kernel/softirq.c

   1 /*
   2  *      linux/kernel/softirq.c
   3  *
   4  *      Copyright (C) 1992 Linus Torvalds
   5  *
   6  *      Distribute under GPLv2.
   7  *
   8  *      Rewritten. Old one was good in 2.2, but in 2.3 it was immoral. --ANK (990903)
   9  */
  10
  11 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  12
  13 #include <linux/export.h>
  14 #include <linux/kernel_stat.h>
  15 #include <linux/interrupt.h>
  16 #include <linux/init.h>
  17 #include <linux/mm.h>
  18 #include <linux/notifier.h>
  19 #include <linux/percpu.h>
  20 #include <linux/cpu.h>
  21 #include <linux/freezer.h>
  22 #include <linux/kthread.h>
  23 #include <linux/rcupdate.h>
  24 #include <linux/ftrace.h>
  25 #include <linux/smp.h>
  26 #include <linux/smpboot.h>
  27 #include <linux/tick.h>
  28 #include <linux/irq.h>
  29
  30 #define CREATE_TRACE_POINTS
  31 #include <trace/events/irq.h>
  32
  33 /*
  34    - No shared variables, all the data are CPU local.
  35    - If a softirq needs serialization, let it serialize itself
  36      by its own spinlocks.
  37    - Even if softirq is serialized, only local cpu is marked for
  38      execution. Hence, we get something sort of weak cpu binding.
  39      Though it is still not clear, will it result in better locality
  40      or will not.
  41
  42    Examples:
  43    - NET RX softirq. It is multithreaded and does not require
  44      any global serialization.
  45    - NET TX softirq. It kicks software netdevice queues, hence
  46      it is logically serialized per device, but this serialization
  47      is invisible to common code.
  48    - Tasklets: serialized wrt itself.
  49  */
  50
  51 #ifndef __ARCH_IRQ_STAT
  52 irq_cpustat_t irq_stat[NR_CPUS] ____cacheline_aligned;
  53 EXPORT_SYMBOL(irq_stat);
  54 #endif
  55
  56 static struct softirq_action softirq_vec[NR_SOFTIRQS] __cacheline_aligned_in_smp;
  57
  58 DEFINE_PER_CPU(struct task_struct *, ksoftirqd);
  59
  60 const char * const softirq_to_name[NR_SOFTIRQS] = {
  61         "HI", "TIMER", "NET_TX", "NET_RX", "BLOCK", "IRQ_POLL",
  62         "TASKLET", "SCHED", "HRTIMER", "RCU"
  63 };
  64
  65 /*
  66  * we cannot loop indefinitely here to avoid userspace starvation,
  67  * but we also don't want to introduce a worst case 1/HZ latency
  68  * to the pending events, so lets the scheduler to balance
  69  * the softirq load for us.
  70  */
  71 static void wakeup_softirqd(void)
  72 {
  73         /* Interrupts are disabled: no need to stop preemption */
  74         struct task_struct *tsk = __this_cpu_read(ksoftirqd);
  75
  76         if (tsk && tsk->state != TASK_RUNNING)
  77                 wake_up_process(tsk);
  78 }
  79
  80 /*
  81  * If ksoftirqd is scheduled, we do not want to process pending softirqs
  82  * right now. Let ksoftirqd handle this at its own rate, to get fairness.
  83  */
  84 static bool ksoftirqd_running(void)
  85 {
  86         struct task_struct *tsk = __this_cpu_read(ksoftirqd);
  87
  88         return tsk && (tsk->state == TASK_RUNNING);
  89 }
  90
  91 /*
  92  * preempt_count and SOFTIRQ_OFFSET usage:
  93  * - preempt_count is changed by SOFTIRQ_OFFSET on entering or leaving
  94  *   softirq processing.
  95  * - preempt_count is changed by SOFTIRQ_DISABLE_OFFSET (= 2 * SOFTIRQ_OFFSET)
  96  *   on local_bh_disable or local_bh_enable.
