x86/kvm: Update spectre-v1 mitigation

[mirror_ubuntu-bionic-kernel.git] / kernel / kthread.c

/* Kernel thread helper functions.
 *   Copyright (C) 2004 IBM Corporation, Rusty Russell.
 *
 * Creation is done via kthreadd, so that we get a clean environment
 * even if we're invoked from userspace (think modprobe, hotplug cpu,
 * etc.).
 */
#include <uapi/linux/sched/types.h>
#include <linux/sched.h>
#include <linux/sched/task.h>
#include <linux/kthread.h>
#include <linux/completion.h>
#include <linux/err.h>
#include <linux/cpuset.h>
#include <linux/unistd.h>
#include <linux/file.h>
#include <linux/export.h>
#include <linux/mutex.h>
#include <linux/slab.h>
#include <linux/freezer.h>
#include <linux/ptrace.h>
#include <linux/uaccess.h>
#include <trace/events/sched.h>

static DEFINE_SPINLOCK(kthread_create_lock);
static LIST_HEAD(kthread_create_list);
struct task_struct *kthreadd_task;

struct kthread_create_info
{
        /* Information passed to kthread() from kthreadd. */
        int (*threadfn)(void *data);
        void *data;
        int node;

        /* Result passed back to kthread_create() from kthreadd. */
        struct task_struct *result;
        struct completion *done;

        struct list_head list;
};

struct kthread {
        unsigned long flags;
        unsigned int cpu;
        void *data;
        struct completion parked;
        struct completion exited;
#ifdef CONFIG_BLK_CGROUP
        struct cgroup_subsys_state *blkcg_css;
#endif
};

enum KTHREAD_BITS {
        KTHREAD_IS_PER_CPU = 0,
        KTHREAD_SHOULD_STOP,
        KTHREAD_SHOULD_PARK,
        KTHREAD_IS_PARKED,
};

static inline void set_kthread_struct(void *kthread)
{
        /*
         * We abuse ->set_child_tid to avoid the new member and because it
         * can't be wrongly copied by copy_process(). We also rely on fact
         * that the caller can't exec, so PF_KTHREAD can't be cleared.
         */
        current->set_child_tid = (__force void __user *)kthread;
}

static inline struct kthread *to_kthread(struct task_struct *k)
{
        WARN_ON(!(k->flags & PF_KTHREAD));
        return (__force void *)k->set_child_tid;
}

void free_kthread_struct(struct task_struct *k)
{
        struct kthread *kthread;

        /*
         * Can be NULL if this kthread was created by kernel_thread()
         * or if kmalloc() in kthread() failed.
         */
        kthread = to_kthread(k);
#ifdef CONFIG_BLK_CGROUP
        WARN_ON_ONCE(kthread && kthread->blkcg_css);
#endif
        kfree(kthread);
}

/**
 * kthread_should_stop - should this kthread return now?
 *
 * When someone calls kthread_stop() on your kthread, it will be woken
 * and this will return true.  You should then return, and your return
 * value will be passed through to kthread_stop().
 */
bool kthread_should_stop(void)
{
        return test_bit(KTHREAD_SHOULD_STOP, &to_kthread(current)->flags);
}
EXPORT_SYMBOL(kthread_should_stop);

/**
 * kthread_should_park - should this kthread park now?
 *
 * When someone calls kthread_park() on your kthread, it will be woken
 * and this will return true.  You should then do the necessary
 * cleanup and call kthread_parkme()
 *
 * Similar to kthread_should_stop(), but this keeps the thread alive
 * and in a park position. kthread_unpark() "restarts" the thread and
 * calls the thread function again.
 */
bool kthread_should_park(void)
{
        return test_bit(KTHREAD_SHOULD_PARK, &to_kthread(current)->flags);
}
EXPORT_SYMBOL_GPL(kthread_should_park);

/**
 * kthread_freezable_should_stop - should this freezable kthread return now?
 * @was_frozen: optional out parameter, indicates whether %current was frozen
 *
 * kthread_should_stop() for freezable kthreads, which will enter
 * refrigerator if necessary.  This function is safe from kthread_stop() /
 * freezer deadlock and freezable kthreads should use this function instead
 * of calling try_to_freeze() directly.
 */
bool kthread_freezable_should_stop(bool *was_frozen)
{
        bool frozen = false;

        might_sleep();

        if (unlikely(freezing(current)))
                frozen = __refrigerator(true);

        if (was_frozen)
                *was_frozen = frozen;

        return kthread_should_stop();
}
EXPORT_SYMBOL_GPL(kthread_freezable_should_stop);

/**
 * kthread_data - return data value specified on kthread creation
 * @task: kthread task in question
 *
 * Return the data value specified when kthread @task was created.
 * The caller is responsible for ensuring the validity of @task when
 * calling this function.
 */
void *kthread_data(struct task_struct *task)
{
        return to_kthread(task)->data;
}

