kernel/sched/membarrier.c - linux/torvalds/linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * Copyright (C) 2010-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  *
  * membarrier system call
  */
 #include "sched.h"

 /*
  * Bitmask made from a "or" of all commands within enum membarrier_cmd,
  * except MEMBARRIER_CMD_QUERY.
  */
 #ifdef CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE
 #define MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK			\
 	(MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE			\
 	| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE)
 #else
 #define MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK	0
 #endif

 #define MEMBARRIER_CMD_BITMASK						\
 	(MEMBARRIER_CMD_GLOBAL | MEMBARRIER_CMD_GLOBAL_EXPEDITED	\
 	| MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED			\
 	| MEMBARRIER_CMD_PRIVATE_EXPEDITED				\
 	| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED			\
 	| MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK)

 static void ipi_mb(void *info)
 {
 	smp_mb();	/* IPIs should be serializing but paranoid. */
 }

 static void ipi_sync_rq_state(void *info)
 {
 	struct mm_struct *mm = (struct mm_struct *) info;

 	if (current->mm != mm)
 		return;
 	this_cpu_write(runqueues.membarrier_state,
 		       atomic_read(&mm->membarrier_state));
 	/*
 	 * Issue a memory barrier after setting
 	 * MEMBARRIER_STATE_GLOBAL_EXPEDITED in the current runqueue to
 	 * guarantee that no memory access following registration is reordered
 	 * before registration.
 	 */
 	smp_mb();
 }

 void membarrier_exec_mmap(struct mm_struct *mm)
 {
 	/*
 	 * Issue a memory barrier before clearing membarrier_state to
 	 * guarantee that no memory access prior to exec is reordered after
 	 * clearing this state.
 	 */
 	smp_mb();
 	atomic_set(&mm->membarrier_state, 0);
 	/*
 	 * Keep the runqueue membarrier_state in sync with this mm
 	 * membarrier_state.
 	 */
 	this_cpu_write(runqueues.membarrier_state, 0);
 }

 static int membarrier_global_expedited(void)
 {
 	int cpu;
 	cpumask_var_t tmpmask;

 	if (num_online_cpus() == 1)
 		return 0;

 	/*
 	 * Matches memory barriers around rq->curr modification in
 	 * scheduler.
 	 */
 	smp_mb();	/* system call entry is not a mb. */

 	if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
 		return -ENOMEM;

 	cpus_read_lock();
 	rcu_read_lock();
 	for_each_online_cpu(cpu) {
 		struct task_struct *p;

 		/*
 		 * Skipping the current CPU is OK even through we can be
 		 * migrated at any point. The current CPU, at the point
 		 * where we read raw_smp_processor_id(), is ensured to
 		 * be in program order with respect to the caller
 		 * thread. Therefore, we can skip this CPU from the
 		 * iteration.
 		 */
 		if (cpu == raw_smp_processor_id())
 			continue;

 		if (!(READ_ONCE(cpu_rq(cpu)->membarrier_state) &
 		    MEMBARRIER_STATE_GLOBAL_EXPEDITED))
 			continue;

 		/*
 		 * Skip the CPU if it runs a kernel thread. The scheduler
 		 * leaves the prior task mm in place as an optimization when
 		 * scheduling a kthread.
 		 */
 		p = rcu_dereference(cpu_rq(cpu)->curr);
 		if (p->flags & PF_KTHREAD)
 			continue;

 		__cpumask_set_cpu(cpu, tmpmask);
 	}
 	rcu_read_unlock();

 	preempt_disable();
 	smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
 	preempt_enable();

 	free_cpumask_var(tmpmask);
 	cpus_read_unlock();

 	/*
 	 * Memory barrier on the caller thread _after_ we finished
 	 * waiting for the last IPI. Matches memory barriers around
 	 * rq->curr modification in scheduler.
 	 */
 	smp_mb();	/* exit from system call is not a mb */
 	return 0;
 }

 static int membarrier_private_expedited(int flags)
 {
 	int cpu;
 	cpumask_var_t tmpmask;
 	struct mm_struct *mm = current->mm;

 	if (flags & MEMBARRIER_FLAG_SYNC_CORE) {
 		if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
 			return -EINVAL;
 		if (!(atomic_read(&mm->membarrier_state) &
 		      MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY))
 			return -EPERM;
 	} else {
 		if (!(atomic_read(&mm->membarrier_state) &
 		      MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY))
 			return -EPERM;
 	}

 	if (atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1)
 		return 0;