  97  * This lets us distinguish between whether we are currently processing
  98  * softirq and whether we just have bh disabled.
  99  */
 100
 101 /*
 102  * This one is for softirq.c-internal use,
 103  * where hardirqs are disabled legitimately:
 104  */
 105 #ifdef CONFIG_TRACE_IRQFLAGS
 106 void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
 107 {
 108         unsigned long flags;
 109
 110         WARN_ON_ONCE(in_irq());
 111
 112         raw_local_irq_save(flags);
 113         /*
 114          * The preempt tracer hooks into preempt_count_add and will break
 115          * lockdep because it calls back into lockdep after SOFTIRQ_OFFSET
 116          * is set and before current->softirq_enabled is cleared.
 117          * We must manually increment preempt_count here and manually
 118          * call the trace_preempt_off later.
 119          */
 120         __preempt_count_add(cnt);
 121         /*
 122          * Were softirqs turned off above:
 123          */
 124         if (softirq_count() == (cnt & SOFTIRQ_MASK))
 125                 trace_softirqs_off(ip);
 126         raw_local_irq_restore(flags);
 127
 128         if (preempt_count() == cnt) {
 129 #ifdef CONFIG_DEBUG_PREEMPT
 130                 current->preempt_disable_ip = get_lock_parent_ip();
 131 #endif
 132                 trace_preempt_off(CALLER_ADDR0, get_lock_parent_ip());
 133         }
 134 }
 135 EXPORT_SYMBOL(__local_bh_disable_ip);
 136 #endif /* CONFIG_TRACE_IRQFLAGS */
 137
 138 static void __local_bh_enable(unsigned int cnt)
 139 {
 140         lockdep_assert_irqs_disabled();
 141
 142         if (softirq_count() == (cnt & SOFTIRQ_MASK))
 143                 trace_softirqs_on(_RET_IP_);
 144         preempt_count_sub(cnt);
 145 }
 146
 147 /*
 148  * Special-case - softirqs can safely be enabled in
 149  * cond_resched_softirq(), or by __do_softirq(),
 150  * without processing still-pending softirqs:
 151  */
 152 void _local_bh_enable(void)
 153 {
 154         WARN_ON_ONCE(in_irq());
 155         __local_bh_enable(SOFTIRQ_DISABLE_OFFSET);
 156 }
 157 EXPORT_SYMBOL(_local_bh_enable);
 158
 159 void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 160 {
 161         WARN_ON_ONCE(in_irq());
 162         lockdep_assert_irqs_enabled();
 163 #ifdef CONFIG_TRACE_IRQFLAGS
 164         local_irq_disable();
 165 #endif
 166         /*
 167          * Are softirqs going to be turned on now:
 168          */
 169         if (softirq_count() == SOFTIRQ_DISABLE_OFFSET)
 170                 trace_softirqs_on(ip);
 171         /*
 172          * Keep preemption disabled until we are done with
 173          * softirq processing:
 174          */
 175         preempt_count_sub(cnt - 1);
 176
 177         if (unlikely(!in_interrupt() && local_softirq_pending())) {
 178                 /*
 179                  * Run softirq if any pending. And do it in its own stack
 180                  * as we may be calling this deep in a task call stack already.
 181                  */
 182                 do_softirq();
 183         }
 184
 185         preempt_count_dec();
 186 #ifdef CONFIG_TRACE_IRQFLAGS
 187         local_irq_enable();
 188 #endif
 189         preempt_check_resched();
 190 }
 191 EXPORT_SYMBOL(__local_bh_enable_ip);
 192
 193 /*
 194  * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
 195  * but break the loop if need_resched() is set or after 2 ms.
 196  * The MAX_SOFTIRQ_TIME provides a nice upper bound in most cases, but in
 197  * certain cases, such as stop_machine(), jiffies may cease to
 198  * increment and so we need the MAX_SOFTIRQ_RESTART limit as
 199  * well to make sure we eventually return from this method.
 200  *
 201  * These limits have been established via experimentation.
 202  * The two things to balance is latency against fairness -
 203  * we want to handle softirqs as soon as possible, but they
 204  * should not be able to lock up the box.
