x86/speculation/mds: Conditionally clear CPU buffers on idle entry

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

/*
 * Generic helpers for smp ipi calls
 *
 * (C) Jens Axboe <jens.axboe@oracle.com> 2008
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/irq_work.h>
#include <linux/rcupdate.h>
#include <linux/rculist.h>
#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/percpu.h>
#include <linux/init.h>
#include <linux/gfp.h>
#include <linux/smp.h>
#include <linux/cpu.h>
#include <linux/sched.h>
#include <linux/sched/idle.h>
#include <linux/hypervisor.h>

#include "smpboot.h"

enum {
        CSD_FLAG_LOCK           = 0x01,
        CSD_FLAG_SYNCHRONOUS    = 0x02,
};

struct call_function_data {
        call_single_data_t      __percpu *csd;
        cpumask_var_t           cpumask;
        cpumask_var_t           cpumask_ipi;
};

static DEFINE_PER_CPU_SHARED_ALIGNED(struct call_function_data, cfd_data);

static DEFINE_PER_CPU_SHARED_ALIGNED(struct llist_head, call_single_queue);

static void flush_smp_call_function_queue(bool warn_cpu_offline);

int smpcfd_prepare_cpu(unsigned int cpu)
{
        struct call_function_data *cfd = &per_cpu(cfd_data, cpu);

        if (!zalloc_cpumask_var_node(&cfd->cpumask, GFP_KERNEL,
                                     cpu_to_node(cpu)))
                return -ENOMEM;
        if (!zalloc_cpumask_var_node(&cfd->cpumask_ipi, GFP_KERNEL,
                                     cpu_to_node(cpu))) {
                free_cpumask_var(cfd->cpumask);
                return -ENOMEM;
        }
        cfd->csd = alloc_percpu(call_single_data_t);
        if (!cfd->csd) {
                free_cpumask_var(cfd->cpumask);
                free_cpumask_var(cfd->cpumask_ipi);
                return -ENOMEM;
        }

        return 0;
}

int smpcfd_dead_cpu(unsigned int cpu)
{
        struct call_function_data *cfd = &per_cpu(cfd_data, cpu);

        free_cpumask_var(cfd->cpumask);
        free_cpumask_var(cfd->cpumask_ipi);
        free_percpu(cfd->csd);
        return 0;
}

int smpcfd_dying_cpu(unsigned int cpu)
{
        /*
         * The IPIs for the smp-call-function callbacks queued by other
         * CPUs might arrive late, either due to hardware latencies or
         * because this CPU disabled interrupts (inside stop-machine)
         * before the IPIs were sent. So flush out any pending callbacks
         * explicitly (without waiting for the IPIs to arrive), to
         * ensure that the outgoing CPU doesn't go offline with work
         * still pending.
         */
        flush_smp_call_function_queue(false);
        return 0;
}

void __init call_function_init(void)
{
        int i;

        for_each_possible_cpu(i)
                init_llist_head(&per_cpu(call_single_queue, i));

        smpcfd_prepare_cpu(smp_processor_id());
}

/*
 * csd_lock/csd_unlock used to serialize access to per-cpu csd resources
 *
 * For non-synchronous ipi calls the csd can still be in use by the
 * previous function call. For multi-cpu calls its even more interesting
 * as we'll have to ensure no other cpu is observing our csd.
 */
static __always_inline void csd_lock_wait(call_single_data_t *csd)
{
        smp_cond_load_acquire(&csd->flags, !(VAL & CSD_FLAG_LOCK));
}

static __always_inline void csd_lock(call_single_data_t *csd)
{
        csd_lock_wait(csd);
        csd->flags |= CSD_FLAG_LOCK;

        /*
         * prevent CPU from reordering the above assignment
         * to ->flags with any subsequent assignments to other
         * fields of the specified call_single_data_t structure:
         */
        smp_wmb();
}

static __always_inline void csd_unlock(call_single_data_t *csd)
{
        WARN_ON(!(csd->flags & CSD_FLAG_LOCK));

        /*
         * ensure we're all done before releasing data:
         */
        smp_store_release(&csd->flags, 0);
}

static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);