 	/*
 	 * Matches memory barriers around rq->curr modification in
 	 * scheduler.
 	 */
 	smp_mb();	/* system call entry is not a mb. */

 	if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
 		return -ENOMEM;

 	cpus_read_lock();
 	rcu_read_lock();
 	for_each_online_cpu(cpu) {
 		struct task_struct *p;

 		/*
 		 * Skipping the current CPU is OK even through we can be
 		 * migrated at any point. The current CPU, at the point
 		 * where we read raw_smp_processor_id(), is ensured to
 		 * be in program order with respect to the caller
 		 * thread. Therefore, we can skip this CPU from the
 		 * iteration.
 		 */
 		if (cpu == raw_smp_processor_id())
 			continue;
 		p = rcu_dereference(cpu_rq(cpu)->curr);
 		if (p && p->mm == mm)
 			__cpumask_set_cpu(cpu, tmpmask);
 	}
 	rcu_read_unlock();

 	preempt_disable();
 	smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
 	preempt_enable();

 	free_cpumask_var(tmpmask);
 	cpus_read_unlock();

 	/*
 	 * Memory barrier on the caller thread _after_ we finished
 	 * waiting for the last IPI. Matches memory barriers around
 	 * rq->curr modification in scheduler.
 	 */
 	smp_mb();	/* exit from system call is not a mb */

 	return 0;
 }

 static int sync_runqueues_membarrier_state(struct mm_struct *mm)
 {
 	int membarrier_state = atomic_read(&mm->membarrier_state);
 	cpumask_var_t tmpmask;
 	int cpu;

 	if (atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1) {
 		this_cpu_write(runqueues.membarrier_state, membarrier_state);

 		/*
 		 * For single mm user, we can simply issue a memory barrier
 		 * after setting MEMBARRIER_STATE_GLOBAL_EXPEDITED in the
 		 * mm and in the current runqueue to guarantee that no memory
 		 * access following registration is reordered before
 		 * registration.
 		 */
 		smp_mb();
 		return 0;
 	}

 	if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
 		return -ENOMEM;

 	/*
 	 * For mm with multiple users, we need to ensure all future
 	 * scheduler executions will observe @mm's new membarrier
 	 * state.
 	 */
 	synchronize_rcu();

 	/*
 	 * For each cpu runqueue, if the task's mm match @mm, ensure that all
 	 * @mm's membarrier state set bits are also set in in the runqueue's
 	 * membarrier state. This ensures that a runqueue scheduling
 	 * between threads which are users of @mm has its membarrier state
 	 * updated.
 	 */
 	cpus_read_lock();
 	rcu_read_lock();
 	for_each_online_cpu(cpu) {
 		struct rq *rq = cpu_rq(cpu);
 		struct task_struct *p;

 		p = rcu_dereference(rq->curr);
 		if (p && p->mm == mm)
 			__cpumask_set_cpu(cpu, tmpmask);
 	}
 	rcu_read_unlock();

 	preempt_disable();
 	smp_call_function_many(tmpmask, ipi_sync_rq_state, mm, 1);
 	preempt_enable();

 	free_cpumask_var(tmpmask);
 	cpus_read_unlock();

 	return 0;
 }

 static int membarrier_register_global_expedited(void)
 {
 	struct task_struct *p = current;
 	struct mm_struct *mm = p->mm;
 	int ret;

 	if (atomic_read(&mm->membarrier_state) &
 	    MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY)
 		return 0;
 	atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED, &mm->membarrier_state);
 	ret = sync_runqueues_membarrier_state(mm);
 	if (ret)
 		return ret;
 	atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY,
 		  &mm->membarrier_state);

 	return 0;
 }

 static int membarrier_register_private_expedited(int flags)
 {
 	struct task_struct *p = current;
 	struct mm_struct *mm = p->mm;
 	int ready_state = MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY,
 	    set_state = MEMBARRIER_STATE_PRIVATE_EXPEDITED,
 	    ret;

 	if (flags & MEMBARRIER_FLAG_SYNC_CORE) {
 		if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
 			return -EINVAL;
 		ready_state =
 			MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY;
 	}