/**
 * kthread_probe_data - speculative version of kthread_data()
 * @task: possible kthread task in question
 *
 * @task could be a kthread task.  Return the data value specified when it
 * was created if accessible.  If @task isn't a kthread task or its data is
 * inaccessible for any reason, %NULL is returned.  This function requires
 * that @task itself is safe to dereference.
 */
void *kthread_probe_data(struct task_struct *task)
{
        struct kthread *kthread = to_kthread(task);
        void *data = NULL;

        probe_kernel_read(&data, &kthread->data, sizeof(data));
        return data;
}

static void __kthread_parkme(struct kthread *self)
{
        __set_current_state(TASK_PARKED);
        while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
                if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
                        complete(&self->parked);
                schedule();
                __set_current_state(TASK_PARKED);
        }
        clear_bit(KTHREAD_IS_PARKED, &self->flags);
        __set_current_state(TASK_RUNNING);
}

void kthread_parkme(void)
{
        __kthread_parkme(to_kthread(current));
}
EXPORT_SYMBOL_GPL(kthread_parkme);

static int kthread(void *_create)
{
        /* Copy data: it's on kthread's stack */
        struct kthread_create_info *create = _create;
        int (*threadfn)(void *data) = create->threadfn;
        void *data = create->data;
        struct completion *done;
        struct kthread *self;
        int ret;

        self = kzalloc(sizeof(*self), GFP_KERNEL);
        set_kthread_struct(self);

        /* If user was SIGKILLed, I release the structure. */
        done = xchg(&create->done, NULL);
        if (!done) {
                kfree(create);
                do_exit(-EINTR);
        }

        if (!self) {
                create->result = ERR_PTR(-ENOMEM);
                complete(done);
                do_exit(-ENOMEM);
        }

        self->data = data;
        init_completion(&self->exited);
        init_completion(&self->parked);
        current->vfork_done = &self->exited;

        /* OK, tell user we're spawned, wait for stop or wakeup */
        __set_current_state(TASK_UNINTERRUPTIBLE);
        create->result = current;
        complete(done);
        schedule();

        ret = -EINTR;
        if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {
                cgroup_kthread_ready();
                __kthread_parkme(self);
                ret = threadfn(data);
        }
        do_exit(ret);
}

/* called from do_fork() to get node information for about to be created task */
int tsk_fork_get_node(struct task_struct *tsk)
{
#ifdef CONFIG_NUMA
        if (tsk == kthreadd_task)
                return tsk->pref_node_fork;
#endif
        return NUMA_NO_NODE;
}

static void create_kthread(struct kthread_create_info *create)
{
        int pid;

#ifdef CONFIG_NUMA
        current->pref_node_fork = create->node;
#endif
        /* We want our own signal handler (we take no signals by default). */
        pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD);
        if (pid < 0) {
                /* If user was SIGKILLed, I release the structure. */
                struct completion *done = xchg(&create->done, NULL);

                if (!done) {
                        kfree(create);
                        return;
                }
                create->result = ERR_PTR(pid);
                complete(done);
        }
}

static __printf(4, 0)
struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
                                                    void *data, int node,
                                                    const char namefmt[],
                                                    va_list args)
{
        DECLARE_COMPLETION_ONSTACK(done);
        struct task_struct *task;
        struct kthread_create_info *create = kmalloc(sizeof(*create),
                                                     GFP_KERNEL);

        if (!create)
                return ERR_PTR(-ENOMEM);
        create->threadfn = threadfn;
        create->data = data;
        create->node = node;
        create->done = &done;

        spin_lock(&kthread_create_lock);
        list_add_tail(&create->list, &kthread_create_list);
        spin_unlock(&kthread_create_lock);

        wake_up_process(kthreadd_task);
        /*
         * Wait for completion in killable state, for I might be chosen by
         * the OOM killer while kthreadd is trying to allocate memory for
         * new kernel thread.
         */
        if (unlikely(wait_for_completion_killable(&done))) {
                int i = 0;

                /*
                 * I got SIGKILL, but wait for 10 more seconds for completion
                 * unless chosen by the OOM killer. This delay is there as a
                 * workaround for boot failure caused by SIGKILL upon device
                 * driver initialization timeout.
                 */
                while (i++ < 10 && !test_tsk_thread_flag(current, TIF_MEMDIE))
                        if (wait_for_completion_timeout(&done, HZ))
                                goto ready;
                /*
                 * If I was SIGKILLed before kthreadd (or new kernel thread)
                 * calls complete(), leave the cleanup of this structure to
                 * that thread.
                 */
                if (xchg(&create->done, NULL))
                        return ERR_PTR(-EINTR);
                /*
                 * kthreadd (or new kernel thread) will call complete()
                 * shortly.
                 */
                wait_for_completion(&done);
        }
ready:
        task = create->result;
        if (!IS_ERR(task)) {
                static const struct sched_param param = { .sched_priority = 0 };

                vsnprintf(task->comm, sizeof(task->comm), namefmt, args);
                /*
                 * root may have changed our (kthreadd's) priority or CPU mask.
                 * The kernel thread should not inherit these properties.
                 */
                sched_setscheduler_nocheck(task, SCHED_NORMAL, &param);
                set_cpus_allowed_ptr(task, cpu_all_mask);
        }
        kfree(create);
        return task;
}