/*
 * Insert a previously allocated call_single_data_t element
 * for execution on the given CPU. data must already have
 * ->func, ->info, and ->flags set.
 */
static int generic_exec_single(int cpu, call_single_data_t *csd,
                               smp_call_func_t func, void *info)
{
        if (cpu == smp_processor_id()) {
                unsigned long flags;

                /*
                 * We can unlock early even for the synchronous on-stack case,
                 * since we're doing this from the same CPU..
                 */
                csd_unlock(csd);
                local_irq_save(flags);
                func(info);
                local_irq_restore(flags);
                return 0;
        }


        if ((unsigned)cpu >= nr_cpu_ids || !cpu_online(cpu)) {
                csd_unlock(csd);
                return -ENXIO;
        }

        csd->func = func;
        csd->info = info;

        /*
         * The list addition should be visible before sending the IPI
         * handler locks the list to pull the entry off it because of
         * normal cache coherency rules implied by spinlocks.
         *
         * If IPIs can go out of order to the cache coherency protocol
         * in an architecture, sufficient synchronisation should be added
         * to arch code to make it appear to obey cache coherency WRT
         * locking and barrier primitives. Generic code isn't really
         * equipped to do the right thing...
         */
        if (llist_add(&csd->llist, &per_cpu(call_single_queue, cpu)))
                arch_send_call_function_single_ipi(cpu);

        return 0;
}

/**
 * generic_smp_call_function_single_interrupt - Execute SMP IPI callbacks
 *
 * Invoked by arch to handle an IPI for call function single.
 * Must be called with interrupts disabled.
 */
void generic_smp_call_function_single_interrupt(void)
{
        flush_smp_call_function_queue(true);
}

/**
 * flush_smp_call_function_queue - Flush pending smp-call-function callbacks
 *
 * @warn_cpu_offline: If set to 'true', warn if callbacks were queued on an
 *                    offline CPU. Skip this check if set to 'false'.
 *
 * Flush any pending smp-call-function callbacks queued on this CPU. This is
 * invoked by the generic IPI handler, as well as by a CPU about to go offline,
 * to ensure that all pending IPI callbacks are run before it goes completely
 * offline.
 *
 * Loop through the call_single_queue and run all the queued callbacks.
 * Must be called with interrupts disabled.
 */
static void flush_smp_call_function_queue(bool warn_cpu_offline)
{
        struct llist_head *head;
        struct llist_node *entry;
        call_single_data_t *csd, *csd_next;
        static bool warned;

        lockdep_assert_irqs_disabled();

        head = this_cpu_ptr(&call_single_queue);
        entry = llist_del_all(head);
        entry = llist_reverse_order(entry);

        /* There shouldn't be any pending callbacks on an offline CPU. */
        if (unlikely(warn_cpu_offline && !cpu_online(smp_processor_id()) &&
                     !warned && !llist_empty(head))) {
                warned = true;
                WARN(1, "IPI on offline CPU %d\n", smp_processor_id());

                /*
                 * We don't have to use the _safe() variant here
                 * because we are not invoking the IPI handlers yet.
                 */
                llist_for_each_entry(csd, entry, llist)
                        pr_warn("IPI callback %pS sent to offline CPU\n",
                                csd->func);
        }

        llist_for_each_entry_safe(csd, csd_next, entry, llist) {
                smp_call_func_t func = csd->func;
                void *info = csd->info;

                /* Do we wait until *after* callback? */
                if (csd->flags & CSD_FLAG_SYNCHRONOUS) {
                        func(info);
                        csd_unlock(csd);
                } else {
                        csd_unlock(csd);
                        func(info);
                }
        }

        /*
         * Handle irq works queued remotely by irq_work_queue_on().
         * Smp functions above are typically synchronous so they
         * better run first since some other CPUs may be busy waiting
         * for them.
         */
        irq_work_run();
}

/*
 * smp_call_function_single - Run a function on a specific CPU
 * @func: The function to run. This must be fast and non-blocking.
 * @info: An arbitrary pointer to pass to the function.
 * @wait: If true, wait until function has completed on other CPUs.
 *
 * Returns 0 on success, else a negative status code.
 */
int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
                             int wait)
{
        call_single_data_t *csd;
        call_single_data_t csd_stack = {
                .flags = CSD_FLAG_LOCK | CSD_FLAG_SYNCHRONOUS,
        };
        int this_cpu;
        int err;

