FreeBSD/Linux Kernel Cross Reference
sys/kernel/smp.c

     /*
      * Generic helpers for smp ipi calls
      *
      * (C) Jens Axboe <jens.axboe@oracle.com> 2008
      */
     #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 "smpboot.h"
    
    #ifdef CONFIG_USE_GENERIC_SMP_HELPERS
    static struct {
            struct list_head        queue;
            raw_spinlock_t          lock;
    } call_function __cacheline_aligned_in_smp =
            {
                    .queue          = LIST_HEAD_INIT(call_function.queue),
                    .lock           = __RAW_SPIN_LOCK_UNLOCKED(call_function.lock),
            };
    
    enum {
            CSD_FLAG_LOCK           = 0x01,
    };
    
    struct call_function_data {
            struct call_single_data csd;
            atomic_t                refs;
            cpumask_var_t           cpumask;
            cpumask_var_t           cpumask_ipi;
    };
    
    static DEFINE_PER_CPU_SHARED_ALIGNED(struct call_function_data, cfd_data);
    
    struct call_single_queue {
            struct list_head        list;
            raw_spinlock_t          lock;
    };
    
    static DEFINE_PER_CPU_SHARED_ALIGNED(struct call_single_queue, call_single_queue);
    
    static int
    hotplug_cfd(struct notifier_block *nfb, unsigned long action, void *hcpu)
    {
            long cpu = (long)hcpu;
            struct call_function_data *cfd = &per_cpu(cfd_data, cpu);
    
            switch (action) {
            case CPU_UP_PREPARE:
            case CPU_UP_PREPARE_FROZEN:
                    if (!zalloc_cpumask_var_node(&cfd->cpumask, GFP_KERNEL,
                                    cpu_to_node(cpu)))
                            return notifier_from_errno(-ENOMEM);
                    if (!zalloc_cpumask_var_node(&cfd->cpumask_ipi, GFP_KERNEL,
                                    cpu_to_node(cpu)))
                            return notifier_from_errno(-ENOMEM);
                    break;
    
    #ifdef CONFIG_HOTPLUG_CPU
            case CPU_UP_CANCELED:
            case CPU_UP_CANCELED_FROZEN:
    
            case CPU_DEAD:
            case CPU_DEAD_FROZEN:
                    free_cpumask_var(cfd->cpumask);
                    free_cpumask_var(cfd->cpumask_ipi);
                    break;
    #endif
            };
    
            return NOTIFY_OK;
    }
    
    static struct notifier_block __cpuinitdata hotplug_cfd_notifier = {
            .notifier_call          = hotplug_cfd,
    };
    
    void __init call_function_init(void)
    {
            void *cpu = (void *)(long)smp_processor_id();
            int i;
    
            for_each_possible_cpu(i) {
                    struct call_single_queue *q = &per_cpu(call_single_queue, i);
    
                    raw_spin_lock_init(&q->lock);
                    INIT_LIST_HEAD(&q->list);
            }
    
            hotplug_cfd(&hotplug_cfd_notifier, CPU_UP_PREPARE, cpu);
            register_cpu_notifier(&hotplug_cfd_notifier);
    }
    
   /*
    * 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 void csd_lock_wait(struct call_single_data *data)
   {
           while (data->flags & CSD_FLAG_LOCK)
                   cpu_relax();
   }
   
   static void csd_lock(struct call_single_data *data)
   {
           csd_lock_wait(data);
           data->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 structure:
            */
           smp_mb();
   }
   
   static void csd_unlock(struct call_single_data *data)
   {
           WARN_ON(!(data->flags & CSD_FLAG_LOCK));
   
           /*
            * ensure we're all done before releasing data:
            */
           smp_mb();
   
           data->flags &= ~CSD_FLAG_LOCK;
   }
   
   /*
    * Insert a previously allocated call_single_data element
    * for execution on the given CPU. data must already have
    * ->func, ->info, and ->flags set.
    */
   static
   void generic_exec_single(int cpu, struct call_single_data *data, int wait)
   {
           struct call_single_queue *dst = &per_cpu(call_single_queue, cpu);
           unsigned long flags;
           int ipi;
   
           raw_spin_lock_irqsave(&dst->lock, flags);
           ipi = list_empty(&dst->list);
           list_add_tail(&data->list, &dst->list);
           raw_spin_unlock_irqrestore(&dst->lock, flags);
   
           /*
            * 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 (ipi)
                   arch_send_call_function_single_ipi(cpu);
   
           if (wait)
                   csd_lock_wait(data);
   }
   
   /*
    * Invoked by arch to handle an IPI for call function. Must be called with
    * interrupts disabled.
    */
   void generic_smp_call_function_interrupt(void)
   {
           struct call_function_data *data;
           int cpu = smp_processor_id();
   