 205  */
 206 #define MAX_SOFTIRQ_TIME  msecs_to_jiffies(2)
 207 #define MAX_SOFTIRQ_RESTART 10
 208
 209 #ifdef CONFIG_TRACE_IRQFLAGS
 210 /*
 211  * When we run softirqs from irq_exit() and thus on the hardirq stack we need
 212  * to keep the lockdep irq context tracking as tight as possible in order to
 213  * not miss-qualify lock contexts and miss possible deadlocks.
 214  */
 215
 216 static inline bool lockdep_softirq_start(void)
 217 {
 218         bool in_hardirq = false;
 219
 220         if (trace_hardirq_context(current)) {
 221                 in_hardirq = true;
 222                 trace_hardirq_exit();
 223         }
 224
 225         lockdep_softirq_enter();
 226
 227         return in_hardirq;
 228 }
 229
 230 static inline void lockdep_softirq_end(bool in_hardirq)
 231 {
 232         lockdep_softirq_exit();
 233
 234         if (in_hardirq)
 235                 trace_hardirq_enter();
 236 }
 237 #else
 238 static inline bool lockdep_softirq_start(void) { return false; }
 239 static inline void lockdep_softirq_end(bool in_hardirq) { }
 240 #endif
 241
 242 asmlinkage __visible void __softirq_entry __do_softirq(void)
 243 {
 244         unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
 245         unsigned long old_flags = current->flags;
 246         int max_restart = MAX_SOFTIRQ_RESTART;
 247         struct softirq_action *h;
 248         bool in_hardirq;
 249         __u32 pending;
 250         int softirq_bit;
 251
 252         /*
 253          * Mask out PF_MEMALLOC s current task context is borrowed for the
 254          * softirq. A softirq handled such as network RX might set PF_MEMALLOC
 255          * again if the socket is related to swap
 256          */
 257         current->flags &= ~PF_MEMALLOC;
 258
 259         pending = local_softirq_pending();
 260         account_irq_enter_time(current);
 261
 262         __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
 263         in_hardirq = lockdep_softirq_start();
 264
 265 restart:
 266         /* Reset the pending bitmask before enabling irqs */
 267         set_softirq_pending(0);
 268
 269         local_irq_enable();
 270
 271         h = softirq_vec;
 272
 273         while ((softirq_bit = ffs(pending))) {
 274                 unsigned int vec_nr;
 275                 int prev_count;
 276
 277                 h += softirq_bit - 1;
 278
 279                 vec_nr = h - softirq_vec;
 280                 prev_count = preempt_count();
 281
 282                 kstat_incr_softirqs_this_cpu(vec_nr);
 283
 284                 trace_softirq_entry(vec_nr);
 285                 h->action(h);
 286                 trace_softirq_exit(vec_nr);
 287                 if (unlikely(prev_count != preempt_count())) {
 288                         pr_err("huh, entered softirq %u %s %p with preempt_count %08x, exited with %08x?\n",
 289                                vec_nr, softirq_to_name[vec_nr], h->action,
 290                                prev_count, preempt_count());
 291                         preempt_count_set(prev_count);
 292                 }
 293                 h++;
 294                 pending >>= softirq_bit;
 295         }
 296
 297         rcu_bh_qs();
 298         local_irq_disable();
 299
 300         pending = local_softirq_pending();
 301         if (pending) {
 302                 if (time_before(jiffies, end) && !need_resched() &&
 303                     --max_restart)
 304                         goto restart;
 305
 306                 wakeup_softirqd();
 307         }
 308
 309         lockdep_softirq_end(in_hardirq);
 310         account_irq_exit_time(current);
 311         __local_bh_enable(SOFTIRQ_OFFSET);
 312         WARN_ON_ONCE(in_interrupt());
 313         current_restore_flags(old_flags, PF_MEMALLOC);
 314 }
 315
 316 asmlinkage __visible void do_softirq(void)
 317 {
 318         __u32 pending;
 319         unsigned long flags;
 320
 321         if (in_interrupt())
 322                 return;
 323
 324         local_irq_save(flags);
 325
 326         pending = local_softirq_pending();
 327
 328         if (pending && !ksoftirqd_running())
 329                 do_softirq_own_stack();
 330
 331         local_irq_restore(flags);
 332 }
 333
 334 /*
 335  * Enter an interrupt context.