/**
 * kthread_create_on_node - create a kthread.
 * @threadfn: the function to run until signal_pending(current).
 * @data: data ptr for @threadfn.
 * @node: task and thread structures for the thread are allocated on this node
 * @namefmt: printf-style name for the thread.
 *
 * Description: This helper function creates and names a kernel
 * thread.  The thread will be stopped: use wake_up_process() to start
 * it.  See also kthread_run().  The new thread has SCHED_NORMAL policy and
 * is affine to all CPUs.
 *
 * If thread is going to be bound on a particular cpu, give its node
 * in @node, to get NUMA affinity for kthread stack, or else give NUMA_NO_NODE.
 * When woken, the thread will run @threadfn() with @data as its
 * argument. @threadfn() can either call do_exit() directly if it is a
 * standalone thread for which no one will call kthread_stop(), or
 * return when 'kthread_should_stop()' is true (which means
 * kthread_stop() has been called).  The return value should be zero
 * or a negative error number; it will be passed to kthread_stop().
 *
 * Returns a task_struct or ERR_PTR(-ENOMEM) or ERR_PTR(-EINTR).
 */
struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
                                           void *data, int node,
                                           const char namefmt[],
                                           ...)
{
        struct task_struct *task;
        va_list args;

        va_start(args, namefmt);
        task = __kthread_create_on_node(threadfn, data, node, namefmt, args);
        va_end(args);

        return task;
}
EXPORT_SYMBOL(kthread_create_on_node);

static void __kthread_bind_mask(struct task_struct *p, const struct cpumask *mask, long state)
{
        unsigned long flags;

        if (!wait_task_inactive(p, state)) {
                WARN_ON(1);
                return;
        }

        /* It's safe because the task is inactive. */
        raw_spin_lock_irqsave(&p->pi_lock, flags);
        do_set_cpus_allowed(p, mask);
        p->flags |= PF_NO_SETAFFINITY;
        raw_spin_unlock_irqrestore(&p->pi_lock, flags);
}

static void __kthread_bind(struct task_struct *p, unsigned int cpu, long state)
{
        __kthread_bind_mask(p, cpumask_of(cpu), state);
}

void kthread_bind_mask(struct task_struct *p, const struct cpumask *mask)
{
        __kthread_bind_mask(p, mask, TASK_UNINTERRUPTIBLE);
}

/**
 * kthread_bind - bind a just-created kthread to a cpu.
 * @p: thread created by kthread_create().
 * @cpu: cpu (might not be online, must be possible) for @k to run on.
 *
 * Description: This function is equivalent to set_cpus_allowed(),
 * except that @cpu doesn't need to be online, and the thread must be
 * stopped (i.e., just returned from kthread_create()).
 */
void kthread_bind(struct task_struct *p, unsigned int cpu)
{
        __kthread_bind(p, cpu, TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(kthread_bind);

/**
 * kthread_create_on_cpu - Create a cpu bound kthread
 * @threadfn: the function to run until signal_pending(current).
 * @data: data ptr for @threadfn.
 * @cpu: The cpu on which the thread should be bound,
 * @namefmt: printf-style name for the thread. Format is restricted
 *           to "name.*%u". Code fills in cpu number.
 *
 * Description: This helper function creates and names a kernel thread
 * The thread will be woken and put into park mode.
 */
struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
                                          void *data, unsigned int cpu,
                                          const char *namefmt)
{
        struct task_struct *p;

        p = kthread_create_on_node(threadfn, data, cpu_to_node(cpu), namefmt,
                                   cpu);
        if (IS_ERR(p))
                return p;
        kthread_bind(p, cpu);
        /* CPU hotplug need to bind once again when unparking the thread. */
        set_bit(KTHREAD_IS_PER_CPU, &to_kthread(p)->flags);
        to_kthread(p)->cpu = cpu;
        return p;
}