        /*
         * prevent preemption and reschedule on another processor,
         * as well as CPU removal
         */
        this_cpu = get_cpu();

        /*
         * Can deadlock when called with interrupts disabled.
         * We allow cpu's that are not yet online though, as no one else can
         * send smp call function interrupt to this cpu and as such deadlocks
         * can't happen.
         */
        WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
                     && !oops_in_progress);

        csd = &csd_stack;
        if (!wait) {
                csd = this_cpu_ptr(&csd_data);
                csd_lock(csd);
        }

        err = generic_exec_single(cpu, csd, func, info);

        if (wait)
                csd_lock_wait(csd);

        put_cpu();

        return err;
}
EXPORT_SYMBOL(smp_call_function_single);

/**
 * smp_call_function_single_async(): Run an asynchronous function on a
 *                               specific CPU.
 * @cpu: The CPU to run on.
 * @csd: Pre-allocated and setup data structure
 *
 * Like smp_call_function_single(), but the call is asynchonous and
 * can thus be done from contexts with disabled interrupts.
 *
 * The caller passes his own pre-allocated data structure
 * (ie: embedded in an object) and is responsible for synchronizing it
 * such that the IPIs performed on the @csd are strictly serialized.
 *
 * NOTE: Be careful, there is unfortunately no current debugging facility to
 * validate the correctness of this serialization.
 */
int smp_call_function_single_async(int cpu, call_single_data_t *csd)
{
        int err = 0;

        preempt_disable();

        /* We could deadlock if we have to wait here with interrupts disabled! */
        if (WARN_ON_ONCE(csd->flags & CSD_FLAG_LOCK))
                csd_lock_wait(csd);

        csd->flags = CSD_FLAG_LOCK;
        smp_wmb();

        err = generic_exec_single(cpu, csd, csd->func, csd->info);
        preempt_enable();

        return err;
}
EXPORT_SYMBOL_GPL(smp_call_function_single_async);

/*
 * smp_call_function_any - Run a function on any of the given cpus
 * @mask: The mask of cpus it can run on.
 * @func: The function to run. This must be fast and non-blocking.
 * @info: An arbitrary pointer to pass to the function.
 * @wait: If true, wait until function has completed.
 *
 * Returns 0 on success, else a negative status code (if no cpus were online).
 *
 * Selection preference:
 *      1) current cpu if in @mask
 *      2) any cpu of current node if in @mask
 *      3) any other online cpu in @mask
 */
int smp_call_function_any(const struct cpumask *mask,
                          smp_call_func_t func, void *info, int wait)
{
        unsigned int cpu;
        const struct cpumask *nodemask;
        int ret;

        /* Try for same CPU (cheapest) */
        cpu = get_cpu();
        if (cpumask_test_cpu(cpu, mask))
                goto call;

        /* Try for same node. */
        nodemask = cpumask_of_node(cpu_to_node(cpu));
        for (cpu = cpumask_first_and(nodemask, mask); cpu < nr_cpu_ids;
             cpu = cpumask_next_and(cpu, nodemask, mask)) {
                if (cpu_online(cpu))
                        goto call;
        }

        /* Any online will do: smp_call_function_single handles nr_cpu_ids. */
        cpu = cpumask_any_and(mask, cpu_online_mask);
call:
        ret = smp_call_function_single(cpu, func, info, wait);
        put_cpu();
        return ret;
}
EXPORT_SYMBOL_GPL(smp_call_function_any);

/**
 * smp_call_function_many(): Run a function on a set of other CPUs.
 * @mask: The set of cpus to run on (only runs on online subset).
 * @func: The function to run. This must be fast and non-blocking.
 * @info: An arbitrary pointer to pass to the function.
 * @wait: If true, wait (atomically) until function has completed
 *        on other CPUs.
 *
 * If @wait is true, then returns once @func has returned.
 *
 * You must not call this function with disabled interrupts or from a
 * hardware interrupt handler or from a bottom half handler. Preemption
 * must be disabled when calling this function.
 */
void smp_call_function_many(const struct cpumask *mask,
                            smp_call_func_t func, void *info, bool wait)
{
        struct call_function_data *cfd;
        int cpu, next_cpu, this_cpu = smp_processor_id();

