[PATCH v0.9.1 0/6] sched,mm,x86/uaccess: implement User Managed Concurrency Groups

@@ -3977,8 +3977,7 @@ bool ttwu_state_match(struct task_struct *p, unsigned int state, int *success)
  * Return: %true if @p->state changes (an actual wakeup was done),
  *	   %false otherwise.
  */
-static int
-try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
+int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 {
 	unsigned long flags;
 	int cpu, success = 0;

@@ -6890,6 +6890,10 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 	if (wake_flags & WF_TTWU) {
 		record_wakee(p);

+		if ((wake_flags & WF_CURRENT_CPU) &&
+		    cpumask_test_cpu(cpu, p->cpus_ptr))
+			return cpu;
+
 		if (sched_energy_enabled()) {
 			new_cpu = find_energy_efficient_cpu(p, prev_cpu);
 			if (new_cpu >= 0)

@@ -2052,13 +2052,14 @@ static inline int task_on_rq_migrating(struct task_struct *p)
 }

 /* Wake flags. The first three directly map to some SD flag value */
-#define WF_EXEC     0x02 /* Wakeup after exec; maps to SD_BALANCE_EXEC */
-#define WF_FORK     0x04 /* Wakeup after fork; maps to SD_BALANCE_FORK */
-#define WF_TTWU     0x08 /* Wakeup;            maps to SD_BALANCE_WAKE */
+#define WF_EXEC         0x02 /* Wakeup after exec; maps to SD_BALANCE_EXEC */
+#define WF_FORK         0x04 /* Wakeup after fork; maps to SD_BALANCE_FORK */
+#define WF_TTWU         0x08 /* Wakeup;            maps to SD_BALANCE_WAKE */

-#define WF_SYNC     0x10 /* Waker goes to sleep after wakeup */
-#define WF_MIGRATED 0x20 /* Internal use, task got migrated */
-#define WF_ON_CPU   0x40 /* Wakee is on_cpu */
+#define WF_SYNC         0x10 /* Waker goes to sleep after wakeup */
+#define WF_MIGRATED     0x20 /* Internal use, task got migrated */
+#define WF_ON_CPU       0x40 /* Wakee is on_cpu */
+#define WF_CURRENT_CPU  0x80 /* Prefer to move the wakee to the current CPU. */

 #ifdef CONFIG_SMP
 static_assert(WF_EXEC == SD_BALANCE_EXEC);

@@ -3076,6 +3077,8 @@ static inline bool is_per_cpu_kthread(struct task_struct *p)
 extern void swake_up_all_locked(struct swait_queue_head *q);
 extern void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait);

+extern int try_to_wake_up(struct task_struct *tsk, unsigned int state, int wake_flags);
+
 #ifdef CONFIG_PREEMPT_DYNAMIC
 extern int preempt_dynamic_mode;
 extern int sched_dynamic_mode(const char *str);
--

@@ -79,4 +79,97 @@ __copy_from_user_flushcache(void *dst, const void __user *src, unsigned size)
 	kasan_check_write(dst, size);
 	return __copy_user_flushcache(dst, src, size);
 }
+
+#define ARCH_HAS_ATOMIC_UACCESS_HELPERS 1
+
+static inline int __try_cmpxchg_user_32(u32 *uval, u32 __user *uaddr,
+						u32 oldval, u32 newval)
+{
+	int ret = 0;
+
+	asm volatile("\n"
+		"1:\t" LOCK_PREFIX "cmpxchgl %4, %2\n"
+		"2:\n"
+		"\t.section .fixup, \"ax\"\n"
+		"3:\tmov     %3, %0\n"
+		"\tjmp     2b\n"
+		"\t.previous\n"
+		_ASM_EXTABLE_UA(1b, 3b)
+		: "+r" (ret), "=a" (oldval), "+m" (*uaddr)
+		: "i" (-EFAULT), "r" (newval), "1" (oldval)
+		: "memory"
+	);
+	*uval = oldval;
+	return ret;
+}
+
+static inline int __try_cmpxchg_user_64(u64 *uval, u64 __user *uaddr,
+						u64 oldval, u64 newval)
+{
+	int ret = 0;
+
+	asm volatile("\n"
+		"1:\t" LOCK_PREFIX "cmpxchgq %4, %2\n"
+		"2:\n"
+		"\t.section .fixup, \"ax\"\n"
+		"3:\tmov     %3, %0\n"
+		"\tjmp     2b\n"
+		"\t.previous\n"
+		_ASM_EXTABLE_UA(1b, 3b)
+		: "+r" (ret), "=a" (oldval), "+m" (*uaddr)
+		: "i" (-EFAULT), "r" (newval), "1" (oldval)
+		: "memory"
+	);
+	*uval = oldval;
+	return ret;
+}
+
+static inline int __try_xchg_user_32(u32 *oval, u32 __user *uaddr, u32 newval)
+{
+	u32 oldval = 0;
+	int ret = 0;
+
+	asm volatile("\n"
+		"1:\txchgl %0, %2\n"
+		"2:\n"
+		"\t.section .fixup, \"ax\"\n"
+		"3:\tmov     %3, %1\n"
+		"\tjmp     2b\n"
+		"\t.previous\n"
+		_ASM_EXTABLE_UA(1b, 3b)
+		: "=r" (oldval), "=r" (ret), "+m" (*uaddr)
+		: "i" (-EFAULT), "0" (newval), "1" (0)
+	);
+
+	if (ret)
+		return ret;
+
+	*oval = oldval;
+	return 0;
+}
+
+static inline int __try_xchg_user_64(u64 *oval, u64 __user *uaddr, u64 newval)
+{
+	u64 oldval = 0;
+	int ret = 0;
+
+	asm volatile("\n"
+		"1:\txchgq %0, %2\n"
+		"2:\n"
+		"\t.section .fixup, \"ax\"\n"
+		"3:\tmov     %3, %1\n"
+		"\tjmp     2b\n"
+		"\t.previous\n"
+		_ASM_EXTABLE_UA(1b, 3b)
+		: "=r" (oldval), "=r" (ret), "+m" (*uaddr)
+		: "i" (-EFAULT), "0" (newval), "1" (0)
+	);
+
+	if (ret)
+		return ret;
+
+	*oval = oldval;
+	return 0;
+}
+
 #endif /* _ASM_X86_UACCESS_64_H */

@@ -408,4 +408,50 @@ void __noreturn usercopy_abort(const char *name, const char *detail,
 			       unsigned long len);
 #endif