 	/*
 	 * We need to consider threads belonging to different thread
 	 * groups, which use the same mm. (CLONE_VM but not
 	 * CLONE_THREAD).
 	 */
 	if ((atomic_read(&mm->membarrier_state) & ready_state) == ready_state)
 		return 0;
 	if (flags & MEMBARRIER_FLAG_SYNC_CORE)
 		set_state |= MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE;
 	atomic_or(set_state, &mm->membarrier_state);
 	ret = sync_runqueues_membarrier_state(mm);
 	if (ret)
 		return ret;
 	atomic_or(ready_state, &mm->membarrier_state);

 	return 0;
 }

 /**
  * sys_membarrier - issue memory barriers on a set of threads
  * @cmd:   Takes command values defined in enum membarrier_cmd.
  * @flags: Currently needs to be 0. For future extensions.
  *
  * If this system call is not implemented, -ENOSYS is returned. If the
  * command specified does not exist, not available on the running
  * kernel, or if the command argument is invalid, this system call
  * returns -EINVAL. For a given command, with flags argument set to 0,
  * if this system call returns -ENOSYS or -EINVAL, it is guaranteed to
  * always return the same value until reboot. In addition, it can return
  * -ENOMEM if there is not enough memory available to perform the system
  * call.
  *
  * All memory accesses performed in program order from each targeted thread
  * is guaranteed to be ordered with respect to sys_membarrier(). If we use
  * the semantic "barrier()" to represent a compiler barrier forcing memory
  * accesses to be performed in program order across the barrier, and
  * smp_mb() to represent explicit memory barriers forcing full memory
  * ordering across the barrier, we have the following ordering table for
  * each pair of barrier(), sys_membarrier() and smp_mb():
  *
  * The pair ordering is detailed as (O: ordered, X: not ordered):
  *
  *                        barrier()   smp_mb() sys_membarrier()
  *        barrier()          X           X            O
  *        smp_mb()           X           O            O
  *        sys_membarrier()   O           O            O
  */
 SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
 {
 	if (unlikely(flags))
 		return -EINVAL;
 	switch (cmd) {
 	case MEMBARRIER_CMD_QUERY:
 	{
 		int cmd_mask = MEMBARRIER_CMD_BITMASK;

 		if (tick_nohz_full_enabled())
 			cmd_mask &= ~MEMBARRIER_CMD_GLOBAL;
 		return cmd_mask;
 	}
 	case MEMBARRIER_CMD_GLOBAL:
 		/* MEMBARRIER_CMD_GLOBAL is not compatible with nohz_full. */
 		if (tick_nohz_full_enabled())
 			return -EINVAL;
 		if (num_online_cpus() > 1)
 			synchronize_rcu();
 		return 0;
 	case MEMBARRIER_CMD_GLOBAL_EXPEDITED:
 		return membarrier_global_expedited();
 	case MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED:
 		return membarrier_register_global_expedited();
 	case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
 		return membarrier_private_expedited(0);
 	case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED:
 		return membarrier_register_private_expedited(0);
 	case MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE:
 		return membarrier_private_expedited(MEMBARRIER_FLAG_SYNC_CORE);
 	case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE:
 		return membarrier_register_private_expedited(MEMBARRIER_FLAG_SYNC_CORE);
 	default:
 		return -EINVAL;
 	}
 }
	// SPDX-License-Identifier: GPL-2.0-or-later
	/*
	* Copyright (C) 2010-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
	*
	* membarrier system call
	*/
	#include "sched.h"

	/*
	* Bitmask made from a "or" of all commands within enum membarrier_cmd,
	* except MEMBARRIER_CMD_QUERY.
	*/
	#ifdef CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE
	#define MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK \
	(MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE \
	\| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE)
	#else
	#define MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK 0
	#endif

	#define MEMBARRIER_CMD_BITMASK \
	(MEMBARRIER_CMD_GLOBAL \| MEMBARRIER_CMD_GLOBAL_EXPEDITED \
	\| MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED \
	\| MEMBARRIER_CMD_PRIVATE_EXPEDITED \
	\| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED \
	\| MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK)

	static void ipi_mb(void *info)
	{
	smp_mb(); /* IPIs should be serializing but paranoid. */
	}

	static void ipi_sync_rq_state(void *info)
	{
	struct mm_struct mm = (struct mm_struct ) info;

	if (current->mm != mm)
	return;
	this_cpu_write(runqueues.membarrier_state,
	atomic_read(&mm->membarrier_state));
	/*
	* Issue a memory barrier after setting
	* MEMBARRIER_STATE_GLOBAL_EXPEDITED in the current runqueue to
	* guarantee that no memory access following registration is reordered
	* before registration.
	*/
	smp_mb();
	}

	void membarrier_exec_mmap(struct mm_struct *mm)
	{
	/*
	* Issue a memory barrier before clearing membarrier_state to
	* guarantee that no memory access prior to exec is reordered after
	* clearing this state.
	*/
	smp_mb();
	atomic_set(&mm->membarrier_state, 0);
	/*
	* Keep the runqueue membarrier_state in sync with this mm
	* membarrier_state.
	*/
	this_cpu_write(runqueues.membarrier_state, 0);
	}

	static int membarrier_global_expedited(void)
	{
	int cpu;
	cpumask_var_t tmpmask;

	if (num_online_cpus() == 1)
	return 0;