/**
 * kthread_unpark - unpark a thread created by kthread_create().
 * @k:          thread created by kthread_create().
 *
 * Sets kthread_should_park() for @k to return false, wakes it, and
 * waits for it to return. If the thread is marked percpu then its
 * bound to the cpu again.
 */
void kthread_unpark(struct task_struct *k)
{
        struct kthread *kthread = to_kthread(k);

        clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
        /*
         * We clear the IS_PARKED bit here as we don't wait
         * until the task has left the park code. So if we'd
         * park before that happens we'd see the IS_PARKED bit
         * which might be about to be cleared.
         */
        if (test_and_clear_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
                /*
                 * Newly created kthread was parked when the CPU was offline.
                 * The binding was lost and we need to set it again.
                 */
                if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags))
                        __kthread_bind(k, kthread->cpu, TASK_PARKED);
                wake_up_state(k, TASK_PARKED);
        }
}
EXPORT_SYMBOL_GPL(kthread_unpark);

/**
 * kthread_park - park a thread created by kthread_create().
 * @k: thread created by kthread_create().
 *
 * Sets kthread_should_park() for @k to return true, wakes it, and
 * waits for it to return. This can also be called after kthread_create()
 * instead of calling wake_up_process(): the thread will park without
 * calling threadfn().
 *
 * Returns 0 if the thread is parked, -ENOSYS if the thread exited.
 * If called by the kthread itself just the park bit is set.
 */
int kthread_park(struct task_struct *k)
{
        struct kthread *kthread = to_kthread(k);

        if (WARN_ON(k->flags & PF_EXITING))
                return -ENOSYS;

        if (!test_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
                set_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
                if (k != current) {
                        wake_up_process(k);
                        wait_for_completion(&kthread->parked);
                }
        }

        return 0;
}
EXPORT_SYMBOL_GPL(kthread_park);

/**
 * kthread_stop - stop a thread created by kthread_create().
 * @k: thread created by kthread_create().
 *
 * Sets kthread_should_stop() for @k to return true, wakes it, and
 * waits for it to exit. This can also be called after kthread_create()
 * instead of calling wake_up_process(): the thread will exit without
 * calling threadfn().
 *
 * If threadfn() may call do_exit() itself, the caller must ensure
 * task_struct can't go away.
 *
 * Returns the result of threadfn(), or %-EINTR if wake_up_process()
 * was never called.
 */
int kthread_stop(struct task_struct *k)
{
        struct kthread *kthread;
        int ret;

        trace_sched_kthread_stop(k);

        get_task_struct(k);
        kthread = to_kthread(k);
        set_bit(KTHREAD_SHOULD_STOP, &kthread->flags);
        kthread_unpark(k);
        wake_up_process(k);
        wait_for_completion(&kthread->exited);
        ret = k->exit_code;
        put_task_struct(k);

        trace_sched_kthread_stop_ret(ret);
        return ret;
}
EXPORT_SYMBOL(kthread_stop);

int kthreadd(void *unused)
{
        struct task_struct *tsk = current;

        /* Setup a clean context for our children to inherit. */
        set_task_comm(tsk, "kthreadd");
        ignore_signals(tsk);
        set_cpus_allowed_ptr(tsk, cpu_all_mask);
        set_mems_allowed(node_states[N_MEMORY]);

        current->flags |= PF_NOFREEZE;
        cgroup_init_kthreadd();

        for (;;) {
                set_current_state(TASK_INTERRUPTIBLE);
                if (list_empty(&kthread_create_list))
                        schedule();
                __set_current_state(TASK_RUNNING);

                spin_lock(&kthread_create_lock);
                while (!list_empty(&kthread_create_list)) {
                        struct kthread_create_info *create;

                        create = list_entry(kthread_create_list.next,
                                            struct kthread_create_info, list);
                        list_del_init(&create->list);
                        spin_unlock(&kthread_create_lock);

                        create_kthread(create);

                        spin_lock(&kthread_create_lock);
                }
                spin_unlock(&kthread_create_lock);
        }

        return 0;
}

void __kthread_init_worker(struct kthread_worker *worker,
                                const char *name,
                                struct lock_class_key *key)
{
        memset(worker, 0, sizeof(struct kthread_worker));
        spin_lock_init(&worker->lock);
        lockdep_set_class_and_name(&worker->lock, key, name);
        INIT_LIST_HEAD(&worker->work_list);
        INIT_LIST_HEAD(&worker->delayed_work_list);
}
EXPORT_SYMBOL_GPL(__kthread_init_worker);

/**
 * kthread_worker_fn - kthread function to process kthread_worker
 * @worker_ptr: pointer to initialized kthread_worker
 *
 * This function implements the main cycle of kthread worker. It processes
 * work_list until it is stopped with kthread_stop(). It sleeps when the queue
 * is empty.
 *
 * The works are not allowed to keep any locks, disable preemption or interrupts
 * when they finish. There is defined a safe point for freezing when one work
 * finishes and before a new one is started.
 *
 * Also the works must not be handled by more than one worker at the same time,
 * see also kthread_queue_work().
 */
int kthread_worker_fn(void *worker_ptr)
{
        struct kthread_worker *worker = worker_ptr;
        struct kthread_work *work;

