umcg_server);
+		tsk->umcg_server = NULL;
+	}
+}
+
+static void umcg_clear_task(struct task_struct *tsk)
+{
+	/*
+	 * This is either called for the current task, or for a newly forked
+	 * task that is not yet running, so we don't need strict atomicity
+	 * below.
+	 */
+	if (tsk->umcg_task) {
+		WRITE_ONCE(tsk->umcg_task, NULL);
+		tsk->umcg_page = NULL;
+
+		tsk->umcg_server = NULL;
+		tsk->umcg_server_page = NULL;
+		tsk->umcg_server_task = NULL;
+
+		tsk->flags &= ~PF_UMCG_WORKER;
+		clear_task_syscall_work(tsk, SYSCALL_UMCG);
+		clear_tsk_thread_flag(tsk, TIF_UMCG);
+	}
+}
+
+/* Called for a forked or execve-ed child. */
+void umcg_clear_child(struct task_struct *tsk)
+{
+	umcg_clear_task(tsk);
+}
+
+/* Called both by normally (unregister) and abnormally exiting workers. */
+void umcg_worker_exit(void)
+{
+	umcg_unpin_pages();
+	umcg_clear_task(current);
+}
+
+/*
+ * Do a state transition: @from -> @to.
+ *
+ * Will clear UMCG_TF_PREEMPT, UMCG_TF_COND_WAIT.
+ *
+ * When @to == {BLOCKED,RUNNABLE}, update timestamps.
+ *
+ * Returns:
+ *   0: success
+ *   -EAGAIN: when self->state != @from
+ *   -EFAULT
+ */
+static int umcg_update_state(struct task_struct *tsk,
+			     struct umcg_task __user *self,
+			     u32 from, u32 to)
+{
+	u32 old, new;
+	u64 now;
+
+	if (to >= UMCG_TASK_RUNNABLE) {
+		switch (tsk->umcg_clock) {
+		case CLOCK_REALTIME:      now = ktime_get_real_ns();     break;
+		case CLOCK_MONOTONIC:     now = ktime_get_ns();          break;
+		case CLOCK_BOOTTIME:      now = ktime_get_boottime_ns(); break;
+		case CLOCK_TAI:           now = ktime_get_clocktai_ns(); break;
+		}
+	}
+
+	if (!user_access_begin(self, sizeof(*self)))
+		return -EFAULT;
+
+	unsafe_get_user(old, &self->state, Efault);
+	do {
+		if ((old & UMCG_TASK_MASK) != from)
+			goto fail;
+
+		new = old & ~(UMCG_TASK_MASK |
+			      UMCG_TF_PREEMPT | UMCG_TF_COND_WAIT);
+		new |= to & UMCG_TASK_MASK;
+
+	} while (!unsafe_try_cmpxchg_user(&self->state, &old, new, Efault));
+
+	if (to == UMCG_TASK_BLOCKED)
+		unsafe_put_user(now, &self->blocked_ts, Efault);
+	if (to == UMCG_TASK_RUNNABLE)
+		unsafe_put_user(now, &self->runnable_ts, Efault);
+
+	user_access_end();
+	return 0;
+
+fail:
+	user_access_end();
+	return -EAGAIN;
+
+Efault:
+	user_access_end();
+	return -EFAULT;
+}
+
+#define __UMCG_DIE(stmt, reason)	do {				\
+	stmt;								\
+	pr_warn_ratelimited("%s: killing task %s/%d because: " reason "\n",\
+			    __func__, current->comm, current->pid);	\
+	force_sig(SIGKILL);						\
+	return;								\
+} while (0)
+
+#define UMCG_DIE(reason)	__UMCG_DIE(,reason)
+#define UMCG_DIE_PF(reason)	__UMCG_DIE(pagefault_enable(), reason)
+#define UMCG_DIE_UNPIN(reason)	__UMCG_DIE(umcg_unpin_pages(), reason)
+
+/* Called from syscall enter path and exceptions that can schedule */
+void umcg_sys_enter(struct pt_regs *regs, long syscall)
+{
+	/* avoid recursion vs our own syscalls */
+	if (syscall == __NR_umcg_wait ||
+	    syscall == __NR_umcg_ctl)
+		return;
+
+	/* avoid recursion vs schedule() */
+	current->flags &= ~PF_UMCG_WORKER;
+
+	/*
+	 * Pin all the state on sys_enter() such that we can rely on it
+	 * from dodgy contexts. It is either unpinned from pre-schedule()
+	 * or sys_exit(), whichever comes first, thereby ensuring the pin
+	 * is temporary.
