kernel/stop_machine.c

   1 #include <linux/stop_machine.h>
   2 #include <linux/kthread.h>
   3 #include <linux/sched.h>
   4 #include <linux/cpu.h>
   5 #include <linux/err.h>
   6 #include <linux/syscalls.h>
   7 #include <linux/kthread.h>
   8 #include <asm/atomic.h>
   9 #include <asm/semaphore.h>
  10 #include <asm/uaccess.h>
  11
  12 /* Since we effect priority and affinity (both of which are visible
  13  * to, and settable by outside processes) we do indirection via a
  14  * kthread. */
  15
  16 /* Thread to stop each CPU in user context. */
  17 enum stopmachine_state {
  18         STOPMACHINE_WAIT,
  19         STOPMACHINE_PREPARE,
  20         STOPMACHINE_DISABLE_IRQ,
  21         STOPMACHINE_EXIT,
  22 };
  23
  24 static enum stopmachine_state stopmachine_state;
  25 static unsigned int stopmachine_num_threads;
  26 static atomic_t stopmachine_thread_ack;
  27 static DECLARE_MUTEX(stopmachine_mutex);
  28
  29 static int stopmachine(void *unused)
  30 {
  31         int irqs_disabled = 0;
  32         int prepared = 0;
  33
  34         /* Ack: we are alive */
  35         smp_mb(); /* Theoretically the ack = 0 might not be on this CPU yet. */
  36         atomic_inc(&stopmachine_thread_ack);
  37
  38         /* Simple state machine */
  39         while (stopmachine_state != STOPMACHINE_EXIT) {
  40                 if (stopmachine_state == STOPMACHINE_DISABLE_IRQ
  41                     && !irqs_disabled) {
  42                         local_irq_disable();
  43                         irqs_disabled = 1;
  44                         /* Ack: irqs disabled. */
  45                         smp_mb(); /* Must read state first. */
  46                         atomic_inc(&stopmachine_thread_ack);
  47                 } else if (stopmachine_state == STOPMACHINE_PREPARE
  48                            && !prepared) {
  49                         /* Everyone is in place, hold CPU. */
  50                         preempt_disable();
  51                         prepared = 1;
  52                         smp_mb(); /* Must read state first. */
  53                         atomic_inc(&stopmachine_thread_ack);
  54                 }
  55                 /* Yield in first stage: migration threads need to
  56                  * help our sisters onto their CPUs. */
  57                 if (!prepared && !irqs_disabled)
  58                         yield();
  59                 else
  60                         cpu_relax();
  61         }
  62
  63         /* Ack: we are exiting. */
  64         smp_mb(); /* Must read state first. */
  65         atomic_inc(&stopmachine_thread_ack);
  66
  67         if (irqs_disabled)
  68                 local_irq_enable();
  69         if (prepared)
  70                 preempt_enable();
  71
  72         return 0;
  73 }
  74
  75 /* Change the thread state */
  76 static void stopmachine_set_state(enum stopmachine_state state)
  77 {
  78         atomic_set(&stopmachine_thread_ack, 0);
  79         smp_wmb();
  80         stopmachine_state = state;
  81         while (atomic_read(&stopmachine_thread_ack) != stopmachine_num_threads)
  82                 cpu_relax();
  83 }
  84
  85 static int stop_machine(void)
  86 {
  87         int ret = 0;
  88         unsigned int i;
  89         struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
  90
  91         /* One high-prio thread per cpu.  We'll do this one. */
  92         sched_setscheduler(current, SCHED_FIFO, &param);
  93
  94         atomic_set(&stopmachine_thread_ack, 0);
  95         stopmachine_num_threads = 0;
  96         stopmachine_state = STOPMACHINE_WAIT;
  97
  98         for_each_online_cpu(i) {
  99                 struct task_struct *tsk;
 100                 if (i == raw_smp_processor_id())
 101                         continue;
 102                 tsk = kthread_create(stopmachine, NULL, "stopmachine");
 103                 if (IS_ERR(tsk)) {
 104                         ret = PTR_ERR(tsk);
 105                         break;
 106                 }
 107                 kthread_bind(tsk, i);
 108                 wake_up_process(tsk);
 109                 stopmachine_num_threads++;
 110         }
 111
 112         /* Wait for them all to come to life. */
 113         while (atomic_read(&stopmachine_thread_ack) != stopmachine_num_threads)
 114                 yield();
 115
 116         /* If some failed, kill them all. */
 117         if (ret < 0) {
 118                 stopmachine_set_state(STOPMACHINE_EXIT);
 119                 up(&stopmachine_mutex);
 120                 return ret;
 121         }
 122
 123         /* Now they are all started, make them hold the CPUs, ready. */
 124         preempt_disable();
 125         stopmachine_set_state(STOPMACHINE_PREPARE);
 126
 127         /* Make them disable irqs. */
 128         local_irq_disable();
 129         stopmachine_set_state(STOPMACHINE_DISABLE_IRQ);
 130
 131         return 0;
 132 }
 133
 134 static void restart_machine(void)
 135 {
 136         stopmachine_set_state(STOPMACHINE_EXIT);
 137         local_irq_enable();
 138         preempt_enable_no_resched();
 139 }
 140
 141 struct stop_machine_data
 142 {
 143         int (*fn)(void *);
 144         void *data;
 145         struct completion done;
 146 };
 147
 148 static int do_stop(void *_smdata)
 149 {
 150         struct stop_machine_data *smdata = _smdata;
 151         int ret;
 152
 153         ret = stop_machine();
 154         if (ret == 0) {
 155                 ret = smdata->fn(smdata->data);
 156                 restart_machine();
 157         }
 158
 159         /* We're done: you can kthread_stop us now */
 160         complete(&smdata->done);
 161
 162         /* Wait for kthread_stop */
 163         set_current_state(TASK_INTERRUPTIBLE);
 164         while (!kthread_should_stop()) {
 165                 schedule();
 166                 set_current_state(TASK_INTERRUPTIBLE);
 167         }
 168         __set_current_state(TASK_RUNNING);
 169         return ret;
 170 }
 171
 172 struct task_struct *__stop_machine_run(int (*fn)(void *), void *data,
 173                                        unsigned int cpu)
 174 {
 175         struct stop_machine_data smdata;
 176         struct task_struct *p;
 177
 178         smdata.fn = fn;
 179         smdata.data = data;
 180         init_completion(&smdata.done);
 181
 182         down(&stopmachine_mutex);
 183
 184         /* If they don't care which CPU fn runs on, bind to any online one. */
 185         if (cpu == NR_CPUS)
 186                 cpu = raw_smp_processor_id();
 187
 188         p = kthread_create(do_stop, &smdata, "kstopmachine");
 189         if (!IS_ERR(p)) {
 190                 kthread_bind(p, cpu);
 191                 wake_up_process(p);
 192                 wait_for_completion(&smdata.done);
 193         }
 194         up(&stopmachine_mutex);
 195         return p;
 196 }
 197
 198 int stop_machine_run(int (*fn)(void *), void *data, unsigned int cpu)
 199 {
 200         struct task_struct *p;
 201         int ret;
 202
 203         /* No CPUs can come up or down during this. */
 204         lock_cpu_hotplug();
 205         p = __stop_machine_run(fn, data, cpu);
 206         if (!IS_ERR(p))
 207                 ret = kthread_stop(p);
 208         else
 209                 ret = PTR_ERR(p);
 210         unlock_cpu_hotplug();
 211
 212         return ret;
 213 }