+#ifdef ARCH_HAS_ATOMIC_UACCESS_HELPERS
+/**
+ * cmpxchg_user_[32|64][_nofault|]() - compare_exchange 32/64-bit values
+ * @uaddr:     Destination address, in user space;
+ * @curr_val:  Source address, in kernel space;
+ * @new_val:   The value to write to the destination address.
+ *
+ * This is the standard cmpxchg: atomically: compare *@uaddr to *@curr_val;
+ * if the values match, write @new_val to @uaddr, return 0; if the values
+ * do not match, write *@uaddr to @curr_val, return -EAGAIN.
+ *
+ * The _nofault versions don't fault and can be used in
+ * atomic/preempt-disabled contexts.
+ *
+ * Return:
+ * 0      : OK/success;
+ * -EINVAL: @uaddr is not properly aligned ('may fault' versions only);
+ * -EFAULT: memory access error (including mis-aligned @uaddr in _nofault);
+ * -EAGAIN: @old did not match.
+ */
+int cmpxchg_user_32_nofault(u32 __user *uaddr, u32 *curr_val, u32 new_val);
+int cmpxchg_user_64_nofault(u64 __user *uaddr, u64 *curr_val, u64 new_val);
+int cmpxchg_user_32(u32 __user *uaddr, u32 *curr_val, u32 new_val);
+int cmpxchg_user_64(u64 __user *uaddr, u64 *curr_val, u64 new_val);
+
+/**
+ * xchg_user_[32|64][_nofault|]() - exchange 32/64-bit values
+ * @uaddr:   Destination address, in user space;
+ * @val:     Source address, in kernel space.
+ *
+ * This is the standard atomic xchg: exchange values pointed to by @uaddr and @val.
+ *
+ * The _nofault versions don't fault and can be used in
+ * atomic/preempt-disabled contexts.
+ *
+ * Return:
+ * 0      : OK/success;
+ * -EINVAL: @uaddr is not properly aligned ('may fault' versions only);
+ * -EFAULT: memory access error (including mis-aligned @uaddr in _nofault).
+ */
+int xchg_user_32_nofault(u32 __user *uaddr, u32 *val);
+int xchg_user_64_nofault(u64 __user *uaddr, u64 *val);
+int xchg_user_32(u32 __user *uaddr, u32 *val);
+int xchg_user_64(u64 __user *uaddr, u64 *val);
+#endif		/* ARCH_HAS_ATOMIC_UACCESS_HELPERS */
+
 #endif		/* __LINUX_UACCESS_H__ */

@@ -335,3 +335,267 @@ long strnlen_user_nofault(const void __user *unsafe_addr, long count)

 	return ret;
 }
+
+#ifdef ARCH_HAS_ATOMIC_UACCESS_HELPERS
+
+static int fix_pagefault(unsigned long uaddr, bool write_fault, int bytes)
+{
+	struct mm_struct *mm = current->mm;
+	int ret;
+
+	mmap_read_lock(mm);
+	ret = fixup_user_fault(mm, uaddr, write_fault ? FAULT_FLAG_WRITE : 0,
+			NULL);
+	mmap_read_unlock(mm);
+
+	return ret < 0 ? ret : 0;
+}
+
+int cmpxchg_user_32_nofault(u32 __user *uaddr, u32 *curr_val, u32 new_val)
+{
+	int ret = -EFAULT;
+	u32 __old = *curr_val;
+
+	if (unlikely(!access_ok(uaddr, sizeof(*uaddr))))
+		return -EFAULT;
+
+	pagefault_disable();
+
+	if (!user_access_begin(uaddr, sizeof(*uaddr))) {
+		pagefault_enable();
+		return -EFAULT;
+	}
+	ret = __try_cmpxchg_user_32(curr_val, uaddr, __old, new_val);
+	user_access_end();
+
+	if (!ret)
+		ret =  *curr_val == __old ? 0 : -EAGAIN;
+
+	pagefault_enable();
+	return ret;
+}
+
+int cmpxchg_user_64_nofault(u64 __user *uaddr, u64 *curr_val, u64 new_val)
+{
+	int ret = -EFAULT;
+	u64 __old = *curr_val;
+
+	if (unlikely(!access_ok(uaddr, sizeof(*uaddr))))
+		return -EFAULT;
+
+	pagefault_disable();
+
+	if (!user_access_begin(uaddr, sizeof(*uaddr))) {
+		pagefault_enable();
+		return -EFAULT;
+	}
+	ret = __try_cmpxchg_user_64(curr_val, uaddr, __old, new_val);
+	user_access_end();
+
+	if (!ret)
+		ret =  *curr_val == __old ? 0 : -EAGAIN;
+
+	pagefault_enable();
+
+	return ret;
+}
+
+int cmpxchg_user_32(u32 __user *uaddr, u32 *curr_val, u32 new_val)
+{
+	int ret = -EFAULT;
+	u32 __old = *curr_val;
+
+	/* Validate proper alignment. */
+	if (unlikely(((unsigned long)uaddr % sizeof(*uaddr)) ||
+			((unsigned long)curr_val % sizeof(*curr_val))))
+		return -EINVAL;
+
+	if (unlikely(!access_ok(uaddr, sizeof(*uaddr))))
+		return -EFAULT;
+
+	pagefault_disable();
+
+	while (true) {
+		ret = -EFAULT;
+		if (!user_access_begin(uaddr, sizeof(*uaddr)))
+			break;
+
+		ret = __try_cmpxchg_user_32(curr_val, uaddr, __old, new_val);
+		user_access_end();
+
+		if (!ret) {
+			ret =  *curr_val == __old ? 0 : -EAGAIN;
+			break;
+		}
+
+		if (fix_pagefault((unsigned long)uaddr, true, sizeof(*uaddr)) < 0)
+			break;
+	}
+
+	pagefault_enable();
+	return ret;
+}
+
+int cmpxchg_user_64(u64 __user *uaddr, u64 *curr_val, u64 new_val)
+{
+	int ret = -EFAULT;
+	u64 __old = *curr_val;
+
+	/* Validate proper alignment. */
+	if (unlikely(((unsigned long)uaddr % sizeof(*uaddr)) ||
+			((unsigned long)curr_val % sizeof(*curr_val))))
+		return -EINVAL;
+
+	if (unlikely(!access_ok(uaddr, sizeof(*uaddr))))
+		return -EFAULT;
+
+	pagefault_disable();
+
+	while (true) {
+		ret = -EFAULT;
+		if (!user_access_begin(uaddr, sizeof(*uaddr)))
+			break;
+
+		ret = __try_cmpxchg_user_64(curr_val, uaddr, __old, new_val);
+		user_access_end();
+
+		if (!ret) {
+			ret =  *curr_val == __old ? 0 : -EAGAIN;
+			break;
+		}
+
+		if (fix_pagefault((unsigned long)uaddr, true, sizeof(*uaddr)) < 0)
+			break;
+	}
+
+	pagefault_enable();
+	return ret;
+}
+
+/**
+ * xchg_user_[32|64][_nofault|]() - exchange 32/64-bit values
+ * @uaddr:   Destination address, in user space;
+ * @val:     Source address, in kernel space.
+ *
+ * This is the standard atomic xchg: exchange values pointed to by @uaddr and @val.
+ *
+ * The _nofault versions don't fault and can be used in
+ * atomic/preempt-disabled contexts.
+ *
+ * Return:
+ * 0      : OK/success;
+ * -EINVAL: @uaddr is not properly aligned ('may fault' versions only);
+ * -EFAULT: memory access error (including mis-aligned @uaddr in _nofault).
+ */
+int xchg_user_32_nofault(u32 __user *uaddr, u32 *val)
+{
+	int ret;
+
+	if (unlikely(!access_ok(uaddr, sizeof(*uaddr))))
+		return -EFAULT;
+
+	pagefault_disable();
+
+	if (!user_access_begin(uaddr, sizeof(*uaddr))) {
+		pagefault_enable();
+		return -EFAULT;
+	}
+
+	ret = __try_xchg_user_32(val, uaddr, *val);
+	user_access_end();
+
+	pagefault_enable();
+
+	return ret;
+}
+
+int xchg_user_64_nofault(u64 __user *uaddr, u64 *val)
+{
+	int ret;
+
+	if (unlikely(!access_ok(uaddr, sizeof(*uaddr))))
+		return -EFAULT;
+
+	pagefault_disable();
+
+	if (!user_access_begin(uaddr, sizeof(*uaddr))) {
+		pagefault_enable();
+		return -EFAULT;
+	}
+
+	ret = __try_xchg_user_64(val, uaddr, *val);
+	user_access_end();
+
+	pagefault_enable();
+
+	return ret;
+}
+
+int xchg_user_32(u32 __user *uaddr, u32 *val)
+{
+	int ret = -EFAULT;
+
+	/* Validate proper alignment. */
+	if (unlikely(((unsigned long)uaddr % sizeof(*uaddr)) ||
+			((unsigned long)val % sizeof(*val))))
+		return -EINVAL;
+
+	if (unlikely(!access_ok(uaddr, sizeof(*uaddr))))
+		return -EFAULT;
+
+	pagefault_disable();
+
+	while (true) {
+		ret = -EFAULT;
+		if (!user_access_begin(uaddr, sizeof(*uaddr)))
+			break;
+
+		ret = __try_xchg_user_32(val, uaddr, *val);
+		user_access_end();
+
+		if (!ret)
+			break;
+
+		if (fix_pagefault((unsigned long)uaddr, true, sizeof(*uaddr)) < 0)
+			break;
+	}
+
+	pagefault_enable();
+
+	return ret;
+}
+
+int xchg_user_64(u64 __user *uaddr, u64 *val)
+{
+	int ret = -EFAULT;
+
+	/* Validate proper alignment. */
+	if (unlikely(((unsigned long)uaddr % sizeof(*uaddr)) ||
+			((unsigned long)val % sizeof(*val))))
+		return -EINVAL;
+
+	if (unlikely(!access_ok(uaddr, sizeof(*uaddr))))
+		return -EFAULT;
+
+	pagefault_disable();
+
+	while (true) {
+		ret = -EFAULT;
+		if (!user_access_begin(uaddr, sizeof(*uaddr)))
+			break;
+
+		ret = __try_xchg_user_64(val, uaddr, *val);
+		user_access_end();
+
+		if (!ret)
+			break;
+
+		if (fix_pagefault((unsigned long)uaddr, true, sizeof(*uaddr)) < 0)
+			break;
+	}
+
+	pagefault_enable();
+
+	return ret;
+}
+#endif		/* ARCH_HAS_ATOMIC_UACCESS_HELPERS */
--