+	 */
+	if (umcg_pin_pages())
+		UMCG_DIE("pin");
+
+	current->flags |= PF_UMCG_WORKER;
+}
+
+static int umcg_wake_task(struct task_struct *tsk, struct umcg_task __user *self)
+{
+	int ret = umcg_update_state(tsk, self, UMCG_TASK_RUNNABLE, UMCG_TASK_RUNNING);
+	if (ret)
+		return ret;
+
+	try_to_wake_up(tsk, TASK_NORMAL, WF_CURRENT_CPU);
+	return 0;
+}
+
+static int umcg_wake_server(struct task_struct *tsk)
+{
+	int ret = umcg_wake_task(tsk->umcg_server, tsk->umcg_server_task);
+	if (ret == -EAGAIN) {
+		/*
+		 * Server could have timed-out or already be running
+		 * due to a runnable enqueue. See umcg_sys_exit().
+		 */
+		ret = 0;
+	}
+	return ret;
+}
+
+/* pre-schedule() */
+void umcg_wq_worker_sleeping(struct task_struct *tsk)
+{
+	struct umcg_task __user *self = READ_ONCE(tsk->umcg_task);
+	int ret;
+
+	if (!tsk->umcg_server) {
+		/*
+		 * Already blocked before, the pages are unpinned.
+		 */
+		return;
+	}
+
+	/* Must not fault, mmap_sem might be held. */
+	pagefault_disable();
+
+	ret = umcg_update_state(tsk, self, UMCG_TASK_RUNNING, UMCG_TASK_BLOCKED);
+	if (ret == -EAGAIN) {
+		/*
+		 * Consider:
+		 *
+		 *   self->state = UMCG_TASK_RUNNABLE | UMCG_TF_COND_WAIT;
+		 *   ...
+		 *   sys_umcg_wait();
+		 *
+		 * and the '...' code doing a blocking syscall/fault. This
+		 * ensures that returns with UMCG_TASK_RUNNING, which will make
+		 * sys_umcg_wait() return with -EAGAIN.
+		 */
+		ret = umcg_update_state(tsk, self, UMCG_TASK_RUNNABLE, UMCG_TASK_BLOCKED);
+	}
+	if (ret)
+		UMCG_DIE_PF("state");
+
+	if (umcg_wake_server(tsk))
+		UMCG_DIE_PF("wake");
+
+	pagefault_enable();
+
+	/*
+	 * We're going to sleep, make sure to unpin the pages, this ensures
+	 * the pins are temporary. Also see umcg_sys_exit().
+	 */
+	umcg_unpin_pages();
+}
+
+/* post-schedule() */
+void umcg_wq_worker_running(struct task_struct *tsk)
+{
+	/* nothing here, see umcg_sys_exit() */
+}
+
+/*
+ * Enqueue @tsk on it's server's runnable list
+ *
+ * Must be called in umcg_pin_pages() context, relies on tsk->umcg_server.
+ *
+ * cmpxchg based single linked list add such that list integrity is never
+ * violated.  Userspace *MUST* remove it from the list before changing ->state.
+ * As such, we must change state to RUNNABLE before enqueue.
+ *
+ * Returns:
+ *   0: success
+ *   -EFAULT
+ */
+static int umcg_enqueue_runnable(struct task_struct *tsk)
+{
+	struct umcg_task __user *server = tsk->umcg_server_task;
+	struct umcg_task __user *self = tsk->umcg_task;
+	u64 first_ptr, *head = &server->runnable_workers_ptr;
+	u64 self_ptr = (unsigned long)self;
+
+	/*
+	 * umcg_pin_pages() did access_ok() on both pointers, use self here
+	 * only because __user_access_begin() isn't available in generic code.