        /*
         * Can deadlock when called with interrupts disabled.
         * We allow cpu's that are not yet online though, as no one else can
         * send smp call function interrupt to this cpu and as such deadlocks
         * can't happen.
         */
        WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
                     && !oops_in_progress && !early_boot_irqs_disabled);

        /* Try to fastpath.  So, what's a CPU they want? Ignoring this one. */
        cpu = cpumask_first_and(mask, cpu_online_mask);
        if (cpu == this_cpu)
                cpu = cpumask_next_and(cpu, mask, cpu_online_mask);

        /* No online cpus?  We're done. */
        if (cpu >= nr_cpu_ids)
                return;

        /* Do we have another CPU which isn't us? */
        next_cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
        if (next_cpu == this_cpu)
                next_cpu = cpumask_next_and(next_cpu, mask, cpu_online_mask);

        /* Fastpath: do that cpu by itself. */
        if (next_cpu >= nr_cpu_ids) {
                smp_call_function_single(cpu, func, info, wait);
                return;
        }

        cfd = this_cpu_ptr(&cfd_data);

        cpumask_and(cfd->cpumask, mask, cpu_online_mask);
        __cpumask_clear_cpu(this_cpu, cfd->cpumask);

        /* Some callers race with other cpus changing the passed mask */
        if (unlikely(!cpumask_weight(cfd->cpumask)))
                return;

        cpumask_clear(cfd->cpumask_ipi);
        for_each_cpu(cpu, cfd->cpumask) {
                call_single_data_t *csd = per_cpu_ptr(cfd->csd, cpu);

                csd_lock(csd);
                if (wait)
                        csd->flags |= CSD_FLAG_SYNCHRONOUS;
                csd->func = func;
                csd->info = info;
                if (llist_add(&csd->llist, &per_cpu(call_single_queue, cpu)))
                        __cpumask_set_cpu(cpu, cfd->cpumask_ipi);
        }

        /* Send a message to all CPUs in the map */
        arch_send_call_function_ipi_mask(cfd->cpumask_ipi);

        if (wait) {
                for_each_cpu(cpu, cfd->cpumask) {
                        call_single_data_t *csd;

                        csd = per_cpu_ptr(cfd->csd, cpu);
                        csd_lock_wait(csd);
                }
        }
}
EXPORT_SYMBOL(smp_call_function_many);

/**
 * smp_call_function(): Run a function on all other CPUs.
 * @func: The function to run. This must be fast and non-blocking.
 * @info: An arbitrary pointer to pass to the function.
 * @wait: If true, wait (atomically) until function has completed
 *        on other CPUs.
 *
 * Returns 0.
 *
 * If @wait is true, then returns once @func has returned; otherwise
 * it returns just before the target cpu calls @func.
 *
 * You must not call this function with disabled interrupts or from a
 * hardware interrupt handler or from a bottom half handler.
 */
int smp_call_function(smp_call_func_t func, void *info, int wait)
{
        preempt_disable();
        smp_call_function_many(cpu_online_mask, func, info, wait);
        preempt_enable();

        return 0;
}
EXPORT_SYMBOL(smp_call_function);

/* Setup configured maximum number of CPUs to activate */
unsigned int setup_max_cpus = NR_CPUS;
EXPORT_SYMBOL(setup_max_cpus);


/*
 * Setup routine for controlling SMP activation
 *
 * Command-line option of "nosmp" or "maxcpus=0" will disable SMP
 * activation entirely (the MPS table probe still happens, though).
 *
 * Command-line option of "maxcpus=<NUM>", where <NUM> is an integer
 * greater than 0, limits the maximum number of CPUs activated in
 * SMP mode to <NUM>.
 */

void __weak arch_disable_smp_support(void) { }

static int __init nosmp(char *str)
{
        setup_max_cpus = 0;
        arch_disable_smp_support();

        return 0;
}

early_param("nosmp", nosmp);

/* this is hard limit */
static int __init nrcpus(char *str)
{
        int nr_cpus;

        get_option(&str, &nr_cpus);
        if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
                nr_cpu_ids = nr_cpus;

        return 0;
}

early_param("nr_cpus", nrcpus);

static int __init maxcpus(char *str)
{
        get_option(&str, &setup_max_cpus);
        if (setup_max_cpus == 0)
                arch_disable_smp_support();

        return 0;
}

early_param("maxcpus", maxcpus);