 336  */
 337 void irq_enter(void)
 338 {
 339         rcu_irq_enter();
 340         if (is_idle_task(current) && !in_interrupt()) {
 341                 /*
 342                  * Prevent raise_softirq from needlessly waking up ksoftirqd
 343                  * here, as softirq will be serviced on return from interrupt.
 344                  */
 345                 local_bh_disable();
 346                 tick_irq_enter();
 347                 _local_bh_enable();
 348         }
 349
 350         __irq_enter();
 351 }
 352
 353 static inline void invoke_softirq(void)
 354 {
 355         if (ksoftirqd_running())
 356                 return;
 357
 358         if (!force_irqthreads) {
 359 #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
 360                 /*
 361                  * We can safely execute softirq on the current stack if
 362                  * it is the irq stack, because it should be near empty
 363                  * at this stage.
 364                  */
 365                 __do_softirq();
 366 #else
 367                 /*
 368                  * Otherwise, irq_exit() is called on the task stack that can
 369                  * be potentially deep already. So call softirq in its own stack
 370                  * to prevent from any overrun.
 371                  */
 372                 do_softirq_own_stack();
 373 #endif
 374         } else {
 375                 wakeup_softirqd();
 376         }
 377 }
 378
 379 static inline void tick_irq_exit(void)
 380 {
 381 #ifdef CONFIG_NO_HZ_COMMON
 382         int cpu = smp_processor_id();
 383
 384         /* Make sure that timer wheel updates are propagated */
 385         if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) {
 386                 if (!in_interrupt())
 387                         tick_nohz_irq_exit();
 388         }
 389 #endif
 390 }
 391
 392 /*
 393  * Exit an interrupt context. Process softirqs if needed and possible:
 394  */
 395 void irq_exit(void)
 396 {
 397 #ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED
 398         local_irq_disable();
 399 #else
 400         lockdep_assert_irqs_disabled();
 401 #endif
 402         account_irq_exit_time(current);
 403         preempt_count_sub(HARDIRQ_OFFSET);
 404         if (!in_interrupt() && local_softirq_pending())
 405                 invoke_softirq();
 406
 407         tick_irq_exit();
 408         rcu_irq_exit();
 409         trace_hardirq_exit(); /* must be last! */
 410 }
 411
 412 /*
 413  * This function must run with irqs disabled!
 414  */
 415 inline void raise_softirq_irqoff(unsigned int nr)
 416 {
 417         __raise_softirq_irqoff(nr);
 418
 419         /*
 420          * If we're in an interrupt or softirq, we're done
 421          * (this also catches softirq-disabled code). We will
 422          * actually run the softirq once we return from
 423          * the irq or softirq.
 424          *
 425          * Otherwise we wake up ksoftirqd to make sure we
 426          * schedule the softirq soon.