+	 */
+	if (!user_access_begin(self, sizeof(*self)))
+		return -EFAULT;
+
+	unsafe_get_user(first_ptr, head, Efault);
+	do {
+		unsafe_put_user(first_ptr, &self->runnable_workers_ptr, Efault);
+	} while (!unsafe_try_cmpxchg_user(head, &first_ptr, self_ptr, Efault));
+
+	user_access_end();
+	return 0;
+
+Efault:
+	user_access_end();
+	return -EFAULT;
+}
+
+static int umcg_enqueue_and_wake(struct task_struct *tsk)
+{
+	int ret;
+
+	ret = umcg_enqueue_runnable(tsk);
+	if (!ret)
+		ret = umcg_wake_server(tsk);
+
+	return ret;
+}
+
+/*
+ * umcg_wait: Wait for ->state to become RUNNING
+ *
+ * Returns:
+ * 0		- success
+ * -EINTR	- pending signal
+ * -EINVAL	- ::state is not {RUNNABLE,RUNNING}
+ * -ETIMEDOUT
+ * -EFAULT
+ */
+static int umcg_wait(u64 timo)
+{
+	struct task_struct *tsk = current;
+	struct umcg_task __user *self = tsk->umcg_task;
+	struct page *page = NULL;
+	u32 state;
+	int ret;
+
+	for (;;) {
+		set_current_state(TASK_INTERRUPTIBLE);
+
+		ret = -EINTR;
+		if (signal_pending(current))
+			break;
+
+		/*
+		 * Faults can block and scribble our wait state.
+		 */
+		pagefault_disable();
+		if (get_user(state, &self->state)) {
+			pagefault_enable();
+			__set_current_state(TASK_RUNNING);
+
+			ret = -EFAULT;
+			if (page) {
+				unpin_user_page(page);
+				page = NULL;
+				break;
+			}
+
+			ret = umcg_pin_and_load(self, page, state);
+			if (ret) {
+				page = NULL;
+				break;
+			}
+
+			continue;
+		}
+
+		if (page) {
+			unpin_user_page(page);
+			page = NULL;
+		}
+		pagefault_enable();
+
+		state &= UMCG_TASK_MASK;
+		if (state != UMCG_TASK_RUNNABLE) {
+			ret = 0;
+			if (state == UMCG_TASK_RUNNING)
+				break;
+
+			ret = -EINVAL;
+			break;
+		}
+
+		if (!schedule_hrtimeout_range_clock(timo ? &timo : NULL,
+						    tsk->timer_slack_ns,
+						    HRTIMER_MODE_ABS,
+						    tsk->umcg_clock)) {
+			ret = -ETIMEDOUT;
+			break;
+		}
+	}
+	__set_current_state(TASK_RUNNING);
+
+	return ret;
+}
+
+/*
+ * Blocked case for umcg_sys_exit(), shared with sys_umcg_ctl().
+ */
+static void umcg_unblock_and_wait(void)
+{
+	struct task_struct *tsk = current;
+	struct umcg_task __user *self = READ_ONCE(tsk->umcg_task);
+
+	/* avoid recursion vs schedule() */
+	tsk->flags &= ~PF_UMCG_WORKER;
+
+	if (umcg_pin_pages())
+		UMCG_DIE("pin");
+
+	if (umcg_update_state(tsk, self, UMCG_TASK_BLOCKED, UMCG_TASK_RUNNABLE))
+		UMCG_DIE_UNPIN("state");
+
+	if (umcg_enqueue_and_wake(tsk))
+		UMCG_DIE_UNPIN("enqueue-wake");
+
+	umcg_unpin_pages();
+
+	switch (umcg_wait(0)) {
+	case 0:
+	case -EINTR:
+		/* notify_resume will continue the wait after the signal */
+		break;
+
+	default:
+		UMCG_DIE("wait");
+	}
+
+	tsk->flags |= PF_UMCG_WORKER;
+}
+
+/* Called from syscall exit path and exceptions that can schedule */
+void umcg_sys_exit(struct pt_regs *regs)
+{
+	struct task_struct *tsk = current;
+	long syscall = syscall_get_nr(tsk, regs);
+
+	if (syscall == __NR_umcg_wait ||
+	    syscall == __NR_umcg_ctl)
+		return;
+
+	if (tsk->umcg_server) {
+		/*
+		 * Didn't block, we done.