/* Setup number of possible processor ids */
unsigned int nr_cpu_ids __read_mostly = NR_CPUS;
EXPORT_SYMBOL(nr_cpu_ids);

/* An arch may set nr_cpu_ids earlier if needed, so this would be redundant */
void __init setup_nr_cpu_ids(void)
{
        nr_cpu_ids = find_last_bit(cpumask_bits(cpu_possible_mask),NR_CPUS) + 1;
}

/* Called by boot processor to activate the rest. */
void __init smp_init(void)
{
        int num_nodes, num_cpus;
        unsigned int cpu;

        idle_threads_init();
        cpuhp_threads_init();

        pr_info("Bringing up secondary CPUs ...\n");

        /* FIXME: This should be done in userspace --RR */
        for_each_present_cpu(cpu) {
                if (num_online_cpus() >= setup_max_cpus)
                        break;
                if (!cpu_online(cpu))
                        cpu_up(cpu);
        }

        num_nodes = num_online_nodes();
        num_cpus  = num_online_cpus();
        pr_info("Brought up %d node%s, %d CPU%s\n",
                num_nodes, (num_nodes > 1 ? "s" : ""),
                num_cpus,  (num_cpus  > 1 ? "s" : ""));

        /* Final decision about SMT support */
        cpu_smt_check_topology();
        /* Any cleanup work */
        smp_cpus_done(setup_max_cpus);
}

/*
 * Call a function on all processors.  May be used during early boot while
 * early_boot_irqs_disabled is set.  Use local_irq_save/restore() instead
 * of local_irq_disable/enable().
 */
int on_each_cpu(void (*func) (void *info), void *info, int wait)
{
        unsigned long flags;
        int ret = 0;

        preempt_disable();
        ret = smp_call_function(func, info, wait);
        local_irq_save(flags);
        func(info);
        local_irq_restore(flags);
        preempt_enable();
        return ret;
}
EXPORT_SYMBOL(on_each_cpu);

/**
 * on_each_cpu_mask(): Run a function on processors specified by
 * cpumask, which may include the local processor.
 * @mask: The set of cpus to run on (only runs on online subset).
 * @func: The function to run. This must be fast and non-blocking.
 * @info: An arbitrary pointer to pass to the function.
 * @wait: If true, wait (atomically) until function has completed
 *        on other CPUs.
 *
 * If @wait is true, then returns once @func has returned.
 *
 * You must not call this function with disabled interrupts or from a
 * hardware interrupt handler or from a bottom half handler.  The
 * exception is that it may be used during early boot while
 * early_boot_irqs_disabled is set.
 */
void on_each_cpu_mask(const struct cpumask *mask, smp_call_func_t func,
                        void *info, bool wait)
{
        int cpu = get_cpu();

        smp_call_function_many(mask, func, info, wait);
        if (cpumask_test_cpu(cpu, mask)) {
                unsigned long flags;
                local_irq_save(flags);
                func(info);
                local_irq_restore(flags);
        }
        put_cpu();
}
EXPORT_SYMBOL(on_each_cpu_mask);

/*
 * on_each_cpu_cond(): Call a function on each processor for which
 * the supplied function cond_func returns true, optionally waiting
 * for all the required CPUs to finish. This may include the local
 * processor.
 * @cond_func:  A callback function that is passed a cpu id and
 *              the the info parameter. The function is called
 *              with preemption disabled. The function should
 *              return a blooean value indicating whether to IPI
 *              the specified CPU.
 * @func:       The function to run on all applicable CPUs.
 *              This must be fast and non-blocking.
 * @info:       An arbitrary pointer to pass to both functions.
 * @wait:       If true, wait (atomically) until function has
 *              completed on other CPUs.
 * @gfp_flags:  GFP flags to use when allocating the cpumask
 *              used internally by the function.
 *
 * The function might sleep if the GFP flags indicates a non
 * atomic allocation is allowed.
 *
 * Preemption is disabled to protect against CPUs going offline but not online.
 * CPUs going online during the call will not be seen or sent an IPI.
 *
 * You must not call this function with disabled interrupts or
 * from a hardware interrupt handler or from a bottom half handler.
 */
void on_each_cpu_cond(bool (*cond_func)(int cpu, void *info),
                        smp_call_func_t func, void *info, bool wait,
                        gfp_t gfp_flags)
{
        cpumask_var_t cpus;
        int cpu, ret;

