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Subject[RFC][PATCH 3/3] sched: User Mode Concurency Groups
User Managed Concurrency Groups is an M:N threading toolkit that allows
constructing user space schedulers designed to efficiently manage
heterogeneous in-process workloads while maintaining high CPU
utilization (95%+).

XXX moar changelog explaining how this is moar awesome than
traditional user-space threading.

The big thing that's missing is the SMP wake-to-remote-idle.

The big assumption this whole thing is build on is that
pin_user_pages() preserves user mappings in so far that
pagefault_disable() will never generate EFAULT (unless the user does
munmap() in which case it can keep the pieces).

shrink_page_list() does page_maybe_dma_pinned() before try_to_unmap()
and as such seems to respect this constraint.

unmap_and_move() however seems willing to unmap otherwise pinned (and
hence unmigratable) pages. This might need fixing.

Originally-by: Peter Oskolkov <posk@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/Kconfig | 1
arch/x86/entry/syscalls/syscall_64.tbl | 3
arch/x86/include/asm/thread_info.h | 2
fs/exec.c | 1
include/linux/entry-common.h | 6
include/linux/sched.h | 86 +++
include/linux/syscalls.h | 4
include/linux/thread_info.h | 2
include/uapi/asm-generic/unistd.h | 9
include/uapi/linux/umcg.h | 143 +++++
init/Kconfig | 15
kernel/entry/common.c | 18
kernel/exit.c | 5
kernel/sched/Makefile | 1
kernel/sched/core.c | 9
kernel/sched/umcg.c | 868 +++++++++++++++++++++++++++++++++
kernel/sys_ni.c | 5
17 files changed, 1171 insertions(+), 7 deletions(-)

--- 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
@@ -22,6 +22,10 @@
# define _TIF_UPROBE (0)
#endif

+#ifndef _TIF_UMCG
+# define _TIF_UMCG (0)
+#endif
+
/*
* SYSCALL_WORK flags handled in syscall_enter_from_user_mode()
*/
@@ -42,11 +46,13 @@
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)

--- 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 +1295,23 @@ 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_worker_page;
+
+ struct task_struct *umcg_server;
+ struct umcg_task __user *umcg_server_task;
+ struct page *umcg_server_page;
+
+ struct task_struct *umcg_next;
+ struct umcg_task __user *umcg_next_task;
+ struct page *umcg_next_page;
+#endif
+
struct tlbflush_unmap_batch tlb_ubc;