+		 */
+		umcg_unpin_pages();
+		return;
+	}
+
+	umcg_unblock_and_wait();
+}
+
+/* return-to-user path */
+void umcg_notify_resume(struct pt_regs *regs)
+{
+	struct task_struct *tsk = current;
+	struct umcg_task __user *self = tsk->umcg_task;
+	bool worker = tsk->flags & PF_UMCG_WORKER;
+	u32 state;
+
+	/* avoid recursion vs schedule() */
+	if (worker)
+		current->flags &= ~PF_UMCG_WORKER;
+
+	if (get_user(state, &self->state))
+		UMCG_DIE("get-state");
+
+	state &= UMCG_TASK_MASK | UMCG_TF_MASK;
+	if (state == UMCG_TASK_RUNNING)
+		goto done;
+
+	/*
+	 * See comment at UMCG_TF_COND_WAIT; TL;DR: user *will* call
+	 * sys_umcg_wait() and signals/interrupts shouldn't block
+	 * return-to-user.
+	 */
+	if (state == (UMCG_TASK_RUNNABLE | UMCG_TF_COND_WAIT))
+		goto done;
+
+	if (state & UMCG_TF_PREEMPT) {
+		if (umcg_pin_pages())
+			UMCG_DIE("pin");
+
+		if (umcg_update_state(tsk, self,
+				      UMCG_TASK_RUNNING,
+				      UMCG_TASK_RUNNABLE))
+			UMCG_DIE_UNPIN("state");
+
+		if (umcg_enqueue_and_wake(tsk))
+			UMCG_DIE_UNPIN("enqueue-wake");
+
+		umcg_unpin_pages();
+	}
+
+	switch (umcg_wait(0)) {
+	case 0:
+	case -EINTR:
+		/* we will resume the wait after the signal */
+		break;
+
+	default:
+		UMCG_DIE("wait");
+	}
+
+done:
+	if (worker)
+		current->flags |= PF_UMCG_WORKER;
+}
+
+/**
+ * sys_umcg_kick: makes a UMCG task cycle through umcg_notify_resume()
+ *
+ * Returns:
+ * 0		- Ok;
+ * -ESRCH	- not a related UMCG task
+ * -EINVAL	- another error happened (unknown flags, etc..)
+ */
+SYSCALL_DEFINE2(umcg_kick, u32, flags, pid_t, tid)
+{
+	struct task_struct *task = umcg_get_task(tid);
+	if (!task)
+		return -ESRCH;
+
+	if (flags)
+		return -EINVAL;
+
+#ifdef CONFIG_SMP
+	smp_send_reschedule(task_cpu(task));
+#endif
+
+	return 0;
+}
+
+/*
+ * Handles ::next_tid as per sys_umcg_wait().