 427          */
 428         if (!in_interrupt())
 429                 wakeup_softirqd();
 430 }
 431
 432 void raise_softirq(unsigned int nr)
 433 {
 434         unsigned long flags;
 435
 436         local_irq_save(flags);
 437         raise_softirq_irqoff(nr);
 438         local_irq_restore(flags);
 439 }
 440
 441 void __raise_softirq_irqoff(unsigned int nr)
 442 {
 443         trace_softirq_raise(nr);
 444         or_softirq_pending(1UL << nr);
 445 }
 446
 447 void open_softirq(int nr, void (*action)(struct softirq_action *))
 448 {
 449         softirq_vec[nr].action = action;
 450 }
 451
 452 /*
 453  * Tasklets
 454  */
 455 struct tasklet_head {
 456         struct tasklet_struct *head;
 457         struct tasklet_struct **tail;
 458 };
 459
 460 static DEFINE_PER_CPU(struct tasklet_head, tasklet_vec);
 461 static DEFINE_PER_CPU(struct tasklet_head, tasklet_hi_vec);
 462
 463 static void __tasklet_schedule_common(struct tasklet_struct *t,
 464                                       struct tasklet_head __percpu *headp,
 465                                       unsigned int softirq_nr)
 466 {
 467         struct tasklet_head *head;
 468         unsigned long flags;
 469
 470         local_irq_save(flags);
 471         head = this_cpu_ptr(headp);
 472         t->next = NULL;
 473         *head->tail = t;
 474         head->tail = &(t->next);
 475         raise_softirq_irqoff(softirq_nr);
 476         local_irq_restore(flags);
 477 }
 478
 479 void __tasklet_schedule(struct tasklet_struct *t)
 480 {
 481         __tasklet_schedule_common(t, &tasklet_vec,
 482                                   TASKLET_SOFTIRQ);
 483 }
 484 EXPORT_SYMBOL(__tasklet_schedule);
 485
 486 void __tasklet_hi_schedule(struct tasklet_struct *t)
 487 {
 488         __tasklet_schedule_common(t, &tasklet_hi_vec,
 489                                   HI_SOFTIRQ);
 490 }
 491 EXPORT_SYMBOL(__tasklet_hi_schedule);
 492
 493 static void tasklet_action_common(struct softirq_action *a,
 494                                   struct tasklet_head *tl_head,
 495                                   unsigned int softirq_nr)
 496 {
 497         struct tasklet_struct *list;
 498
 499         local_irq_disable();
 500         list = tl_head->head;
 501         tl_head->head = NULL;
 502         tl_head->tail = &tl_head->head;
 503         local_irq_enable();
 504
 505         while (list) {
 506                 struct tasklet_struct *t = list;
 507
 508                 list = list->next;
 509
 510                 if (tasklet_trylock(t)) {
 511                         if (!atomic_read(&t->count)) {
 512                                 if (!test_and_clear_bit(TASKLET_STATE_SCHED,
 513                                                         &t->state))
 514                                         BUG();
 515                                 t->func(t->data);
 516                                 tasklet_unlock(t);
 517                                 continue;
 518                         }
 519                         tasklet_unlock(t);
 520                 }
 521
 522                 local_irq_disable();
 523                 t->next = NULL;
 524                 *tl_head->tail = t;
 525                 tl_head->tail = &t->next;
 526                 __raise_softirq_irqoff(softirq_nr);
 527                 local_irq_enable();
 528         }
 529 }
 530
 531 static __latent_entropy void tasklet_action(struct softirq_action *a)
 532 {
 533         tasklet_action_common(a, this_cpu_ptr(&tasklet_vec), TASKLET_SOFTIRQ);
 534 }
 535
 536 static __latent_entropy void tasklet_hi_action(struct softirq_action *a)
 537 {
 538         tasklet_action_common(a, this_cpu_ptr(&tasklet_hi_vec), HI_SOFTIRQ);
 539 }
 540
 541 void tasklet_init(struct tasklet_struct *t,
 542                   void (*func)(unsigned long), unsigned long data)
 543 {
 544         t->next = NULL;
 545         t->state = 0;
 546         atomic_set(&t->count, 0);
 547         t->func = func;
 548         t->data = data;
 549 }
 550 EXPORT_SYMBOL(tasklet_init);
 551
 552 void tasklet_kill(struct tasklet_struct *t)
 553 {
 554         if (in_interrupt())
 555                 pr_notice("Attempt to kill tasklet from interrupt\n");
 556
 557         while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
 558                 do {
 559                         yield();
 560                 } while (test_bit(TASKLET_STATE_SCHED, &t->state));
 561         }
 562         tasklet_unlock_wait(t);
 563         clear_bit(TASKLET_STATE_SCHED, &t->state);
 564 }
 565 EXPORT_SYMBOL(tasklet_kill);
 566
 567 /*
 568  * tasklet_hrtimer
 569  */
 570
 571 /*
 572  * The trampoline is called when the hrtimer expires. It schedules a tasklet
 573  * to run __tasklet_hrtimer_trampoline() which in turn will call the intended
 574  * hrtimer callback, but from softirq context.