        might_sleep_if(gfpflags_allow_blocking(gfp_flags));

        if (likely(zalloc_cpumask_var(&cpus, (gfp_flags|__GFP_NOWARN)))) {
                preempt_disable();
                for_each_online_cpu(cpu)
                        if (cond_func(cpu, info))
                                cpumask_set_cpu(cpu, cpus);
                on_each_cpu_mask(cpus, func, info, wait);
                preempt_enable();
                free_cpumask_var(cpus);
        } else {
                /*
                 * No free cpumask, bother. No matter, we'll
                 * just have to IPI them one by one.
                 */
                preempt_disable();
                for_each_online_cpu(cpu)
                        if (cond_func(cpu, info)) {
                                ret = smp_call_function_single(cpu, func,
                                                                info, wait);
                                WARN_ON_ONCE(ret);
                        }
                preempt_enable();
        }
}
EXPORT_SYMBOL(on_each_cpu_cond);

static void do_nothing(void *unused)
{
}

/**
 * kick_all_cpus_sync - Force all cpus out of idle
 *
 * Used to synchronize the update of pm_idle function pointer. It's
 * called after the pointer is updated and returns after the dummy
 * callback function has been executed on all cpus. The execution of
 * the function can only happen on the remote cpus after they have
 * left the idle function which had been called via pm_idle function
 * pointer. So it's guaranteed that nothing uses the previous pointer
 * anymore.
 */
void kick_all_cpus_sync(void)
{
        /* Make sure the change is visible before we kick the cpus */
        smp_mb();
        smp_call_function(do_nothing, NULL, 1);
}
EXPORT_SYMBOL_GPL(kick_all_cpus_sync);

/**
 * wake_up_all_idle_cpus - break all cpus out of idle
 * wake_up_all_idle_cpus try to break all cpus which is in idle state even
 * including idle polling cpus, for non-idle cpus, we will do nothing
 * for them.
 */
void wake_up_all_idle_cpus(void)
{
        int cpu;

        preempt_disable();
        for_each_online_cpu(cpu) {
                if (cpu == smp_processor_id())
                        continue;

                wake_up_if_idle(cpu);
        }
        preempt_enable();
}
EXPORT_SYMBOL_GPL(wake_up_all_idle_cpus);

/**
 * smp_call_on_cpu - Call a function on a specific cpu
 *
 * Used to call a function on a specific cpu and wait for it to return.
 * Optionally make sure the call is done on a specified physical cpu via vcpu
 * pinning in order to support virtualized environments.
 */
struct smp_call_on_cpu_struct {
        struct work_struct      work;
        struct completion       done;
        int                     (*func)(void *);
        void                    *data;
        int                     ret;
        int                     cpu;
};

static void smp_call_on_cpu_callback(struct work_struct *work)
{
        struct smp_call_on_cpu_struct *sscs;

        sscs = container_of(work, struct smp_call_on_cpu_struct, work);
        if (sscs->cpu >= 0)
                hypervisor_pin_vcpu(sscs->cpu);
        sscs->ret = sscs->func(sscs->data);
        if (sscs->cpu >= 0)
                hypervisor_pin_vcpu(-1);

        complete(&sscs->done);
}

int smp_call_on_cpu(unsigned int cpu, int (*func)(void *), void *par, bool phys)
{
        struct smp_call_on_cpu_struct sscs = {
                .done = COMPLETION_INITIALIZER_ONSTACK(sscs.done),
                .func = func,
                .data = par,
                .cpu  = phys ? cpu : -1,
        };

        INIT_WORK_ONSTACK(&sscs.work, smp_call_on_cpu_callback);

        if (cpu >= nr_cpu_ids || !cpu_online(cpu))
                return -ENXIO;

        queue_work_on(cpu, system_wq, &sscs.work);
        wait_for_completion(&sscs.done);

        return sscs.ret;
}
EXPORT_SYMBOL_GPL(smp_call_on_cpu);