+ *
+ * ::next_tid		- return
+ * -----------------------------
+ * 0			- 0 (success)
+ *
+ * tid			- -ESRCH (no such task, or not of this UMCG)
+ *			- -EAGAIN (next::state != RUNNABLE)
+ *			- 0 (success, ::next_tid |= RUNNING)
+ *
+ * tid|RUNNING		- -EAGAIN (next::state != RUNNING)
+ *			- 0 (success)
+ *
+ * Returns:
+ *  0: success
+ *  -EFAULT
+ *  -ESRCH
+ *  -EAGAIN
+ */
+static int umcg_wake_next(struct task_struct *tsk, struct umcg_task __user *self)
+{
+	struct umcg_task __user *next_task;
+	struct task_struct *next;
+	u32 next_tid, state;
+	int ret;
+
+	if (get_user(next_tid, &self->next_tid))
+		return -EFAULT;
+
+	if (!next_tid)
+		return 0;
+
+	next = umcg_get_task(next_tid);
+	if (!next)
+		return -ESRCH;
+
+	next_task = READ_ONCE(next->umcg_task);
+
+	if (next_tid & UMCG_TID_RUNNING) {
+		ret = -EFAULT;
+		if (get_user(state, &next_task->state))
+			goto put_next;
+
+		ret = 0;
+		if ((state & UMCG_TASK_MASK) != UMCG_TASK_RUNNING)
+			ret = -EAGAIN;
+
+	} else {
+		ret = umcg_wake_task(next, next_task);
+		if (ret)
+			goto put_next;
+
+		ret = -EFAULT;
+		if (put_user(next_tid | UMCG_TID_RUNNING, &self->next_tid))
+			goto put_next;
+
+		/*
+		 * If this is a worker doing sys_umcg_wait() switching to
+		 * another worker, userspace has the responsibility to update
+		 * server::next_tid.
+		 */
+
+		ret = 0;
+	}
+
+put_next:
+	put_task_struct(next);
+	return ret;
+}
+
+/**
+ * sys_umcg_wait: transfer running context
+ *
+ * Called like:
+ *
+ *	self->state = UMCG_TASK_RUNNABLE | UMCG_TF_COND_WAIT;
+ *	...
+ *	sys_umcg_wait(0, time);
+ *
+ * The syscall will clear TF_COND_WAIT and wait until state becomes RUNNING.
+ * The code '...' must not contain syscalls
+ *
+ * If self->next_tid is set and indicates a valid UMCG task with RUNNABLE state
+ * that task will be made RUNNING and woken -- transfering the running context
+ * to that task. In this case self->next_tid is modified with TID_RUNNING to
+ * indicate self->next_tid is consumed.
+ *
+ * If self->next has TID_RUNNING set, it is validated the related task has
+ * RUNNING state, otherwise -EAGAIN is returned to indicate a new task needs to
+ * be selected.
+ *
+ * If the caller is a worker:
+ *
+ *  - it will be enqueued on the associated server's runnable_workers_ptr list
+ *    and the server will be woken.
+ *
+ *  - when ::next_tid is used to affect a worker-to-worker transfer, it is up
+ *    to userspace to keep server::next_tid consistent.
+ *
+ * The corrolary is that a server setting ::next_tid to 0 will idle.
+ *
+ * Returns:
+ * 0		- OK;
+ * -ETIMEDOUT	- the timeout expired;
+ * -ERANGE	- the timeout is out of range (worker);
+ * -EAGAIN	- ::state wasn't RUNNABLE, concurrent wakeup;
+ * -EFAULT	- failed accessing struct umcg_task __user of the current
+ *		  task, the server or next;
+ * -ESRCH	- the task to wake not found or not a UMCG task;
+ * -EINVAL	- another error happened (e.g. the current task is not a
+ *		  UMCG task, etc.)
+ */
+SYSCALL_DEFINE2(umcg_wait, u32, flags, u64, timo)
+{
+	struct task_struct *tsk = current;
+	struct umcg_task __user *self = READ_ONCE(tsk->umcg_task);
+	bool worker = tsk->flags & PF_UMCG_WORKER;
+	int ret;
+
+	if (!self || flags)
+		return -EINVAL;
+
+	if (worker) {
+		tsk->flags &= ~PF_UMCG_WORKER;
+		if (timo)
+			return -ERANGE;
+	}
+
+	/* see umcg_sys_{enter,exit}() syscall exceptions */
+	ret = umcg_pin_pages();
+	if (ret)
+		goto unblock;
+
+	/*
+	 * Clear UMCG_TF_COND_WAIT *and* check state == RUNNABLE.