	/*
	* Matches memory barriers around rq->curr modification in
	* scheduler.
	*/
	smp_mb(); /* system call entry is not a mb. */

	if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
	return -ENOMEM;

	cpus_read_lock();
	rcu_read_lock();
	for_each_online_cpu(cpu) {
	struct task_struct *p;

	/*
	* Skipping the current CPU is OK even through we can be
	* migrated at any point. The current CPU, at the point
	* where we read raw_smp_processor_id(), is ensured to
	* be in program order with respect to the caller
	* thread. Therefore, we can skip this CPU from the
	* iteration.
	*/
	if (cpu == raw_smp_processor_id())
	continue;

	if (!(READ_ONCE(cpu_rq(cpu)->membarrier_state) &
	MEMBARRIER_STATE_GLOBAL_EXPEDITED))
	continue;

	/*
	* Skip the CPU if it runs a kernel thread. The scheduler
	* leaves the prior task mm in place as an optimization when
	* scheduling a kthread.
	*/
	p = rcu_dereference(cpu_rq(cpu)->curr);
	if (p->flags & PF_KTHREAD)
	continue;

	__cpumask_set_cpu(cpu, tmpmask);
	}
	rcu_read_unlock();

	preempt_disable();
	smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
	preempt_enable();

	free_cpumask_var(tmpmask);
	cpus_read_unlock();

	/*
	* Memory barrier on the caller thread _after_ we finished
	* waiting for the last IPI. Matches memory barriers around
	* rq->curr modification in scheduler.
	*/
	smp_mb(); /* exit from system call is not a mb */
	return 0;
	}

	static int membarrier_private_expedited(int flags)
	{
	int cpu;
	cpumask_var_t tmpmask;
	struct mm_struct *mm = current->mm;

	if (flags & MEMBARRIER_FLAG_SYNC_CORE) {
	if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
	return -EINVAL;
	if (!(atomic_read(&mm->membarrier_state) &
	MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY))
	return -EPERM;
	} else {
	if (!(atomic_read(&mm->membarrier_state) &
	MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY))
	return -EPERM;
	}

	if (atomic_read(&mm->mm_users) == 1 \|\| num_online_cpus() == 1)
	return 0;

	/*
	* Matches memory barriers around rq->curr modification in
	* scheduler.
	*/
	smp_mb(); /* system call entry is not a mb. */

	if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
	return -ENOMEM;

	cpus_read_lock();
	rcu_read_lock();
	for_each_online_cpu(cpu) {
	struct task_struct *p;

	/*
	* Skipping the current CPU is OK even through we can be
	* migrated at any point. The current CPU, at the point
	* where we read raw_smp_processor_id(), is ensured to
	* be in program order with respect to the caller
	* thread. Therefore, we can skip this CPU from the
	* iteration.
	*/
	if (cpu == raw_smp_processor_id())
	continue;
	p = rcu_dereference(cpu_rq(cpu)->curr);
	if (p && p->mm == mm)
	__cpumask_set_cpu(cpu, tmpmask);
	}
	rcu_read_unlock();

	preempt_disable();
	smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
	preempt_enable();

	free_cpumask_var(tmpmask);
	cpus_read_unlock();

	/*
	* Memory barrier on the caller thread _after_ we finished
	* waiting for the last IPI. Matches memory barriers around
	* rq->curr modification in scheduler.
	*/
	smp_mb(); /* exit from system call is not a mb */

	return 0;
	}

	static int sync_runqueues_membarrier_state(struct mm_struct *mm)
	{
	int membarrier_state = atomic_read(&mm->membarrier_state);
	cpumask_var_t tmpmask;
	int cpu;

	if (atomic_read(&mm->mm_users) == 1 \|\| num_online_cpus() == 1) {
	this_cpu_write(runqueues.membarrier_state, membarrier_state);

	/*
	* For single mm user, we can simply issue a memory barrier
	* after setting MEMBARRIER_STATE_GLOBAL_EXPEDITED in the
	* mm and in the current runqueue to guarantee that no memory
	* access following registration is reordered before
	* registration.
	*/
	smp_mb();
	return 0;
	}

	if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
	return -ENOMEM;

	/*
	* For mm with multiple users, we need to ensure all future
	* scheduler executions will observe @mm's new membarrier
	* state.
	*/
	synchronize_rcu();