@@ -371,6 +371,8 @@
 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

 #
 # Due to a historical design error, certain syscalls are numbered differently

@@ -1838,6 +1838,7 @@ static int bprm_execve(struct linux_binprm *bprm,
 	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;

@@ -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 +1295,12 @@ struct task_struct {
 	unsigned long rseq_event_mask;
 #endif

+#ifdef CONFIG_UMCG
+	struct umcg_task __user	*umcg_task;
+	struct page		*pinned_umcg_worker_page;  /* self */
+	struct page		*pinned_umcg_server_page;
+#endif
+
 	struct tlbflush_unmap_batch	tlb_ubc;

 	union {

@@ -1687,6 +1694,13 @@ 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. */

@@ -2287,6 +2301,63 @@ static inline void rseq_execve(struct task_struct *t)

 #endif

+#ifdef CONFIG_UMCG
+
+void umcg_handle_resuming_worker(void);
+void umcg_handle_exiting_worker(void);
+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 exit_to_user_mode_loop() in kernel/entry/common.c.*/
+static inline void umcg_handle_notify_resume(void)
+{
+	if (current->flags & PF_UMCG_WORKER)
+		umcg_handle_resuming_worker();
+}
+
+/* Called by do_exit() in kernel/exit.c. */
+static inline void umcg_handle_exit(void)
+{
+	if (current->flags & PF_UMCG_WORKER)
+		umcg_handle_exiting_worker();
+}
+
+/*
+ * umcg_wq_worker_[sleeping|running] are called in core.c by
+ * sched_submit_work() and sched_update_worker().
+ */
+void umcg_wq_worker_sleeping(struct task_struct *tsk);
+void umcg_wq_worker_running(struct task_struct *tsk);
+
+#else  /* CONFIG_UMCG */
+
+static inline void umcg_clear_child(struct task_struct *tsk)
+{
+}
+static inline void umcg_execve(struct task_struct *tsk)
+{
+}
+static inline void umcg_handle_notify_resume(void)
+{
+}
+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

 void rseq_syscall(struct pt_regs *regs);

@@ -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 +1058,8 @@ asmlinkage long sys_landlock_add_rule(int ruleset_fd, enum landlock_rule_type ru
 		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);
+asmlinkage long sys_umcg_wait(u32 flags, u64 abs_timeout);

 /*
  * Architecture-specific system calls

@@ -883,8 +883,13 @@ __SYSCALL(__NR_process_mrelease, sys_process_mrelease)
 #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)
 #undef __NR_syscalls
-#define __NR_syscalls 450
+
+#define __NR_syscalls 452