+	 */
+	ret = umcg_update_state(tsk, self, UMCG_TASK_RUNNABLE, UMCG_TASK_RUNNABLE);
+	if (ret)
+		goto unpin;
+
+	ret = umcg_wake_next(tsk, self);
+	if (ret)
+		goto unpin;
+
+	if (worker) {
+		/*
+		 * If this fails it is possible ::next_tid is already running
+		 * while this task is not going to block. This violates our
+		 * constraints.
+		 *
+		 * That said, pretty much the only way to make this fail is by
+		 * force munmap()'ing things. In which case one is most welcome
+		 * to the pieces.
+		 */
+		ret = umcg_enqueue_and_wake(tsk);
+		if (ret)
+			goto unpin;
+	}
+
+	umcg_unpin_pages();
+
+	ret = umcg_wait(timo);
+	switch (ret) {
+	case 0:		/* all done */
+	case -EINTR:	/* umcg_notify_resume() will continue the wait */
+		ret = 0;
+		break;
+
+	default:
+		goto unblock;
+	}
+out:
+	if (worker)
+		tsk->flags |= PF_UMCG_WORKER;
+	return ret;
+
+unpin:
+	umcg_unpin_pages();
+unblock:
+	umcg_update_state(tsk, self, UMCG_TASK_RUNNABLE, UMCG_TASK_RUNNING);
+	goto out;
+}
+
+static int umcg_register(struct umcg_task __user *self, u32 flags, clockid_t which_clock)
+{
+	struct task_struct *server;
+	struct umcg_task ut;
+
+	switch (which_clock) {
+	case CLOCK_REALTIME:
+	case CLOCK_MONOTONIC:
+	case CLOCK_BOOTTIME:
+	case CLOCK_TAI:
+		current->umcg_clock = which_clock;
+		break;
+
+	default:
+		return -EINVAL;
+	}
+
+	if (current->umcg_task || !self)
+		return -EINVAL;
+
+	if (copy_from_user(&ut, self, sizeof(ut)))
+		return -EFAULT;
+
+	if (ut.next_tid || ut.__hole[0] || ut.__zero[0] || ut.__zero[1] || ut.__zero[2])
+		return -EINVAL;
+
+	rcu_read_lock();
+	server = find_task_by_vpid(ut.server_tid);
+	if (server && server->mm == current->mm) {
+		if (flags == UMCG_CTL_WORKER) {
+			if (!server->umcg_task ||
+			    (server->flags & PF_UMCG_WORKER))
+				server = NULL;
+		} else {
+			if (server != current)
+				server = NULL;
+		}
+	} else {
+		server = NULL;
+	}
+	rcu_read_unlock();
+
+	if (!server)
+		return -ESRCH;
+
+	if (flags == UMCG_CTL_WORKER) {
+		if ((ut.state & (UMCG_TASK_MASK | UMCG_TF_MASK)) != UMCG_TASK_BLOCKED)
+			return -EINVAL;
+
+		WRITE_ONCE(current->umcg_task, self);
+		current->flags |= PF_UMCG_WORKER;	/* hook schedule() */
+		set_syscall_work(SYSCALL_UMCG);		/* hook syscall */
+		set_thread_flag(TIF_UMCG);		/* hook return-to-user */
+
+		umcg_unblock_and_wait();
+
+	} else {
+		if ((ut.state & (UMCG_TASK_MASK | UMCG_TF_MASK)) != UMCG_TASK_RUNNING)
+			return -EINVAL;
+
+		WRITE_ONCE(current->umcg_task, self);
+		set_thread_flag(TIF_UMCG);		/* hook return-to-user */
+
+		/* umcg_notify_resume() would block if not RUNNING */
+	}
+
+	return 0;
+}
+
+static int umcg_unregister(struct umcg_task __user *self, u32 flags)
+{
+	bool worker = current->flags & PF_UMCG_WORKER;
+	int ret;
+
+	if (!self || self != current->umcg_task)
+		return -EINVAL;
+
+	if (!worker != !(flags & UMCG_CTL_WORKER))
+		return -EINVAL;
+
+	current->flags &= ~PF_UMCG_WORKER;
+
+	ret = umcg_pin_pages();
+	if (ret) {
+		if (worker)
+			current->flags |= PF_UMCG_WORKER;
+		return ret;
+	}
+
+	ret = umcg_update_state(current, self, UMCG_TASK_RUNNING, UMCG_TASK_NONE);
+	if (ret) {
+		if (worker)
+			current->flags |= PF_UMCG_WORKER;
+		return ret;
+	}
+
+	if (worker)
+		umcg_wake_server(current);
+
+	umcg_unpin_pages();
+	umcg_clear_task(current);
+	return 0;
+}
+
+#define UMCG_CTL_CMD	0xff
+
+/**
+ * sys_umcg_ctl: (un)register the current task as a UMCG task.