           /*
            * Shouldn't receive this interrupt on a cpu that is not yet online.
            */
           WARN_ON_ONCE(!cpu_online(cpu));
   
           /*
            * Ensure entry is visible on call_function_queue after we have
            * entered the IPI. See comment in smp_call_function_many.
            * If we don't have this, then we may miss an entry on the list
            * and never get another IPI to process it.
            */
           smp_mb();
   
           /*
            * It's ok to use list_for_each_rcu() here even though we may
            * delete 'pos', since list_del_rcu() doesn't clear ->next
            */
           list_for_each_entry_rcu(data, &call_function.queue, csd.list) {
                   int refs;
                   smp_call_func_t func;
   
                   /*
                    * Since we walk the list without any locks, we might
                    * see an entry that was completed, removed from the
                    * list and is in the process of being reused.
                    *
                    * We must check that the cpu is in the cpumask before
                    * checking the refs, and both must be set before
                    * executing the callback on this cpu.
                    */
   
                   if (!cpumask_test_cpu(cpu, data->cpumask))
                           continue;
   
                   smp_rmb();
   
                   if (atomic_read(&data->refs) == 0)
                           continue;
   
                   func = data->csd.func;          /* save for later warn */
                   func(data->csd.info);
   
                   /*
                    * If the cpu mask is not still set then func enabled
                    * interrupts (BUG), and this cpu took another smp call
                    * function interrupt and executed func(info) twice
                    * on this cpu.  That nested execution decremented refs.
                    */
                   if (!cpumask_test_and_clear_cpu(cpu, data->cpumask)) {
                           WARN(1, "%pf enabled interrupts and double executed\n", func);
                           continue;
                   }
   
                   refs = atomic_dec_return(&data->refs);
                   WARN_ON(refs < 0);
   
                   if (refs)
                           continue;
   
                   WARN_ON(!cpumask_empty(data->cpumask));
   
                   raw_spin_lock(&call_function.lock);
                   list_del_rcu(&data->csd.list);
                   raw_spin_unlock(&call_function.lock);
   
                   csd_unlock(&data->csd);
           }
   
   }
   
   /*
    * Invoked by arch to handle an IPI for call function single. Must be
    * called from the arch with interrupts disabled.
    */
   void generic_smp_call_function_single_interrupt(void)
   {
           struct call_single_queue *q = &__get_cpu_var(call_single_queue);
           unsigned int data_flags;
           LIST_HEAD(list);
   
           /*
            * Shouldn't receive this interrupt on a cpu that is not yet online.
            */
           WARN_ON_ONCE(!cpu_online(smp_processor_id()));
   
           raw_spin_lock(&q->lock);
           list_replace_init(&q->list, &list);
           raw_spin_unlock(&q->lock);
   
           while (!list_empty(&list)) {
                   struct call_single_data *data;
   
                   data = list_entry(list.next, struct call_single_data, list);
                   list_del(&data->list);
   
                   /*
                    * 'data' can be invalid after this call if flags == 0
                    * (when called through generic_exec_single()),
                    * so save them away before making the call:
                    */
                   data_flags = data->flags;
   
                   data->func(data->info);
   
                   /*
                    * Unlocked CSDs are valid through generic_exec_single():
                    */
                   if (data_flags & CSD_FLAG_LOCK)
                           csd_unlock(data);
           }
   }
   
   static DEFINE_PER_CPU_SHARED_ALIGNED(struct call_single_data, csd_data);
   
   /*
    * 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)
   {
           struct call_single_data d = {
                   .flags = 0,
           };
           unsigned long flags;
           int this_cpu;
           int err = 0;
   
           /*
            * 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);
   
           if (cpu == this_cpu) {
                   local_irq_save(flags);
                   func(info);
                   local_irq_restore(flags);
           } else {
                   if ((unsigned)cpu < nr_cpu_ids && cpu_online(cpu)) {
                           struct call_single_data *data = &d;
   
                           if (!wait)
                                   data = &__get_cpu_var(csd_data);
   
                           csd_lock(data);
   
                           data->func = func;
                           data->info = info;
                           generic_exec_single(cpu, data, wait);
                   } else {
                           err = -ENXIO;   /* CPU not online */
                   }
           }
   
           put_cpu();
   
           return err;
   }
   EXPORT_SYMBOL(smp_call_function_single);
   