        /*
         * FIXME: Update the check and remove the assignment when all kthread
         * worker users are created using kthread_create_worker*() functions.
         */
        WARN_ON(worker->task && worker->task != current);
        worker->task = current;

        if (worker->flags & KTW_FREEZABLE)
                set_freezable();

repeat:
        set_current_state(TASK_INTERRUPTIBLE);  /* mb paired w/ kthread_stop */

        if (kthread_should_stop()) {
                __set_current_state(TASK_RUNNING);
                spin_lock_irq(&worker->lock);
                worker->task = NULL;
                spin_unlock_irq(&worker->lock);
                return 0;
        }

        work = NULL;
        spin_lock_irq(&worker->lock);
        if (!list_empty(&worker->work_list)) {
                work = list_first_entry(&worker->work_list,
                                        struct kthread_work, node);
                list_del_init(&work->node);
        }
        worker->current_work = work;
        spin_unlock_irq(&worker->lock);

        if (work) {
                __set_current_state(TASK_RUNNING);
                work->func(work);
        } else if (!freezing(current))
                schedule();

        try_to_freeze();
        cond_resched();
        goto repeat;
}
EXPORT_SYMBOL_GPL(kthread_worker_fn);

static __printf(3, 0) struct kthread_worker *
__kthread_create_worker(int cpu, unsigned int flags,
                        const char namefmt[], va_list args)
{
        struct kthread_worker *worker;
        struct task_struct *task;
        int node = -1;

        worker = kzalloc(sizeof(*worker), GFP_KERNEL);
        if (!worker)
                return ERR_PTR(-ENOMEM);

        kthread_init_worker(worker);

        if (cpu >= 0)
                node = cpu_to_node(cpu);

        task = __kthread_create_on_node(kthread_worker_fn, worker,
                                                node, namefmt, args);
        if (IS_ERR(task))
                goto fail_task;

        if (cpu >= 0)
                kthread_bind(task, cpu);

        worker->flags = flags;
        worker->task = task;
        wake_up_process(task);
        return worker;

fail_task:
        kfree(worker);
        return ERR_CAST(task);
}

/**
 * kthread_create_worker - create a kthread worker
 * @flags: flags modifying the default behavior of the worker
 * @namefmt: printf-style name for the kthread worker (task).
 *
 * Returns a pointer to the allocated worker on success, ERR_PTR(-ENOMEM)
 * when the needed structures could not get allocated, and ERR_PTR(-EINTR)
 * when the worker was SIGKILLed.
 */
struct kthread_worker *
kthread_create_worker(unsigned int flags, const char namefmt[], ...)
{
        struct kthread_worker *worker;
        va_list args;

        va_start(args, namefmt);
        worker = __kthread_create_worker(-1, flags, namefmt, args);
        va_end(args);

        return worker;
}
EXPORT_SYMBOL(kthread_create_worker);

/**
 * kthread_create_worker_on_cpu - create a kthread worker and bind it
 *      it to a given CPU and the associated NUMA node.
 * @cpu: CPU number
 * @flags: flags modifying the default behavior of the worker
 * @namefmt: printf-style name for the kthread worker (task).
 *
 * Use a valid CPU number if you want to bind the kthread worker
 * to the given CPU and the associated NUMA node.
 *
 * A good practice is to add the cpu number also into the worker name.
 * For example, use kthread_create_worker_on_cpu(cpu, "helper/%d", cpu).
 *
 * Returns a pointer to the allocated worker on success, ERR_PTR(-ENOMEM)
 * when the needed structures could not get allocated, and ERR_PTR(-EINTR)
 * when the worker was SIGKILLed.
 */
struct kthread_worker *
kthread_create_worker_on_cpu(int cpu, unsigned int flags,
                             const char namefmt[], ...)
{
        struct kthread_worker *worker;
        va_list args;

        va_start(args, namefmt);
        worker = __kthread_create_worker(cpu, flags, namefmt, args);
        va_end(args);

        return worker;
}
EXPORT_SYMBOL(kthread_create_worker_on_cpu);

/*
 * Returns true when the work could not be queued at the moment.
 * It happens when it is already pending in a worker list
 * or when it is being cancelled.
 */
static inline bool queuing_blocked(struct kthread_worker *worker,
                                   struct kthread_work *work)
{
        lockdep_assert_held(&worker->lock);

        return !list_empty(&work->node) || work->canceling;
}

static void kthread_insert_work_sanity_check(struct kthread_worker *worker,
                                             struct kthread_work *work)
{
        lockdep_assert_held(&worker->lock);
        WARN_ON_ONCE(!list_empty(&work->node));
        /* Do not use a work with >1 worker, see kthread_queue_work() */
        WARN_ON_ONCE(work->worker && work->worker != worker);
}