	/*
	* For each cpu runqueue, if the task's mm match @mm, ensure that all
	* @mm's membarrier state set bits are also set in in the runqueue's
	* membarrier state. This ensures that a runqueue scheduling
	* between threads which are users of @mm has its membarrier state
	* updated.
	*/
	cpus_read_lock();
	rcu_read_lock();
	for_each_online_cpu(cpu) {
	struct rq *rq = cpu_rq(cpu);
	struct task_struct *p;

	p = rcu_dereference(rq->curr);
	if (p && p->mm == mm)
	__cpumask_set_cpu(cpu, tmpmask);
	}
	rcu_read_unlock();

	preempt_disable();
	smp_call_function_many(tmpmask, ipi_sync_rq_state, mm, 1);
	preempt_enable();

	free_cpumask_var(tmpmask);
	cpus_read_unlock();

	return 0;
	}

	static int membarrier_register_global_expedited(void)
	{
	struct task_struct *p = current;
	struct mm_struct *mm = p->mm;
	int ret;

	if (atomic_read(&mm->membarrier_state) &
	MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY)
	return 0;
	atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED, &mm->membarrier_state);
	ret = sync_runqueues_membarrier_state(mm);
	if (ret)
	return ret;
	atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY,
	&mm->membarrier_state);

	return 0;
	}

	static int membarrier_register_private_expedited(int flags)
	{
	struct task_struct *p = current;
	struct mm_struct *mm = p->mm;
	int ready_state = MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY,
	set_state = MEMBARRIER_STATE_PRIVATE_EXPEDITED,
	ret;

	if (flags & MEMBARRIER_FLAG_SYNC_CORE) {
	if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
	return -EINVAL;
	ready_state =
	MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY;
	}

	/*
	* We need to consider threads belonging to different thread
	* groups, which use the same mm. (CLONE_VM but not
	* CLONE_THREAD).
	*/
	if ((atomic_read(&mm->membarrier_state) & ready_state) == ready_state)
	return 0;
	if (flags & MEMBARRIER_FLAG_SYNC_CORE)
	set_state \|= MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE;
	atomic_or(set_state, &mm->membarrier_state);
	ret = sync_runqueues_membarrier_state(mm);
	if (ret)
	return ret;
	atomic_or(ready_state, &mm->membarrier_state);

	return 0;
	}

	/**
	* sys_membarrier - issue memory barriers on a set of threads
	* @cmd: Takes command values defined in enum membarrier_cmd.
	* @flags: Currently needs to be 0. For future extensions.
	*
	* If this system call is not implemented, -ENOSYS is returned. If the
	* command specified does not exist, not available on the running
	* kernel, or if the command argument is invalid, this system call
	* returns -EINVAL. For a given command, with flags argument set to 0,
	* if this system call returns -ENOSYS or -EINVAL, it is guaranteed to
	* always return the same value until reboot. In addition, it can return
	* -ENOMEM if there is not enough memory available to perform the system
	* call.
	*
	* All memory accesses performed in program order from each targeted thread
	* is guaranteed to be ordered with respect to sys_membarrier(). If we use
	* the semantic "barrier()" to represent a compiler barrier forcing memory
	* accesses to be performed in program order across the barrier, and
	* smp_mb() to represent explicit memory barriers forcing full memory
	* ordering across the barrier, we have the following ordering table for
	* each pair of barrier(), sys_membarrier() and smp_mb():
	*
	* The pair ordering is detailed as (O: ordered, X: not ordered):
	*
	* barrier() smp_mb() sys_membarrier()
	* barrier() X X O
	* smp_mb() X O O
	* sys_membarrier() O O O
	*/
	SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
	{
	if (unlikely(flags))
	return -EINVAL;
	switch (cmd) {
	case MEMBARRIER_CMD_QUERY:
	{
	int cmd_mask = MEMBARRIER_CMD_BITMASK;

	if (tick_nohz_full_enabled())
	cmd_mask &= ~MEMBARRIER_CMD_GLOBAL;
	return cmd_mask;
	}
	case MEMBARRIER_CMD_GLOBAL:
	/* MEMBARRIER_CMD_GLOBAL is not compatible with nohz_full. */
	if (tick_nohz_full_enabled())
	return -EINVAL;
	if (num_online_cpus() > 1)
	synchronize_rcu();
	return 0;
	case MEMBARRIER_CMD_GLOBAL_EXPEDITED:
	return membarrier_global_expedited();
	case MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED:
	return membarrier_register_global_expedited();
	case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
	return membarrier_private_expedited(0);
	case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED:
	return membarrier_register_private_expedited(0);
	case MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE:
	return membarrier_private_expedited(MEMBARRIER_FLAG_SYNC_CORE);
	case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE:
	return membarrier_register_private_expedited(MEMBARRIER_FLAG_SYNC_CORE);
	default:
	return -EINVAL;
	}
	}