 575  */
 576 static enum hrtimer_restart __hrtimer_tasklet_trampoline(struct hrtimer *timer)
 577 {
 578         struct tasklet_hrtimer *ttimer =
 579                 container_of(timer, struct tasklet_hrtimer, timer);
 580
 581         tasklet_hi_schedule(&ttimer->tasklet);
 582         return HRTIMER_NORESTART;
 583 }
 584
 585 /*
 586  * Helper function which calls the hrtimer callback from
 587  * tasklet/softirq context
 588  */
 589 static void __tasklet_hrtimer_trampoline(unsigned long data)
 590 {
 591         struct tasklet_hrtimer *ttimer = (void *)data;
 592         enum hrtimer_restart restart;
 593
 594         restart = ttimer->function(&ttimer->timer);
 595         if (restart != HRTIMER_NORESTART)
 596                 hrtimer_restart(&ttimer->timer);
 597 }
 598
 599 /**
 600  * tasklet_hrtimer_init - Init a tasklet/hrtimer combo for softirq callbacks
 601  * @ttimer:      tasklet_hrtimer which is initialized
 602  * @function:    hrtimer callback function which gets called from softirq context
 603  * @which_clock: clock id (CLOCK_MONOTONIC/CLOCK_REALTIME)
 604  * @mode:        hrtimer mode (HRTIMER_MODE_ABS/HRTIMER_MODE_REL)
 605  */
 606 void tasklet_hrtimer_init(struct tasklet_hrtimer *ttimer,
 607                           enum hrtimer_restart (*function)(struct hrtimer *),
 608                           clockid_t which_clock, enum hrtimer_mode mode)
 609 {
 610         hrtimer_init(&ttimer->timer, which_clock, mode);
 611         ttimer->timer.function = __hrtimer_tasklet_trampoline;
 612         tasklet_init(&ttimer->tasklet, __tasklet_hrtimer_trampoline,
 613                      (unsigned long)ttimer);
 614         ttimer->function = function;
 615 }
 616 EXPORT_SYMBOL_GPL(tasklet_hrtimer_init);
 617
 618 void __init softirq_init(void)
 619 {
 620         int cpu;
 621
 622         for_each_possible_cpu(cpu) {
 623                 per_cpu(tasklet_vec, cpu).tail =
 624                         &per_cpu(tasklet_vec, cpu).head;
 625                 per_cpu(tasklet_hi_vec, cpu).tail =
 626                         &per_cpu(tasklet_hi_vec, cpu).head;
 627         }
 628
 629         open_softirq(TASKLET_SOFTIRQ, tasklet_action);
 630         open_softirq(HI_SOFTIRQ, tasklet_hi_action);
 631 }
 632
 633 static int ksoftirqd_should_run(unsigned int cpu)
 634 {
 635         return local_softirq_pending();
 636 }
 637
 638 static void run_ksoftirqd(unsigned int cpu)
 639 {
 640         local_irq_disable();
 641         if (local_softirq_pending()) {
 642                 /*
 643                  * We can safely run softirq on inline stack, as we are not deep
 644                  * in the task stack here.
 645                  */
 646                 __do_softirq();
 647                 local_irq_enable();
 648                 cond_resched();
 649                 return;
 650         }
 651         local_irq_enable();
 652 }
 653
 654 #ifdef CONFIG_HOTPLUG_CPU
 655 /*
 656  * tasklet_kill_immediate is called to remove a tasklet which can already be
 657  * scheduled for execution on @cpu.
 658  *
 659  * Unlike tasklet_kill, this function removes the tasklet
 660  * _immediately_, even if the tasklet is in TASKLET_STATE_SCHED state.
 661  *
 662  * When this function is called, @cpu must be in the CPU_DEAD state.