union {
@@ -1687,6 +1705,13 @@ 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 +2319,67 @@ 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 +1,143 @@
+/* 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 6 bits of struct umcg_task.state_ts.
+ * The states represent the user space point of view.
+ *
+ * ,--------(TF_PREEMPT + notify_resume)-------. ,------------.
+ * | v | |
+ * RUNNING -(schedule)-> BLOCKED -(sys_exit)-> RUNNABLE (signal + notify_resume)
+ * ^ | ^ |
+ * `--------------(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->status = UMCG_TASK_RUNNABLE | UMCG_TF_COND_WAIT;
+ * // smp_mb() implied by xchg()
+ *
+ * runnable_ptr = xchg(self->runnable_workers_ptr, NULL);
+ * while (runnable_ptr) {
+ * next = runnable_ptr->runnable_workers_ptr;
+ *
+ * umcg_server_add_runnable(self, runnable_ptr);
+ *
+ * runnable_ptr = next;
+ * }
+ *
+ * self->next = 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_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 - 7: task state;
+ * bits 8 - 15: state flags;
+ * bits 16 - 31: for userspace use;
+ */
+ __u32 state; /* r/w */
+
+ /**
+ * @next_tid: the TID of the UMCG task that should be context-switched
+ * into in sys_umcg_wait(). Can be zero, in which case
+ * it'll switch to server_tid.
+ *
+ * @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, in CLOCK_MONOTONIC.
+ */
+ __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 +1,868 @@
+// 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 <asm/syscall.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);
+ if (tsk && current->mm == tsk->mm && tsk->umcg_task)
+ get_task_struct(tsk);
+ else
+ tsk = NULL;
+ rcu_read_unlock();
+ }
+
+ return tsk;
+}
+
+/**
+ * umcg_pin_pages: pin pages containing struct umcg_task of
+ * this task, its server (possibly this task again)
+ * and the next (possibly NULL).
+ */
+static int umcg_pin_pages(void)
+{
+ struct task_struct *server = NULL, *next = NULL, *tsk = current;
+ struct umcg_task __user *self = READ_ONCE(tsk->umcg_task);
+ int server_tid, next_tid;
+ int ret;
+
+ /* must not have stale state */
+ if (WARN_ON_ONCE(tsk->umcg_worker_page ||
+ tsk->umcg_server_page ||
+ tsk->umcg_next_page ||
+ tsk->umcg_server_task ||
+ tsk->umcg_next_task ||
+ tsk->umcg_server ||
+ tsk->umcg_next))
+ return -EBUSY;
+
+ ret = -EFAULT;
+ if (pin_user_pages_fast((unsigned long)self, 1, 0,
+ &tsk->umcg_worker_page) != 1)
+ goto clear_self;
+
+ if (get_user(server_tid, &self->server_tid))
+ goto unpin_self;
+
+ ret = -ESRCH;
+ server = umcg_get_task(server_tid);
+ if (!server)
+ goto unpin_self;
+
+ ret = -EFAULT;
+ /* must cache due to possible concurrent change vs access_ok() */
+ tsk->umcg_server_task = READ_ONCE(server->umcg_task);
+ if (pin_user_pages_fast((unsigned long)tsk->umcg_server_task, 1, 0,
+ &tsk->umcg_server_page) != 1)
+ goto clear_server;
+
+ tsk->umcg_server = server;
+
+ if (get_user(next_tid, &self->next_tid))
+ goto unpin_server;
+
+ if (!next_tid)
+ goto done;
+
+ ret = -ESRCH;
+ next = umcg_get_task(next_tid);
+ if (!next)
+ goto unpin_server;
+
+ ret = -EFAULT;
+ tsk->umcg_next_task = READ_ONCE(next->umcg_task);
+ if (pin_user_pages_fast((unsigned long)tsk->umcg_next_task, 1, 0,
+ &tsk->umcg_next_page) != 1)
+ goto clear_next;
+
+ tsk->umcg_next = next;
+
+done:
+ return 0;
+
+clear_next:
+ tsk->umcg_next_task = NULL;
+ tsk->umcg_next_page = NULL;
+
+unpin_server:
+ unpin_user_page(tsk->umcg_server_page);
+
+clear_server:
+ tsk->umcg_server_task = NULL;
+ tsk->umcg_server_page = NULL;
+
+unpin_self:
+ unpin_user_page(tsk->umcg_worker_page);
+clear_self:
+ tsk->umcg_worker_page = NULL;
+
+ return ret;
+}
+
+static void umcg_unpin_pages(void)
+{
+ struct task_struct *tsk = current;
+
+ if (tsk->umcg_server) {
+ unpin_user_page(tsk->umcg_worker_page);
+ tsk->umcg_worker_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;
+
+ if (tsk->umcg_next) {
+ unpin_user_page(tsk->umcg_next_page);
+ tsk->umcg_next_page = NULL;
+ tsk->umcg_next_task = NULL;
+
+ put_task_struct(tsk->umcg_next);
+ tsk->umcg_next = 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_worker_page = NULL;
+
+ tsk->umcg_server = NULL;
+ tsk->umcg_server_page = NULL;
+ tsk->umcg_server_task = NULL;
+
+ tsk->umcg_next = NULL;
+ tsk->umcg_next_page = NULL;
+ tsk->umcg_next_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 */
+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_next(struct task_struct *tsk)
+{
+ int ret = umcg_wake_task(tsk->umcg_next, tsk->umcg_next_task);
+ if (ret)
+ return ret;
+
+ /*
+ * If userspace sets umcg_task::next_tid, it needs to remove
+ * that task from the ready-queue to avoid another server
+ * selecting it. However, that also means it needs to put it
+ * back in case it went unused.
+ *
+ * By clearing the field on use, userspace can detect this case
+ * and DTRT.
+ */
+ if (put_user(0u, &tsk->umcg_task->next_tid))
+ return -EFAULT;
+
+ return 0;
+}
+
+static int umcg_wake_server(struct task_struct *tsk)
+{
+ int ret = umcg_wake_task(tsk->umcg_server, tsk->umcg_server_task);
+ switch (ret) {
+ case 0:
+ case -EAGAIN:
+ /*
+ * Server could have timed-out or already be running
+ * due to a runnable enqueue. See umcg_sys_exit().
+ */
+ break;
+
+ default:
+ return ret;
+ }
+
+ return 0;
+}
+
+/*
+ * Wake ::next_tid or ::server_tid.
+ *
+ * Must be called in umcg_pin_pages() context, relies on
+ * tsk->umcg_{server,next}.
+ *
+ * Returns:
+ * 0: success
+ * -EAGAIN
+ * -EFAULT
+ */
+static int umcg_wake(struct task_struct *tsk)
+{
+ if (tsk->umcg_next)
+ return umcg_wake_next(tsk);
+
+ return umcg_wake_server(tsk);
+}
+
+/* pre-schedule() */
+void umcg_wq_worker_sleeping(struct task_struct *tsk)
+{
+ struct umcg_task __user *self = READ_ONCE(tsk->umcg_task);
+
+ /* Must not fault, mmap_sem might be held. */
+ pagefault_disable();
+
+ if (WARN_ON_ONCE(!tsk->umcg_server))
+ UMCG_DIE_PF("no server");
+
+ if (umcg_update_state(tsk, self, UMCG_TASK_RUNNING, UMCG_TASK_BLOCKED))
+ UMCG_DIE_PF("state");
+
+ if (umcg_wake(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 self_ptr = (unsigned long)self;
+ u64 first_ptr;
+
+ /*
+ * 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, &server->runnable_workers_ptr, Efault);
+ do {
+ unsafe_put_user(first_ptr, &self->runnable_workers_ptr, Efault);
+ } while (!unsafe_try_cmpxchg_user(&server->runnable_workers_ptr, &first_ptr, self_ptr, Efault));
+
+ user_access_end();
+ return 0;
+
+Efault:
+ user_access_end();
+ return -EFAULT;
+}
+
+/*
+ * umcg_wait: Wait for ->state to become RUNNING
+ *
+ * Returns:
+ * 0 - success
+ * -EINTR - pending signal
+ * -EINVAL - ::state is not {RUNNABLE,RUNNING}
+ * -ETIMEDOUT
+ * -EFAULT
+ */
+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();
+
+ ret = -EFAULT;
+ if (page) {
+ unpin_user_page(page);
+ page = NULL;
+ break;
+ }
+
+ if (pin_user_pages_fast((unsigned long)self, 1, 0, &page) != 1) {
+ 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;
+}
+
+void umcg_sys_exit(struct pt_regs *regs)
+{
+ struct task_struct *tsk = current;
+ struct umcg_task __user *self = READ_ONCE(tsk->umcg_task);
+ long syscall = syscall_get_nr(tsk, regs);
+
+ if (syscall == __NR_umcg_wait)
+ return;
+
+ /*