/* insert @work before @pos in @worker */
static void kthread_insert_work(struct kthread_worker *worker,
                                struct kthread_work *work,
                                struct list_head *pos)
{
        kthread_insert_work_sanity_check(worker, work);

        list_add_tail(&work->node, pos);
        work->worker = worker;
        if (!worker->current_work && likely(worker->task))
                wake_up_process(worker->task);
}

/**
 * kthread_queue_work - queue a kthread_work
 * @worker: target kthread_worker
 * @work: kthread_work to queue
 *
 * Queue @work to work processor @task for async execution.  @task
 * must have been created with kthread_worker_create().  Returns %true
 * if @work was successfully queued, %false if it was already pending.
 *
 * Reinitialize the work if it needs to be used by another worker.
 * For example, when the worker was stopped and started again.
 */
bool kthread_queue_work(struct kthread_worker *worker,
                        struct kthread_work *work)
{
        bool ret = false;
        unsigned long flags;

        spin_lock_irqsave(&worker->lock, flags);
        if (!queuing_blocked(worker, work)) {
                kthread_insert_work(worker, work, &worker->work_list);
                ret = true;
        }
        spin_unlock_irqrestore(&worker->lock, flags);
        return ret;
}
EXPORT_SYMBOL_GPL(kthread_queue_work);

/**
 * kthread_delayed_work_timer_fn - callback that queues the associated kthread
 *      delayed work when the timer expires.
 * @t: pointer to the expired timer
 *
 * The format of the function is defined by struct timer_list.
 * It should have been called from irqsafe timer with irq already off.
 */
void kthread_delayed_work_timer_fn(struct timer_list *t)
{
        struct kthread_delayed_work *dwork = from_timer(dwork, t, timer);
        struct kthread_work *work = &dwork->work;
        struct kthread_worker *worker = work->worker;

        /*
         * This might happen when a pending work is reinitialized.
         * It means that it is used a wrong way.
         */
        if (WARN_ON_ONCE(!worker))
                return;

        spin_lock(&worker->lock);
        /* Work must not be used with >1 worker, see kthread_queue_work(). */
        WARN_ON_ONCE(work->worker != worker);

        /* Move the work from worker->delayed_work_list. */
        WARN_ON_ONCE(list_empty(&work->node));
        list_del_init(&work->node);
        kthread_insert_work(worker, work, &worker->work_list);

        spin_unlock(&worker->lock);
}
EXPORT_SYMBOL(kthread_delayed_work_timer_fn);

void __kthread_queue_delayed_work(struct kthread_worker *worker,
                                  struct kthread_delayed_work *dwork,
                                  unsigned long delay)
{
        struct timer_list *timer = &dwork->timer;
        struct kthread_work *work = &dwork->work;

        WARN_ON_ONCE(timer->function != kthread_delayed_work_timer_fn);

        /*
         * If @delay is 0, queue @dwork->work immediately.  This is for
         * both optimization and correctness.  The earliest @timer can
         * expire is on the closest next tick and delayed_work users depend
         * on that there's no such delay when @delay is 0.
         */
        if (!delay) {
                kthread_insert_work(worker, work, &worker->work_list);
                return;
        }

        /* Be paranoid and try to detect possible races already now. */
        kthread_insert_work_sanity_check(worker, work);

        list_add(&work->node, &worker->delayed_work_list);
        work->worker = worker;
        timer->expires = jiffies + delay;
        add_timer(timer);
}

/**
 * kthread_queue_delayed_work - queue the associated kthread work
 *      after a delay.
 * @worker: target kthread_worker
 * @dwork: kthread_delayed_work to queue
 * @delay: number of jiffies to wait before queuing
 *
 * If the work has not been pending it starts a timer that will queue
 * the work after the given @delay. If @delay is zero, it queues the
 * work immediately.
 *
 * Return: %false if the @work has already been pending. It means that
 * either the timer was running or the work was queued. It returns %true
 * otherwise.
 */
bool kthread_queue_delayed_work(struct kthread_worker *worker,
                                struct kthread_delayed_work *dwork,
                                unsigned long delay)
{
        struct kthread_work *work = &dwork->work;
        unsigned long flags;
        bool ret = false;

        spin_lock_irqsave(&worker->lock, flags);

        if (!queuing_blocked(worker, work)) {
                __kthread_queue_delayed_work(worker, dwork, delay);
                ret = true;
        }

        spin_unlock_irqrestore(&worker->lock, flags);
        return ret;
}
EXPORT_SYMBOL_GPL(kthread_queue_delayed_work);

struct kthread_flush_work {
        struct kthread_work     work;
        struct completion       done;
};

static void kthread_flush_work_fn(struct kthread_work *work)
{
        struct kthread_flush_work *fwork =
                container_of(work, struct kthread_flush_work, work);
        complete(&fwork->done);
}