 /*
  * 32 bit systems traditionally used different

@@ -0,0 +1,137 @@
+/* SPDX-License-Identifier: GPL-2.0+ WITH Linux-syscall-note */
+#ifndef _UAPI_LINUX_UMCG_H
+#define _UAPI_LINUX_UMCG_H
+
+#include <linux/limits.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.
+ *
+ * struct umcg_task (below): controls the state of UMCG tasks.
+ *
+ * See Documentation/userspace-api/umcg.txt for detals.
+ */
+
+/*
+ * UMCG task states, the first 6 bits of struct umcg_task.state_ts.
+ * The states represent the user space point of view.
+ */
+#define UMCG_TASK_NONE			0ULL
+#define UMCG_TASK_RUNNING		1ULL
+#define UMCG_TASK_IDLE			2ULL
+#define UMCG_TASK_BLOCKED		3ULL
+
+/* UMCG task state flags, bits 7-8 */
+
+/*
+ * UMCG_TF_LOCKED: locked by the userspace in preparation to calling umcg_wait.
+ */
+#define UMCG_TF_LOCKED			(1ULL << 6)
+
+/*
+ * UMCG_TF_PREEMPTED: the userspace indicates the worker should be preempted.
+ */
+#define UMCG_TF_PREEMPTED		(1ULL << 7)
+
+/* The first six bits: RUNNING, IDLE, or BLOCKED. */
+#define UMCG_TASK_STATE_MASK		0x3fULL
+
+/* The full state mask: the first 18 bits. */
+#define UMCG_TASK_STATE_MASK_FULL	0x3ffffULL
+
+/*
+ * The number of bits reserved for UMCG state timestamp in
+ * struct umcg_task.state_ts.
+ */
+#define UMCG_STATE_TIMESTAMP_BITS	46
+
+/* The number of bits truncated from UMCG state timestamp. */
+#define UMCG_STATE_TIMESTAMP_GRANULARITY	4
+
+/**
+ * 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_ts: the current state of the UMCG task described by
+	 *            this struct, with a unique timestamp indicating
+	 *            when the last state change happened.
+	 *
+	 * Readable/writable by both the kernel and the userspace.
+	 *
+	 * UMCG task state:
+	 *   bits  0 -  5: task state;
+	 *   bits  6 -  7: state flags;
+	 *   bits  8 - 12: reserved; must be zeroes;
+	 *   bits 13 - 17: for userspace use;
+	 *   bits 18 - 63: timestamp (see below).
+	 *
+	 * Timestamp: a 46-bit CLOCK_MONOTONIC timestamp, at 16ns resolution.
+	 * See Documentation/userspace-api/umcg.txt for detals.
+	 */
+	__u64	state_ts;		/* r/w */
+
+	/**
+	 * @next_tid: the TID of the UMCG task that should be context-switched
+	 *            into in sys_umcg_wait(). Can be zero.
+	 *
+	 * Running UMCG workers must have next_tid set to point to IDLE
+	 * UMCG servers.
+	 *
+	 * Read-only for the kernel, read/write for the userspace.
+	 */
+	__u32	next_tid;		/* r   */
+
+	__u32	flags;			/* Reserved; must be zero. */
+
+	/**
+	 * @idle_workers_ptr: a single-linked list of idle workers. Can be NULL.
+	 *
+	 * Readable/writable by both the kernel and the userspace: the
+	 * kernel adds items to the list, the userspace removes them.
+	 */
+	__u64	idle_workers_ptr;	/* r/w */
+
+	/**
+	 * @idle_server_tid_ptr: a pointer pointing to a single idle server.
+	 *                       Readonly.
+	 */
+	__u64	idle_server_tid_ptr;	/* r   */
+} __attribute__((packed, aligned(8 * sizeof(__u64))));
+
+/**
+ * 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,
+};
+
+/**
+ * enum umcg_wait_flag - flags to pass to sys_umcg_wait
+ * @UMCG_WAIT_WAKE_ONLY:      wake @self->next_tid, don't put @self to sleep;
+ * @UMCG_WAIT_WF_CURRENT_CPU: wake @self->next_tid on the current CPU
+ *                            (use WF_CURRENT_CPU); @UMCG_WAIT_WAKE_ONLY
+ *                            must be set.
+ */
+enum umcg_wait_flag {
+	UMCG_WAIT_WAKE_ONLY			= 1,
+	UMCG_WAIT_WF_CURRENT_CPU		= 2,
+};
+
+/* See Documentation/userspace-api/umcg.txt.*/
+#define UMCG_IDLE_NODE_PENDING (1ULL)
+
+#endif /* _UAPI_LINUX_UMCG_H */

@@ -1693,6 +1693,16 @@ config MEMBARRIER

 	  If unsure, say Y.

+config UMCG
+	bool "Enable User Managed Concurrency Groups API"
+	depends on X86_64
+	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

@@ -171,8 +171,10 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
 		if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL))
 			handle_signal_work(regs, ti_work);

-		if (ti_work & _TIF_NOTIFY_RESUME)
+		if (ti_work & _TIF_NOTIFY_RESUME) {
+			umcg_handle_notify_resume();
 			tracehook_notify_resume(regs);
+		}

 		/* Architecture specific TIF work */
 		arch_exit_to_user_mode_work(regs, ti_work);

@@ -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)

@@ -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

@@ -4269,6 +4269,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 	p->wake_entry.u_flags = CSD_TYPE_TTWU;
 	p->migration_pending = NULL;
 #endif
+	umcg_clear_child(p);
 }

 DEFINE_STATIC_KEY_FALSE(sched_numa_balancing);

@@ -6327,9 +6328,11 @@ static inline void sched_submit_work(struct task_struct *tsk)
 	 * 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);
 	}

@@ -6347,9 +6350,11 @@ static inline void sched_submit_work(struct task_struct *tsk)