+ * @flags:       ORed values from enum umcg_ctl_flag; see below;
+ * @self:        a pointer to struct umcg_task that describes this
+ *               task and governs the behavior of sys_umcg_wait.
+ * @which_clock: clockid to use for timestamps and timeouts
+ *
+ * @flags & UMCG_CTL_REGISTER: register a UMCG task:
+ *
+ *	UMCG workers:
+ *	 - @flags & UMCG_CTL_WORKER
+ *	 - self->state must be UMCG_TASK_BLOCKED
+ *
+ *	UMCG servers:
+ *	 - !(@flags & UMCG_CTL_WORKER)
+ *	 - self->state must be UMCG_TASK_RUNNING
+ *
+ *	All tasks:
+ *	 - self->server_tid must be a valid server
+ *	 - self->next_tid must be zero
+ *
+ *	If the conditions above are met, sys_umcg_ctl() immediately returns
+ *	if the registered task is a server. If the registered task is a
+ *	worker it will be added to it's server's runnable_workers_ptr list
+ *	and the server will be woken.
+ *
+ * @flags & UMCG_CTL_UNREGISTER: unregister a UMCG task.
+ *
+ *	UMCG workers:
+ *	 - @flags & UMCG_CTL_WORKER
+ *
+ *	UMCG servers:
+ *	 - !(@flags & UMCG_CTL_WORKER)
+ *
+ *	All tasks:
+ *	 - self must match with UMCG_CTL_REGISTER
+ *	 - self->state must be UMCG_TASK_RUNNING
+ *	 - self->server_tid must be a valid server
+ *
+ *	If the conditions above are met, sys_umcg_ctl() will change state to
+ *	UMCG_TASK_NONE, and for workers, wake either next or server.
+ *
+ * Return:
+ * 0		- success
+ * -EFAULT	- failed to read @self
+ * -EINVAL	- some other error occurred
+ * -ESRCH	- no such server_tid
+ */
+SYSCALL_DEFINE3(umcg_ctl, u32, flags, struct umcg_task __user *, self, clockid_t, which_clock)
+{
+	int cmd = flags & UMCG_CTL_CMD;
+
+	if ((unsigned long)self % UMCG_TASK_ALIGN)
+		return -EINVAL;
+
+	flags &= ~UMCG_CTL_CMD;
+
+	if (flags & ~(UMCG_CTL_WORKER))
+		return -EINVAL;
+
+	switch (cmd) {
+	case UMCG_CTL_REGISTER:
+		return umcg_register(self, flags, which_clock);
+
+	case UMCG_CTL_UNREGISTER:
+		return umcg_unregister(self, flags);
+
+	default:
+		break;
+	}
+
+	return -EINVAL;
+}
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -273,6 +273,11 @@ COND_SYSCALL(landlock_create_ruleset);
 COND_SYSCALL(landlock_add_rule);
 COND_SYSCALL(landlock_restrict_self);
 
+/* kernel/sched/umcg.c */
+COND_SYSCALL(umcg_ctl);
+COND_SYSCALL(umcg_wait);
+COND_SYSCALL(umcg_kick);
+
 /* arch/example/kernel/sys_example.c */
 
 /* mm/fadvise.c */