/**
 * kthread_flush_work - flush a kthread_work
 * @work: work to flush
 *
 * If @work is queued or executing, wait for it to finish execution.
 */
void kthread_flush_work(struct kthread_work *work)
{
        struct kthread_flush_work fwork = {
                KTHREAD_WORK_INIT(fwork.work, kthread_flush_work_fn),
                COMPLETION_INITIALIZER_ONSTACK(fwork.done),
        };
        struct kthread_worker *worker;
        bool noop = false;

        worker = work->worker;
        if (!worker)
                return;

        spin_lock_irq(&worker->lock);
        /* Work must not be used with >1 worker, see kthread_queue_work(). */
        WARN_ON_ONCE(work->worker != worker);

        if (!list_empty(&work->node))
                kthread_insert_work(worker, &fwork.work, work->node.next);
        else if (worker->current_work == work)
                kthread_insert_work(worker, &fwork.work,
                                    worker->work_list.next);
        else
                noop = true;

        spin_unlock_irq(&worker->lock);

        if (!noop)
                wait_for_completion(&fwork.done);
}
EXPORT_SYMBOL_GPL(kthread_flush_work);

/*
 * This function removes the work from the worker queue. Also it makes sure
 * that it won't get queued later via the delayed work's timer.
 *
 * The work might still be in use when this function finishes. See the
 * current_work proceed by the worker.
 *
 * Return: %true if @work was pending and successfully canceled,
 *      %false if @work was not pending
 */
static bool __kthread_cancel_work(struct kthread_work *work, bool is_dwork,
                                  unsigned long *flags)
{
        /* Try to cancel the timer if exists. */
        if (is_dwork) {
                struct kthread_delayed_work *dwork =
                        container_of(work, struct kthread_delayed_work, work);
                struct kthread_worker *worker = work->worker;

                /*
                 * del_timer_sync() must be called to make sure that the timer
                 * callback is not running. The lock must be temporary released
                 * to avoid a deadlock with the callback. In the meantime,
                 * any queuing is blocked by setting the canceling counter.
                 */
                work->canceling++;
                spin_unlock_irqrestore(&worker->lock, *flags);
                del_timer_sync(&dwork->timer);
                spin_lock_irqsave(&worker->lock, *flags);
                work->canceling--;
        }

        /*
         * Try to remove the work from a worker list. It might either
         * be from worker->work_list or from worker->delayed_work_list.
         */
        if (!list_empty(&work->node)) {
                list_del_init(&work->node);
                return true;
        }

        return false;
}

/**
 * kthread_mod_delayed_work - modify delay of or queue a kthread delayed work
 * @worker: kthread worker to use
 * @dwork: kthread delayed work to queue
 * @delay: number of jiffies to wait before queuing
 *
 * If @dwork is idle, equivalent to kthread_queue_delayed_work(). Otherwise,
 * modify @dwork's timer so that it expires after @delay. If @delay is zero,
 * @work is guaranteed to be queued immediately.
 *
 * Return: %true if @dwork was pending and its timer was modified,
 * %false otherwise.
 *
 * A special case is when the work is being canceled in parallel.
 * It might be caused either by the real kthread_cancel_delayed_work_sync()
 * or yet another kthread_mod_delayed_work() call. We let the other command
 * win and return %false here. The caller is supposed to synchronize these
 * operations a reasonable way.
 *
 * This function is safe to call from any context including IRQ handler.
 * See __kthread_cancel_work() and kthread_delayed_work_timer_fn()
 * for details.
 */
bool kthread_mod_delayed_work(struct kthread_worker *worker,
                              struct kthread_delayed_work *dwork,
                              unsigned long delay)
{
        struct kthread_work *work = &dwork->work;
        unsigned long flags;
        int ret = false;

        spin_lock_irqsave(&worker->lock, flags);

        /* Do not bother with canceling when never queued. */
        if (!work->worker)
                goto fast_queue;

        /* Work must not be used with >1 worker, see kthread_queue_work() */
        WARN_ON_ONCE(work->worker != worker);

        /* Do not fight with another command that is canceling this work. */
        if (work->canceling)
                goto out;

        ret = __kthread_cancel_work(work, true, &flags);
fast_queue:
        __kthread_queue_delayed_work(worker, dwork, delay);
out:
        spin_unlock_irqrestore(&worker->lock, flags);
        return ret;
}
EXPORT_SYMBOL_GPL(kthread_mod_delayed_work);

static bool __kthread_cancel_work_sync(struct kthread_work *work, bool is_dwork)
{
        struct kthread_worker *worker = work->worker;
        unsigned long flags;
        int ret = false;

        if (!worker)
                goto out;

        spin_lock_irqsave(&worker->lock, flags);
        /* Work must not be used with >1 worker, see kthread_queue_work(). */
        WARN_ON_ONCE(work->worker != worker);

        ret = __kthread_cancel_work(work, is_dwork, &flags);

        if (worker->current_work != work)
                goto out_fast;