 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);
 	}

@@ -0,0 +1,949 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+/*
+ * User Managed Concurrency Groups (UMCG).
+ *
+ * See Documentation/userspace-api/umcg.txt for detals.
+ */
+
+#include <linux/syscalls.h>
+#include <linux/types.h>
+#include <linux/uaccess.h>
+#include <linux/umcg.h>
+
+#include "sched.h"
+
+/**
+ * get_user_nofault - get user value without sleeping.
+ *
+ * get_user() might sleep and therefore cannot be used in preempt-disabled
+ * regions.
+ */
+#define get_user_nofault(out, uaddr)			\
+({							\
+	int ret = -EFAULT;				\
+							\
+	if (access_ok((uaddr), sizeof(*(uaddr)))) {	\
+		pagefault_disable();			\
+							\
+		if (!__get_user((out), (uaddr)))	\
+			ret = 0;			\
+							\
+		pagefault_enable();			\
+	}						\
+	ret;						\
+})
+
+/**
+ * umcg_pin_pages: pin pages containing struct umcg_task of this worker
+ *                 and its server.
+ *
+ * The pages are pinned when the worker exits to the userspace and unpinned
+ * when the worker is in sched_submit_work(), i.e. when the worker is
+ * about to be removed from its runqueue. Thus at most NR_CPUS UMCG pages
+ * are pinned at any one time across the whole system.
+ *
+ * The pinning is needed so that going-to-sleep workers can access
+ * their and their servers' userspace umcg_task structs without page faults,
+ * as the code path can be executed in the context of a pagefault, with
+ * mm lock held.
+ */
+static int umcg_pin_pages(u32 server_tid)
+{
+	struct umcg_task __user *worker_ut = current->umcg_task;
+	struct umcg_task __user *server_ut = NULL;
+	struct task_struct *tsk;
+
+	rcu_read_lock();
+	tsk = find_task_by_vpid(server_tid);
+	/* Server/worker interaction is allowed only within the same mm. */
+	if (tsk && current->mm == tsk->mm)
+		server_ut = READ_ONCE(tsk->umcg_task);
+	rcu_read_unlock();
+
+	if (!server_ut)
+		return -EINVAL;
+
+	tsk = current;
+
+	/* worker_ut is stable, don't need to repin */
+	if (!tsk->pinned_umcg_worker_page)
+		if (1 != pin_user_pages_fast((unsigned long)worker_ut, 1, 0,
+					&tsk->pinned_umcg_worker_page))
+			return -EFAULT;
+
+	/* server_ut may change, need to repin */
+	if (tsk->pinned_umcg_server_page) {
+		unpin_user_page(tsk->pinned_umcg_server_page);
+		tsk->pinned_umcg_server_page = NULL;
+	}
+
+	if (1 != pin_user_pages_fast((unsigned long)server_ut, 1, 0,
+				&tsk->pinned_umcg_server_page))
+		return -EFAULT;
+
+	return 0;
+}
+
+static void umcg_unpin_pages(void)
+{
+	struct task_struct *tsk = current;
+
+	if (tsk->pinned_umcg_worker_page)
+		unpin_user_page(tsk->pinned_umcg_worker_page);
+	if (tsk->pinned_umcg_server_page)
+		unpin_user_page(tsk->pinned_umcg_server_page);
+
+	tsk->pinned_umcg_worker_page = NULL;
+	tsk->pinned_umcg_server_page = 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);
+
+		/* These can be simple writes - see the commment above. */
+		tsk->pinned_umcg_worker_page = NULL;
+		tsk->pinned_umcg_server_page = NULL;
+		tsk->flags &= ~PF_UMCG_WORKER;
+	}
+}
+
+/* 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_handle_exiting_worker(void)
+{
+	umcg_unpin_pages();
+	umcg_clear_task(current);
+}
+
+/**
+ * umcg_update_state: atomically update umcg_task.state_ts, set new timestamp.
+ * @state_ts   - points to the state_ts member of struct umcg_task to update;
+ * @expected   - the expected value of state_ts, including the timestamp;
+ * @desired    - the desired value of state_ts, state part only;
+ * @may_fault  - whether to use normal or _nofault cmpxchg.
+ *
+ * The function is basically cmpxchg(state_ts, expected, desired), with extra
+ * code to set the timestamp in @desired.
+ */
+static int umcg_update_state(u64 __user *state_ts, u64 *expected, u64 desired,
+				bool may_fault)
+{
+	u64 curr_ts = (*expected) >> (64 - UMCG_STATE_TIMESTAMP_BITS);
+	u64 next_ts = ktime_get_ns() >> UMCG_STATE_TIMESTAMP_GRANULARITY;
+
+	/* Cut higher order bits. */
+	next_ts &= (1ULL << UMCG_STATE_TIMESTAMP_BITS) - 1;
+
+	if (next_ts == curr_ts)
+		++next_ts;
+
+	/* Remove an old timestamp, if any. */
+	desired &= UMCG_TASK_STATE_MASK_FULL;
+
+	/* Set the new timestamp. */
+	desired |= (next_ts << (64 - UMCG_STATE_TIMESTAMP_BITS));
+
+	if (may_fault)
+		return cmpxchg_user_64(state_ts, expected, desired);
+
+	return cmpxchg_user_64_nofault(state_ts, expected, desired);
+}
+
+/**
+ * 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 if
+ *               registering; must be NULL if unregistering.
+ *
+ * @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->next_tid must be zero
+ *
+ *         If the conditions above are met, sys_umcg_ctl() immediately returns
+ *         if the registered task is a server; a worker will be added to
+ *         idle_workers_ptr, and the worker put to sleep; an idle server
+ *         from idle_server_tid_ptr will be woken, if present.
+ *
+ * @flags == UMCG_CTL_UNREGISTER: unregister a UMCG task. If the current task
+ *           is a UMCG worker, the userspace is responsible for waking its
+ *           server (before or after calling sys_umcg_ctl).
+ *
+ * Return:
+ * 0                - success
+ * -EFAULT          - failed to read @self
+ * -EINVAL          - some other error occurred
+ */
+SYSCALL_DEFINE2(umcg_ctl, u32, flags, struct umcg_task __user *, self)
+{
+	struct umcg_task ut;
+
+	if (flags == UMCG_CTL_UNREGISTER) {
+		if (self || !current->umcg_task)
+			return -EINVAL;
+
+		if (current->flags & PF_UMCG_WORKER)
+			umcg_handle_exiting_worker();
+		else
+			umcg_clear_task(current);
+
+		return 0;
+	}
+
+	if (!(flags & UMCG_CTL_REGISTER))
+		return -EINVAL;
+
+	flags &= ~UMCG_CTL_REGISTER;
+	if (flags && flags != UMCG_CTL_WORKER)
+		return -EINVAL;
+
+	if (current->umcg_task || !self)
+		return -EINVAL;
+
+	if (copy_from_user(&ut, self, sizeof(ut)))
+		return -EFAULT;
+
+	if (ut.next_tid)
+		return -EINVAL;
+
+	if (flags == UMCG_CTL_WORKER) {
+		if ((ut.state_ts & UMCG_TASK_STATE_MASK_FULL) != UMCG_TASK_BLOCKED)
+			return -EINVAL;
+
+		WRITE_ONCE(current->umcg_task, self);
+		current->flags |= PF_UMCG_WORKER;
+
+		/* Trigger umcg_handle_resuming_worker() */