+ * sys_umcg_ctl() will get here without having called umcg_sys_enter()
+ * as such it will look like a syscall that blocked.
+ */
+
+ if (tsk->umcg_server) {
+ /*
+ * Didn't block, we done.
+ */
+ umcg_unpin_pages();
+ return;
+ }
+
+ /* avoid recursion vs schedule() */
+ current->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_runnable(tsk))
+ UMCG_DIE_UNPIN("enqueue");
+
+ /* Server might not be RUNNABLE, means it's already running */
+ if (umcg_wake_server(tsk))
+ UMCG_DIE_UNPIN("wake-server");
+
+ umcg_unpin_pages();
+
+ switch (umcg_wait(0)) {
+ case -EFAULT:
+ case -EINVAL:
+ case -ETIMEDOUT: /* how!?! */
+ default:
+ UMCG_DIE("wait");
+
+ case 0:
+ case -EINTR:
+ /* notify_resume will continue the wait after the signal */
+ break;
+ }
+
+ current->flags |= PF_UMCG_WORKER;
+}
+
+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_runnable(tsk))
+ UMCG_DIE_UNPIN("enqueue");
+
+ /*
+ * XXX do we want a preemption consuming ::next_tid ?
+ * I'm currently leaning towards no.
+ */
+ if (umcg_wake_server(tsk))
+ UMCG_DIE_UNPIN("wake-server");
+
+ umcg_unpin_pages();
+ }
+
+ switch (umcg_wait(0)) {
+ case -EFAULT:
+ case -EINVAL:
+ case -ETIMEDOUT: /* how!?! */
+ default:
+ UMCG_DIE("wait");
+
+ case 0:
+ case -EINTR:
+ /* we'll will resume the wait after the signal */
+ break;
+ }
+
+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;
+}
+
+/**
+ * sys_umcg_wait: transfer running context
+ *
+ * Block until RUNNING. Userspace must already set RUNNABLE to deal with the
+ * sleep condition races (see TF_COND_WAIT).
+ *
+ * Will wake either ::next_tid or ::server_tid to take our place. If this is a
+ * server then not setting ::next_tid will wake self.
+ *
+ * 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;
+
+ if (worker) {
+ ret = umcg_enqueue_runnable(tsk);
+ if (ret)
+ goto unpin;
+ }
+
+ if (worker)
+ ret = umcg_wake(tsk);
+ else if (tsk->umcg_next)
+ ret = umcg_wake_next(tsk);
+
+ if (ret) {
+ /*
+ * XXX already enqueued ourself on ::server_tid; failing now
+ * leaves the lot in an inconsistent state since it'll also
+ * unblock self in order to return the error. !?!?
+ */
+ 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:
+ /*
+ * Workers will still block in umcg_notify_resume() before they can
+ * consume their error, servers however need to get the error asap.
+ *
+ * Still, things might be unrecoverably screwy after this. Not our
+ * problem.
+ */
+ if (!worker)
+ umcg_update_state(tsk, self, UMCG_TASK_RUNNABLE, UMCG_TASK_RUNNING);
+ goto out;
+}
+
+/**
+ * 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->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. 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
+ * -ESRCH - no such server_tid
+ */
+SYSCALL_DEFINE3(umcg_ctl, u32, flags, struct umcg_task __user *, self, clockid_t, which_clock)
+{
+ struct task_struct *server;
+ struct umcg_task ut;
+
+ if ((unsigned long)self % UMCG_TASK_ALIGN)
+ return -EINVAL;
+
+ if (flags & ~(UMCG_CTL_REGISTER |
+ UMCG_CTL_UNREGISTER |
+ UMCG_CTL_WORKER))
+ return -EINVAL;
+
+ if (flags == UMCG_CTL_UNREGISTER) {
+ if (self || !current->umcg_task)
+ return -EINVAL;
+
+ if (current->flags & PF_UMCG_WORKER)
+ umcg_worker_exit();
+ else
+ umcg_clear_task(current);
+
+ return 0;
+ }
+
+ if (!(flags & UMCG_CTL_REGISTER))
+ return -EINVAL;
+
+ flags &= ~UMCG_CTL_REGISTER;
+
+ 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_sys_exit() will transition to RUNNABLE 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;
+}
--- 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 */

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