 663  */
 664 void tasklet_kill_immediate(struct tasklet_struct *t, unsigned int cpu)
 665 {
 666         struct tasklet_struct **i;
 667
 668         BUG_ON(cpu_online(cpu));
 669         BUG_ON(test_bit(TASKLET_STATE_RUN, &t->state));
 670
 671         if (!test_bit(TASKLET_STATE_SCHED, &t->state))
 672                 return;
 673
 674         /* CPU is dead, so no lock needed. */
 675         for (i = &per_cpu(tasklet_vec, cpu).head; *i; i = &(*i)->next) {
 676                 if (*i == t) {
 677                         *i = t->next;
 678                         /* If this was the tail element, move the tail ptr */
 679                         if (*i == NULL)
 680                                 per_cpu(tasklet_vec, cpu).tail = i;
 681                         return;
 682                 }
 683         }
 684         BUG();
 685 }
 686
 687 static int takeover_tasklets(unsigned int cpu)
 688 {
 689         /* CPU is dead, so no lock needed. */
 690         local_irq_disable();
 691
 692         /* Find end, append list for that CPU. */
 693         if (&per_cpu(tasklet_vec, cpu).head != per_cpu(tasklet_vec, cpu).tail) {
 694                 *__this_cpu_read(tasklet_vec.tail) = per_cpu(tasklet_vec, cpu).head;
 695                 this_cpu_write(tasklet_vec.tail, per_cpu(tasklet_vec, cpu).tail);
 696                 per_cpu(tasklet_vec, cpu).head = NULL;
 697                 per_cpu(tasklet_vec, cpu).tail = &per_cpu(tasklet_vec, cpu).head;
 698         }
 699         raise_softirq_irqoff(TASKLET_SOFTIRQ);
 700
 701         if (&per_cpu(tasklet_hi_vec, cpu).head != per_cpu(tasklet_hi_vec, cpu).tail) {
 702                 *__this_cpu_read(tasklet_hi_vec.tail) = per_cpu(tasklet_hi_vec, cpu).head;
 703                 __this_cpu_write(tasklet_hi_vec.tail, per_cpu(tasklet_hi_vec, cpu).tail);
 704                 per_cpu(tasklet_hi_vec, cpu).head = NULL;
 705                 per_cpu(tasklet_hi_vec, cpu).tail = &per_cpu(tasklet_hi_vec, cpu).head;
 706         }
 707         raise_softirq_irqoff(HI_SOFTIRQ);
 708
 709         local_irq_enable();
 710         return 0;
 711 }
 712 #else
 713 #define takeover_tasklets       NULL
 714 #endif /* CONFIG_HOTPLUG_CPU */
 715
 716 static struct smp_hotplug_thread softirq_threads = {
 717         .store                  = &ksoftirqd,
 718         .thread_should_run      = ksoftirqd_should_run,
 719         .thread_fn              = run_ksoftirqd,
 720         .thread_comm            = "ksoftirqd/%u",
 721 };
 722
 723 static __init int spawn_ksoftirqd(void)
 724 {
 725         cpuhp_setup_state_nocalls(CPUHP_SOFTIRQ_DEAD, "softirq:dead", NULL,
 726                                   takeover_tasklets);
 727         BUG_ON(smpboot_register_percpu_thread(&softirq_threads));
 728
 729         return 0;
 730 }
 731 early_initcall(spawn_ksoftirqd);
 732
 733 /*
 734  * [ These __weak aliases are kept in a separate compilation unit, so that
 735  *   GCC does not inline them incorrectly. ]
 736  */
 737
 738 int __init __weak early_irq_init(void)
 739 {
 740         return 0;
 741 }
 742
 743 int __init __weak arch_probe_nr_irqs(void)
 744 {
 745         return NR_IRQS_LEGACY;
 746 }
 747
 748 int __init __weak arch_early_irq_init(void)
 749 {
 750         return 0;
 751 }
 752
 753 unsigned int __weak arch_dynirq_lower_bound(unsigned int from)
 754 {
 755         return from;
 756 }