+		set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
+	} else {
+		if ((ut.state_ts & UMCG_TASK_STATE_MASK_FULL) != UMCG_TASK_RUNNING)
+			return -EINVAL;
+
+		WRITE_ONCE(current->umcg_task, self);
+	}
+
+	return 0;
+}
+
+/**
+ * handle_timedout_worker - make sure the worker is added to idle_workers
+ *                          upon a "clean" timeout.
+ */
+static int handle_timedout_worker(struct umcg_task __user *self)
+{
+	u64 curr_state, next_state;
+	int ret;
+
+	if (get_user(curr_state, &self->state_ts))
+		return -EFAULT;
+
+	if ((curr_state & UMCG_TASK_STATE_MASK) == UMCG_TASK_IDLE) {
+		/* TODO: should we care here about TF_LOCKED or TF_PREEMPTED? */
+
+		next_state = curr_state & ~UMCG_TASK_STATE_MASK;
+		next_state |= UMCG_TASK_BLOCKED;
+
+		ret = umcg_update_state(&self->state_ts, &curr_state, next_state, true);
+		if (ret)
+			return ret;
+
+		return -ETIMEDOUT;
+	}
+
+	return 0;  /* Not really timed out. */
+}
+
+/*
+ * umcg_should_idle - return true if tasks with @state should block in
+ *                    imcg_idle_loop().
+ */
+static bool umcg_should_idle(u64 state)
+{
+	switch (state & UMCG_TASK_STATE_MASK) {
+	case UMCG_TASK_RUNNING:
+		return state & UMCG_TF_LOCKED;
+	case UMCG_TASK_IDLE:
+		return !(state & UMCG_TF_LOCKED);
+	case UMCG_TASK_BLOCKED:
+		return false;
+	default:
+		WARN_ONCE(true, "unknown UMCG task state");
+		return false;
+	}
+}
+
+/**
+ * umcg_idle_loop - sleep until !umcg_should_idle() or a timeout expires
+ * @abs_timeout - absolute timeout in nanoseconds; zero => no timeout
+ *
+ * The function marks the current task as INTERRUPTIBLE and calls
+ * freezable_schedule().
+ *
+ * Note: because UMCG workers should not be running WITHOUT attached servers,
+ *       and because servers should not be running WITH attached workers,
+ *       the function returns only on fatal signal pending and ignores/flushes
+ *       all other signals.
+ */
+static int umcg_idle_loop(u64 abs_timeout)
+{
+	int ret;
+	struct page *pinned_page = NULL;
+	struct hrtimer_sleeper timeout;
+	struct umcg_task __user *self = current->umcg_task;
+	const bool worker = current->flags & PF_UMCG_WORKER;
+
+	/* Clear PF_UMCG_WORKER to elide workqueue handlers. */
+	if (worker)
+		current->flags &= ~PF_UMCG_WORKER;
+
+	if (abs_timeout) {
+		hrtimer_init_sleeper_on_stack(&timeout, CLOCK_REALTIME,
+				HRTIMER_MODE_ABS);
+
+		hrtimer_set_expires_range_ns(&timeout.timer, (s64)abs_timeout,
+				current->timer_slack_ns);
+	}
+
+	while (true) {
+		u64 umcg_state;
+
+		/*
+		 * We need to read from userspace _after_ the task is marked
+		 * TASK_INTERRUPTIBLE, to properly handle concurrent wakeups;
+		 * but faulting is not allowed; so we try a fast no-fault read,
+		 * and if it fails, pin the page temporarily.
+		 */
+retry_once:
+		set_current_state(TASK_INTERRUPTIBLE);
+
+		/* Order set_current_state above with get_user below. */
+		smp_mb();
+		ret = -EFAULT;
+		if (get_user_nofault(umcg_state, &self->state_ts)) {
+			set_current_state(TASK_RUNNING);
+
+			if (pinned_page)
+				goto out;
+			else if (1 != pin_user_pages_fast((unsigned long)self,
+						1, 0, &pinned_page))
+					goto out;
+
+			goto retry_once;
+		}
+
+		if (pinned_page) {
+			unpin_user_page(pinned_page);
+			pinned_page = NULL;
+		}
+
+		ret = 0;
+		if (!umcg_should_idle(umcg_state)) {
+			set_current_state(TASK_RUNNING);
+			goto out;
+		}
+
+		if (abs_timeout)
+			hrtimer_sleeper_start_expires(&timeout, HRTIMER_MODE_ABS);
+
+		if (!abs_timeout || timeout.task)
+			freezable_schedule();
+
+		__set_current_state(TASK_RUNNING);
+
+		/*
+		 * Check for timeout before checking the state, as workers
+		 * are not going to return from freezable_schedule() unless
+		 * they are RUNNING.
+		 */
+		ret = -ETIMEDOUT;
+		if (abs_timeout && !timeout.task)
+			goto out;
+
+		/* Order set_current_state above with get_user below. */
+		smp_mb();
+		ret = -EFAULT;
+		if (get_user(umcg_state, &self->state_ts))
+			goto out;
+
+		ret = 0;
+		if (!umcg_should_idle(umcg_state))
+			goto out;
+
+		ret = -EINTR;
+		if (fatal_signal_pending(current))
+			goto out;
+
+		if (signal_pending(current))
+			flush_signals(current);
+	}
+
+out:
+	if (pinned_page) {
+		unpin_user_page(pinned_page);
+		pinned_page = NULL;
+	}
+	if (abs_timeout) {
+		hrtimer_cancel(&timeout.timer);
+		destroy_hrtimer_on_stack(&timeout.timer);
+	}
+	if (worker) {
+		current->flags |= PF_UMCG_WORKER;
+
+		if (ret == -ETIMEDOUT)
+			ret = handle_timedout_worker(self);
+
+		/* Workers must go through workqueue handlers upon wakeup. */
+		set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
+	}
+	return ret;
+}
+
+/**
+ * umcg_wakeup_allowed - check whether @current can wake @tsk.
+ *
+ * Currently a placeholder that allows wakeups within a single process
+ * only (same mm). In the future the requirement will be relaxed (securely).
+ */
+static bool umcg_wakeup_allowed(struct task_struct *tsk)
+{
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
+	if (tsk->mm && tsk->mm == current->mm && READ_ONCE(tsk->umcg_task))
+		return true;
+
+	return false;
+}
+
+/*
+ * Try to wake up. May be called with preempt_disable set. May be called
+ * cross-process.
+ *
+ * Note: umcg_ttwu succeeds even if ttwu fails: see wait/wake state
+ *       ordering logic.
+ */
+static int umcg_ttwu(u32 next_tid, int wake_flags)
+{
+	struct task_struct *next;
+
+	rcu_read_lock();
+	next = find_task_by_vpid(next_tid);
+	if (!next || !umcg_wakeup_allowed(next)) {
+		rcu_read_unlock();
+		return -ESRCH;
+	}
+
+	/* The result of ttwu below is ignored. */
+	try_to_wake_up(next, TASK_NORMAL, wake_flags);
+	rcu_read_unlock();
+
+	return 0;
+}
+
+/*
+ * At the moment, umcg_do_context_switch simply wakes up @next with
+ * WF_CURRENT_CPU and puts the current task to sleep.
+ *
+ * In the future an optimization will be added to adjust runtime accounting
+ * so that from the kernel scheduling perspective the two tasks are
+ * essentially treated as one. In addition, the context switch may be performed
+ * right here on the fast path, instead of going through the wake/wait pair.
+ */