   /*
    * 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).
    * Note that @wait will be implicitly turned on in case of allocation failures,
    * since we fall back to on-stack allocation.
    *
    * 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_single(): Run a function on a specific CPU
    * @cpu: The CPU to run on.
    * @data: Pre-allocated and setup data structure
    * @wait: If true, wait until function has completed on specified CPU.
    *
    * Like smp_call_function_single(), but allow caller to pass in a
    * pre-allocated data structure. Useful for embedding @data inside
    * other structures, for instance.
    */
   void __smp_call_function_single(int cpu, struct call_single_data *data,
                                   int wait)
   {
           unsigned int this_cpu;
           unsigned long flags;
   
           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(smp_processor_id()) && wait && irqs_disabled()
                        && !oops_in_progress);
   
           if (cpu == this_cpu) {
                   local_irq_save(flags);
                   data->func(data->info);
                   local_irq_restore(flags);
           } else {
                   csd_lock(data);
                   generic_exec_single(cpu, data, wait);
           }
           put_cpu();
   }
   
   /**
    * 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 *data;
           unsigned long flags;
           int refs, 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;
           }
   
           data = &__get_cpu_var(cfd_data);
           csd_lock(&data->csd);
   
           /* This BUG_ON verifies our reuse assertions and can be removed */
           BUG_ON(atomic_read(&data->refs) || !cpumask_empty(data->cpumask));
   
           /*
            * The global call function queue list add and delete are protected
            * by a lock, but the list is traversed without any lock, relying
            * on the rcu list add and delete to allow safe concurrent traversal.
            * We reuse the call function data without waiting for any grace
            * period after some other cpu removes it from the global queue.
            * This means a cpu might find our data block as it is being
            * filled out.
            *
            * We hold off the interrupt handler on the other cpu by
            * ordering our writes to the cpu mask vs our setting of the
            * refs counter.  We assert only the cpu owning the data block
            * will set a bit in cpumask, and each bit will only be cleared
            * by the subject cpu.  Each cpu must first find its bit is
            * set and then check that refs is set indicating the element is
            * ready to be processed, otherwise it must skip the entry.
            *
            * On the previous iteration refs was set to 0 by another cpu.
            * To avoid the use of transitivity, set the counter to 0 here
            * so the wmb will pair with the rmb in the interrupt handler.
            */
           atomic_set(&data->refs, 0);     /* convert 3rd to 1st party write */
   
           data->csd.func = func;
           data->csd.info = info;
   
           /* Ensure 0 refs is visible before mask.  Also orders func and info */
           smp_wmb();
   
           /* We rely on the "and" being processed before the store */
           cpumask_and(data->cpumask, mask, cpu_online_mask);
           cpumask_clear_cpu(this_cpu, data->cpumask);
           refs = cpumask_weight(data->cpumask);
   
           /* Some callers race with other cpus changing the passed mask */
           if (unlikely(!refs)) {
                   csd_unlock(&data->csd);
                   return;
           }
   
           /*
            * After we put an entry into the list, data->cpumask
            * may be cleared again when another CPU sends another IPI for
            * a SMP function call, so data->cpumask will be zero.
            */
           cpumask_copy(data->cpumask_ipi, data->cpumask);
           raw_spin_lock_irqsave(&call_function.lock, flags);
           /*
            * Place entry at the _HEAD_ of the list, so that any cpu still
            * observing the entry in generic_smp_call_function_interrupt()
            * will not miss any other list entries:
            */
           list_add_rcu(&data->csd.list, &call_function.queue);
           /*
            * We rely on the wmb() in list_add_rcu to complete our writes
            * to the cpumask before this write to refs, which indicates
            * data is on the list and is ready to be processed.
            */
           atomic_set(&data->refs, refs);
           raw_spin_unlock_irqrestore(&call_function.lock, flags);
   
           /*
            * Make the list addition visible before sending the ipi.
            * (IPIs must obey or appear to obey normal Linux cache
            * coherency rules -- see comment in generic_exec_single).
            */
           smp_mb();
   
           /* Send a message to all CPUs in the map */
           arch_send_call_function_ipi_mask(data->cpumask_ipi);
   
           /* Optionally wait for the CPUs to complete */
           if (wait)
                   csd_lock_wait(&data->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);
   #endif /* USE_GENERIC_SMP_HELPERS */
   
   /* 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 */
   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)
   {
           unsigned int cpu;
   
           idle_threads_init();
   
           /* 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);
           }
   
           /* Any cleanup work */
           printk(KERN_INFO "Brought up %ld CPUs\n", (long)num_online_cpus());
           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.
    */
   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)) {
                   local_irq_disable();
                   func(info);
                   local_irq_enable();
           }
           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(gfp_flags & __GFP_WAIT);
   
           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);

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