        /*
         * The work is in progress and we need to wait with the lock released.
         * In the meantime, block any queuing by setting the canceling counter.
         */
        work->canceling++;
        spin_unlock_irqrestore(&worker->lock, flags);
        kthread_flush_work(work);
        spin_lock_irqsave(&worker->lock, flags);
        work->canceling--;

out_fast:
        spin_unlock_irqrestore(&worker->lock, flags);
out:
        return ret;
}

/**
 * kthread_cancel_work_sync - cancel a kthread work and wait for it to finish
 * @work: the kthread work to cancel
 *
 * Cancel @work and wait for its execution to finish.  This function
 * can be used even if the work re-queues itself. On return from this
 * function, @work is guaranteed to be not pending or executing on any CPU.
 *
 * kthread_cancel_work_sync(&delayed_work->work) must not be used for
 * delayed_work's. Use kthread_cancel_delayed_work_sync() instead.
 *
 * The caller must ensure that the worker on which @work was last
 * queued can't be destroyed before this function returns.
 *
 * Return: %true if @work was pending, %false otherwise.
 */
bool kthread_cancel_work_sync(struct kthread_work *work)
{
        return __kthread_cancel_work_sync(work, false);
}
EXPORT_SYMBOL_GPL(kthread_cancel_work_sync);

/**
 * kthread_cancel_delayed_work_sync - cancel a kthread delayed work and
 *      wait for it to finish.
 * @dwork: the kthread delayed work to cancel
 *
 * This is kthread_cancel_work_sync() for delayed works.
 *
 * Return: %true if @dwork was pending, %false otherwise.
 */
bool kthread_cancel_delayed_work_sync(struct kthread_delayed_work *dwork)
{
        return __kthread_cancel_work_sync(&dwork->work, true);
}
EXPORT_SYMBOL_GPL(kthread_cancel_delayed_work_sync);

/**
 * kthread_flush_worker - flush all current works on a kthread_worker
 * @worker: worker to flush
 *
 * Wait until all currently executing or pending works on @worker are
 * finished.
 */
void kthread_flush_worker(struct kthread_worker *worker)
{
        struct kthread_flush_work fwork = {
                KTHREAD_WORK_INIT(fwork.work, kthread_flush_work_fn),
                COMPLETION_INITIALIZER_ONSTACK(fwork.done),
        };

        kthread_queue_work(worker, &fwork.work);
        wait_for_completion(&fwork.done);
}
EXPORT_SYMBOL_GPL(kthread_flush_worker);

/**
 * kthread_destroy_worker - destroy a kthread worker
 * @worker: worker to be destroyed
 *
 * Flush and destroy @worker.  The simple flush is enough because the kthread
 * worker API is used only in trivial scenarios.  There are no multi-step state
 * machines needed.
 */
void kthread_destroy_worker(struct kthread_worker *worker)
{
        struct task_struct *task;

        task = worker->task;
        if (WARN_ON(!task))
                return;

        kthread_flush_worker(worker);
        kthread_stop(task);
        WARN_ON(!list_empty(&worker->work_list));
        kfree(worker);
}
EXPORT_SYMBOL(kthread_destroy_worker);

#ifdef CONFIG_BLK_CGROUP
/**
 * kthread_associate_blkcg - associate blkcg to current kthread
 * @css: the cgroup info
 *
 * Current thread must be a kthread. The thread is running jobs on behalf of
 * other threads. In some cases, we expect the jobs attach cgroup info of
 * original threads instead of that of current thread. This function stores
 * original thread's cgroup info in current kthread context for later
 * retrieval.
 */
void kthread_associate_blkcg(struct cgroup_subsys_state *css)
{
        struct kthread *kthread;

        if (!(current->flags & PF_KTHREAD))
                return;
        kthread = to_kthread(current);
        if (!kthread)
                return;

        if (kthread->blkcg_css) {
                css_put(kthread->blkcg_css);
                kthread->blkcg_css = NULL;
        }
        if (css) {
                css_get(css);
                kthread->blkcg_css = css;
        }
}
EXPORT_SYMBOL(kthread_associate_blkcg);

/**
 * kthread_blkcg - get associated blkcg css of current kthread
 *
 * Current thread must be a kthread.
 */
struct cgroup_subsys_state *kthread_blkcg(void)
{
        struct kthread *kthread;

        if (current->flags & PF_KTHREAD) {
                kthread = to_kthread(current);
                if (kthread)
                        return kthread->blkcg_css;
        }
        return NULL;
}
EXPORT_SYMBOL(kthread_blkcg);
#endif