+static int umcg_do_context_switch(u32 next_tid, u64 abs_timeout)
+{
+	int ret;
+
+	ret = umcg_ttwu(next_tid, WF_CURRENT_CPU);
+	if (ret)
+		return ret;
+
+	return umcg_idle_loop(abs_timeout);
+}
+
+/**
+ * sys_umcg_wait: put the current task to sleep and/or wake another task.
+ * @flags:        zero or a value from enum umcg_wait_flag.
+ * @abs_timeout:  when to wake the task, in nanoseconds; zero for no timeout.
+ *
+ * @self->state_ts must be UMCG_TASK_IDLE (where @self is current->umcg_task)
+ * if !(@flags & UMCG_WAIT_WAKE_ONLY) (also see umcg_idle_loop and
+ * umcg_should_idle above).
+ *
+ * If @self->next_tid is not zero, it must point to an IDLE UMCG task.
+ * The userspace must have changed its state from IDLE to RUNNING
+ * before calling sys_umcg_wait() in the current task. This "next"
+ * task will be woken (context-switched-to on the fast path) when the
+ * current task is put to sleep.
+ *
+ * See Documentation/userspace-api/umcg.txt for detals.
+ *
+ * Return:
+ * 0             - OK;
+ * -ETIMEDOUT    - the timeout expired;
+ * -EFAULT       - failed accessing struct umcg_task __user of the current
+ *                 task;
+ * -ESRCH        - the task to wake not found or not a UMCG task;
+ * -EINVAL       - another error happened (e.g. bad @flags, or the current
+ *                 task is not a UMCG task, etc.)
+ */
+SYSCALL_DEFINE2(umcg_wait, u32, flags, u64, abs_timeout)
+{
+	struct umcg_task __user *self = current->umcg_task;
+	u32 next_tid;
+
+	if (!self)
+		return -EINVAL;
+
+	if (get_user(next_tid, &self->next_tid))
+		return -EFAULT;
+
+	if (flags & UMCG_WAIT_WAKE_ONLY) {
+		if (!next_tid || abs_timeout)
+			return -EINVAL;
+
+		flags &= ~UMCG_WAIT_WAKE_ONLY;
+		if (flags & ~UMCG_WAIT_WF_CURRENT_CPU)
+			return -EINVAL;
+
+		return umcg_ttwu(next_tid, flags & UMCG_WAIT_WF_CURRENT_CPU ?
+					WF_CURRENT_CPU : 0);
+	}
+
+	/* Unlock the worker, if locked. */
+	if (current->flags & PF_UMCG_WORKER) {
+		u64 umcg_state;
+
+		if (get_user(umcg_state, &self->state_ts))
+			return -EFAULT;
+
+		if ((umcg_state & UMCG_TF_LOCKED) && umcg_update_state(
+					&self->state_ts, &umcg_state,
+					umcg_state & ~UMCG_TF_LOCKED, true))
+			return -EFAULT;
+	}
+
+	if (next_tid)
+		return umcg_do_context_switch(next_tid, abs_timeout);
+
+	return umcg_idle_loop(abs_timeout);
+}
+
+/*
+ * NOTE: all code below is called from workqueue submit/update, or
+ *       syscall exit to usermode loop, so all errors result in the
+ *       termination of the current task (via SIGKILL).
+ */
+
+/*
+ * Wake idle server: find the task, change its state IDLE=>RUNNING, ttwu.
+ */
+static int umcg_wake_idle_server_nofault(u32 server_tid)
+{
+	struct umcg_task __user *ut_server = NULL;
+	struct task_struct *tsk;
+	int ret = -EINVAL;
+	u64 state;
+
+	rcu_read_lock();
+
+	tsk = find_task_by_vpid(server_tid);
+	/* Server/worker interaction is allowed only within the same mm. */
+	if (tsk && current->mm == tsk->mm)
+		ut_server = READ_ONCE(tsk->umcg_task);
+
+	if (!ut_server)
+		goto out_rcu;
+
+	ret = -EFAULT;
+	if (get_user_nofault(state, &ut_server->state_ts))
+		goto out_rcu;
+
+	ret = -EAGAIN;
+	if ((state & UMCG_TASK_STATE_MASK) != UMCG_TASK_IDLE)
+		goto out_rcu;
+
+	ret = umcg_update_state(&ut_server->state_ts, &state,
+			(state & ~UMCG_TASK_STATE_MASK) | UMCG_TASK_RUNNING,
+			false);
+
+	if (ret)
+		goto out_rcu;
+
+	try_to_wake_up(tsk, TASK_NORMAL, WF_CURRENT_CPU);
+
+out_rcu:
+	rcu_read_unlock();
+	return ret;
+}
+
+/*
+ * Wake idle server: find the task, change its state IDLE=>RUNNING, ttwu.
+ */
+static int umcg_wake_idle_server_may_fault(u32 server_tid)
+{
+	struct umcg_task __user *ut_server = NULL;
+	struct task_struct *tsk;
+	int ret = -EINVAL;
+	u64 state;
+
+	rcu_read_lock();
+	tsk = find_task_by_vpid(server_tid);
+	if (tsk && current->mm == tsk->mm)
+		ut_server = READ_ONCE(tsk->umcg_task);
+	rcu_read_unlock();
+
+	if (!ut_server)
+		return -EINVAL;
+
+	if (get_user(state, &ut_server->state_ts))
+		return -EFAULT;
+
+	if ((state & UMCG_TASK_STATE_MASK) != UMCG_TASK_IDLE)
+		return -EAGAIN;
+
+	ret = umcg_update_state(&ut_server->state_ts, &state,
+			(state & ~UMCG_TASK_STATE_MASK) | UMCG_TASK_RUNNING,
+			true);
+	if (ret)
+		return ret;
+
+	/*
+	 * umcg_ttwu will call find_task_by_vpid again; but we cannot
+	 * elide this, as we cannot do get_user() from an rcu-locked
+	 * code block.
+	 */
+	return umcg_ttwu(server_tid, WF_CURRENT_CPU);
+}
+
+/*
+ * Wake idle server: find the task, change its state IDLE=>RUNNING, ttwu.
+ */
+static int umcg_wake_idle_server(u32 server_tid, bool may_fault)
+{
+	int ret = umcg_wake_idle_server_nofault(server_tid);
+
+	if (!ret)
+		return 0;
+
+	if (!may_fault || ret != -EFAULT)
+		return ret;
+
+	return umcg_wake_idle_server_may_fault(server_tid);
+}
+
+/*
+ * Called in sched_submit_work() context for UMCG workers. In the common case,
+ * the worker's state changes RUNNING => BLOCKED, and its server's state
+ * changes IDLE => RUNNING, and the server is ttwu-ed.
+ *
+ * Under some conditions (e.g. the worker is "locked", see
+ * /Documentation/userspace-api/umcg.txt for more details), the
+ * function does nothing.
+ *
+ * The function is called with preempt disabled to make sure the retry_once
+ * logic below works correctly.
+ */
+static void process_sleeping_worker(struct task_struct *tsk, u32 *server_tid)
+{
+	struct umcg_task __user *ut_worker = tsk->umcg_task;
+	u64 curr_state, next_state;
+	bool retried = false;
+	u32 tid;
+	int ret;
+
+	*server_tid = 0;
+
+	if (WARN_ONCE((tsk != current) || !ut_worker, "Invalid UMCG worker."))
+		return;
+
+	/* If the worker has no server, do nothing. */
+	if (unlikely(!tsk->pinned_umcg_server_page))
+		return;
+
+	if (get_user_nofault(curr_state, &ut_worker->state_ts))
+		goto die;
+
+	/*
+	 * The userspace is allowed to concurrently change a RUNNING worker's
+	 * state only once in a "short" period of time, so we retry state
+	 * change at most once. As this retry block is within a
+	 * preempt_disable region, "short" is truly short here.
+	 *
+	 * See Documentation/userspace-api/umcg.txt for details.
+	 */
+retry_once:
+	if (curr_state & UMCG_TF_LOCKED)
