linux/kernel/stop_machine.c
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   1/* Copyright 2005 Rusty Russell rusty@rustcorp.com.au IBM Corporation.
   2 * GPL v2 and any later version.
   3 */
   4#include <linux/stop_machine.h>
   5#include <linux/kthread.h>
   6#include <linux/sched.h>
   7#include <linux/cpu.h>
   8#include <linux/err.h>
   9#include <linux/syscalls.h>
  10#include <asm/atomic.h>
  11#include <asm/semaphore.h>
  12#include <asm/uaccess.h>
  13
  14/* Since we effect priority and affinity (both of which are visible
  15 * to, and settable by outside processes) we do indirection via a
  16 * kthread. */
  17
  18/* Thread to stop each CPU in user context. */
  19enum stopmachine_state {
  20        STOPMACHINE_WAIT,
  21        STOPMACHINE_PREPARE,
  22        STOPMACHINE_DISABLE_IRQ,
  23        STOPMACHINE_EXIT,
  24};
  25
  26static enum stopmachine_state stopmachine_state;
  27static unsigned int stopmachine_num_threads;
  28static atomic_t stopmachine_thread_ack;
  29static DECLARE_MUTEX(stopmachine_mutex);
  30
  31static int stopmachine(void *cpu)
  32{
  33        int irqs_disabled = 0;
  34        int prepared = 0;
  35
  36        set_cpus_allowed(current, cpumask_of_cpu((int)(long)cpu));
  37
  38        /* Ack: we are alive */
  39        smp_mb(); /* Theoretically the ack = 0 might not be on this CPU yet. */
  40        atomic_inc(&stopmachine_thread_ack);
  41
  42        /* Simple state machine */
  43        while (stopmachine_state != STOPMACHINE_EXIT) {
  44                if (stopmachine_state == STOPMACHINE_DISABLE_IRQ 
  45                    && !irqs_disabled) {
  46                        local_irq_disable();
  47                        irqs_disabled = 1;
  48                        /* Ack: irqs disabled. */
  49                        smp_mb(); /* Must read state first. */
  50                        atomic_inc(&stopmachine_thread_ack);
  51                } else if (stopmachine_state == STOPMACHINE_PREPARE
  52                           && !prepared) {
  53                        /* Everyone is in place, hold CPU. */
  54                        preempt_disable();
  55                        prepared = 1;
  56                        smp_mb(); /* Must read state first. */
  57                        atomic_inc(&stopmachine_thread_ack);
  58                }
  59                /* Yield in first stage: migration threads need to
  60                 * help our sisters onto their CPUs. */
  61                if (!prepared && !irqs_disabled)
  62                        yield();
  63                else
  64                        cpu_relax();
  65        }
  66
  67        /* Ack: we are exiting. */
  68        smp_mb(); /* Must read state first. */
  69        atomic_inc(&stopmachine_thread_ack);
  70
  71        if (irqs_disabled)
  72                local_irq_enable();
  73        if (prepared)
  74                preempt_enable();
  75
  76        return 0;
  77}
  78
  79/* Change the thread state */
  80static void stopmachine_set_state(enum stopmachine_state state)
  81{
  82        atomic_set(&stopmachine_thread_ack, 0);
  83        smp_wmb();
  84        stopmachine_state = state;
  85        while (atomic_read(&stopmachine_thread_ack) != stopmachine_num_threads)
  86                cpu_relax();
  87}
  88
  89static int stop_machine(void)
  90{
  91        int i, ret = 0;
  92        struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
  93
  94        /* One high-prio thread per cpu.  We'll do this one. */
  95        sched_setscheduler(current, SCHED_FIFO, &param);
  96
  97        atomic_set(&stopmachine_thread_ack, 0);
  98        stopmachine_num_threads = 0;
  99        stopmachine_state = STOPMACHINE_WAIT;
 100
 101        for_each_online_cpu(i) {
 102                if (i == raw_smp_processor_id())
 103                        continue;
 104                ret = kernel_thread(stopmachine, (void *)(long)i,CLONE_KERNEL);
 105                if (ret < 0)
 106                        break;
 107                stopmachine_num_threads++;
 108        }
 109
 110        /* Wait for them all to come to life. */
 111        while (atomic_read(&stopmachine_thread_ack) != stopmachine_num_threads)
 112                yield();
 113
 114        /* If some failed, kill them all. */
 115        if (ret < 0) {
 116                stopmachine_set_state(STOPMACHINE_EXIT);
 117                return ret;
 118        }
 119
 120        /* Now they are all started, make them hold the CPUs, ready. */
 121        preempt_disable();
 122        stopmachine_set_state(STOPMACHINE_PREPARE);
 123
 124        /* Make them disable irqs. */
 125        local_irq_disable();
 126        stopmachine_set_state(STOPMACHINE_DISABLE_IRQ);
 127
 128        return 0;
 129}
 130
 131static void restart_machine(void)
 132{
 133        stopmachine_set_state(STOPMACHINE_EXIT);
 134        local_irq_enable();
 135        preempt_enable_no_resched();
 136}
 137
 138struct stop_machine_data
 139{
 140        int (*fn)(void *);
 141        void *data;
 142        struct completion done;
 143};
 144
 145static int do_stop(void *_smdata)
 146{
 147        struct stop_machine_data *smdata = _smdata;
 148        int ret;
 149
 150        ret = stop_machine();
 151        if (ret == 0) {
 152                ret = smdata->fn(smdata->data);
 153                restart_machine();
 154        }
 155
 156        /* We're done: you can kthread_stop us now */
 157        complete(&smdata->done);
 158
 159        /* Wait for kthread_stop */
 160        set_current_state(TASK_INTERRUPTIBLE);
 161        while (!kthread_should_stop()) {
 162                schedule();
 163                set_current_state(TASK_INTERRUPTIBLE);
 164        }
 165        __set_current_state(TASK_RUNNING);
 166        return ret;
 167}
 168
 169struct task_struct *__stop_machine_run(int (*fn)(void *), void *data,
 170                                       unsigned int cpu)
 171{
 172        struct stop_machine_data smdata;
 173        struct task_struct *p;
 174
 175        smdata.fn = fn;
 176        smdata.data = data;
 177        init_completion(&smdata.done);
 178
 179        down(&stopmachine_mutex);
 180
 181        /* If they don't care which CPU fn runs on, bind to any online one. */
 182        if (cpu == NR_CPUS)
 183                cpu = raw_smp_processor_id();
 184
 185        p = kthread_create(do_stop, &smdata, "kstopmachine");
 186        if (!IS_ERR(p)) {
 187                kthread_bind(p, cpu);
 188                wake_up_process(p);
 189                wait_for_completion(&smdata.done);
 190        }
 191        up(&stopmachine_mutex);
 192        return p;
 193}
 194
 195int stop_machine_run(int (*fn)(void *), void *data, unsigned int cpu)
 196{
 197        struct task_struct *p;
 198        int ret;
 199
 200        /* No CPUs can come up or down during this. */
 201        lock_cpu_hotplug();
 202        p = __stop_machine_run(fn, data, cpu);
 203        if (!IS_ERR(p))
 204                ret = kthread_stop(p);
 205        else
 206                ret = PTR_ERR(p);
 207        unlock_cpu_hotplug();
 208
 209        return ret;
 210}
 211