+		return;
+
+	if (WARN_ONCE((curr_state & UMCG_TASK_STATE_MASK) != UMCG_TASK_RUNNING,
+			"Unexpected UMCG worker state."))
+		goto die;
+
+	next_state = curr_state & ~UMCG_TASK_STATE_MASK;
+	next_state |= UMCG_TASK_BLOCKED;
+
+	ret = umcg_update_state(&ut_worker->state_ts, &curr_state, next_state, false);
+	if (ret == -EAGAIN) {
+		if (retried)
+			goto die;
+
+		retried = true;
+		goto retry_once;
+	}
+	if (ret)
+		goto die;
+
+	smp_mb();  /* Order state read/write above and getting next_tid below. */
+	if (get_user_nofault(tid, &ut_worker->next_tid))
+		goto die;
+
+	*server_tid = tid;
+	return;
+
+die:
+	pr_warn("%s: killing task %d\n", __func__, current->pid);
+	force_sig(SIGKILL);
+}
+
+/* Called from sched_submit_work(). Must not fault/sleep. */
+void umcg_wq_worker_sleeping(struct task_struct *tsk)
+{
+	u32 server_tid;
+
+	/*
+	 * Disable preemption so that retry_once in process_sleeping_worker
+	 * works properly.
+	 */
+	preempt_disable();
+	process_sleeping_worker(tsk, &server_tid);
+	preempt_enable();
+
+	if (server_tid) {
+		int ret = umcg_wake_idle_server_nofault(server_tid);
+
+		if (ret && ret != -EAGAIN)
+			goto die;
+	}
+
+	goto out;
+
+die:
+	pr_warn("%s: killing task %d\n", __func__, current->pid);
+	force_sig(SIGKILL);
+out:
+	umcg_unpin_pages();
+}
+
+/**
+ * enqueue_idle_worker - push an idle worker onto idle_workers_ptr list/stack.
+ *
+ * Returns true on success, false on a fatal failure.
+ *
+ * See Documentation/userspace-api/umcg.txt for details.
+ */
+static bool enqueue_idle_worker(struct umcg_task __user *ut_worker)
+{
+	u64 __user *node = &ut_worker->idle_workers_ptr;
+	u64 __user *head_ptr;
+	u64 first = (u64)node;
+	u64 head;
+
+	if (get_user(head, node) || !head)
+		return false;
+
+	head_ptr = (u64 __user *)head;
+
+	/* Mark the worker as pending. */
+	if (put_user(UMCG_IDLE_NODE_PENDING, node))
+		return false;
+
+	/* Make the head point to the worker. */
+	if (xchg_user_64(head_ptr, &first))
+		return false;
+
+	/* Make the worker point to the previous head. */
+	if (put_user(first, node))
+		return false;
+
+	return true;
+}
+
+/**
+ * get_idle_server - retrieve an idle server, if present.
+ *
+ * Returns true on success, false on a fatal failure.
+ */
+static bool get_idle_server(struct umcg_task __user *ut_worker, u32 *server_tid)
+{
+	u64 server_tid_ptr;
+	u32 tid;
+
+	/* Empty result is OK. */
+	*server_tid = 0;
+
+	if (get_user(server_tid_ptr, &ut_worker->idle_server_tid_ptr))
+		return false;
+
+	if (!server_tid_ptr)
+		return false;
+
+	tid = 0;
+	if (xchg_user_32((u32 __user *)server_tid_ptr, &tid))
+		return false;
+
+	*server_tid = tid;
+	return true;
+}
+
+/*
+ * Returns true to wait for the userspace to schedule this worker, false
+ * to return to the userspace.
+ *
+ * In the common case, a BLOCKED worker is marked IDLE and enqueued
+ * to idle_workers_ptr list. The idle server is woken (if present).
+ *
+ * If a RUNNING worker is preempted, this function will trigger, in which
+ * case the worker is moved to IDLE state and its server is woken.
+ *
+ * Sets @server_tid to point to the server to be woken if the worker
+ * is going to sleep; sets @server_tid to point to the server assigned
+ * to this RUNNING worker if the worker is to return to the userspace.
+ */
+static bool process_waking_worker(struct task_struct *tsk, u32 *server_tid)
+{
+	struct umcg_task __user *ut_worker = tsk->umcg_task;
+	u64 curr_state, next_state;
+
+	*server_tid = 0;
+
+	if (WARN_ONCE((tsk != current) || !ut_worker, "Invalid umcg worker"))
+		return false;
+
+	if (fatal_signal_pending(tsk))
+		return false;
+
+	if (get_user(curr_state, &ut_worker->state_ts))
+		goto die;
+
+	if ((curr_state & UMCG_TASK_STATE_MASK) == UMCG_TASK_RUNNING) {
+		u32 tid;
+
+		/* Wakeup: wait but don't enqueue. */
+		if (curr_state & UMCG_TF_LOCKED)
+			return true;
+
+		smp_mb();  /* Order getting state and getting server_tid */
+		if (get_user(tid, &ut_worker->next_tid))
+			goto die;
+
+		if (!tid)
+			/* RUNNING workers must have servers. */
+			goto die;
+
+		*server_tid = tid;
+
+		/* pass-through: RUNNING with a server. */
+		if (!(curr_state & UMCG_TF_PREEMPTED))
+			return false;
+
+		/*
+		 * Fallthrough to mark the worker IDLE: the worker is
+		 * PREEMPTED.
+		 */
+	} else if (unlikely((curr_state & UMCG_TASK_STATE_MASK) == UMCG_TASK_IDLE &&
+			(curr_state & UMCG_TF_LOCKED)))
+		/* The worker prepares to sleep or to unregister. */
+		return false;
+
+	if (unlikely((curr_state & UMCG_TASK_STATE_MASK) == UMCG_TASK_IDLE))
+		goto die;
+
+	next_state = curr_state & ~UMCG_TASK_STATE_MASK;
+	next_state |= UMCG_TASK_IDLE;
+
+	if (umcg_update_state(&ut_worker->state_ts, &curr_state,
+			next_state, true))
+		goto die;
+
+	if (!enqueue_idle_worker(ut_worker))
+		goto die;
+
+	smp_mb();  /* Order enqueuing the worker with getting the server. */
+	if (!(*server_tid) && !get_idle_server(ut_worker, server_tid))
+		goto die;
+
+	return true;
+
+die:
+	pr_warn("umcg_process_waking_worker: killing task %d\n", current->pid);
+	force_sig(SIGKILL);
+	return false;
+}
+
+/*
+ * Called from sched_update_worker(): defer all work until later, as
+ * sched_update_worker() may be called with in-kernel locks held.
+ */
+void umcg_wq_worker_running(struct task_struct *tsk)
+{
+	set_tsk_thread_flag(tsk, TIF_NOTIFY_RESUME);
+}
+
+/* Called via TIF_NOTIFY_RESUME flag from exit_to_user_mode_loop. */
+void umcg_handle_resuming_worker(void)
+{
+	u32 server_tid;
+
+	/* Avoid recursion by removing PF_UMCG_WORKER */
+	current->flags &= ~PF_UMCG_WORKER;
+
+	do {
+		bool should_wait;
+
+		should_wait = process_waking_worker(current, &server_tid);
+		if (!should_wait)
+			break;
+
+		if (server_tid) {
+			int ret = umcg_wake_idle_server(server_tid, true);
+
+			if (ret && ret != -EAGAIN)
+				goto die;
+		}
+
+		umcg_idle_loop(0);
+	} while (true);
+
+	if (!server_tid)
+		/* No server => no reason to pin pages. */
+		umcg_unpin_pages();
+	else if (umcg_pin_pages(server_tid))
+		goto die;
+
+	goto out;
+
+die:
+	pr_warn("%s: killing task %d\n", __func__, current->pid);
+	force_sig(SIGKILL);
+out:
+	current->flags |= PF_UMCG_WORKER;
+}