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 DEFINE_PER_CPU_ALIGNED(irq_cpustat_t, irq_stat);
  53 EXPORT_PER_CPU_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 by __do_softirq(),
 149  * without processing still-pending softirqs:
 150  */
 151 void _local_bh_enable(void)
 152 {
 153         WARN_ON_ONCE(in_irq());
 154         __local_bh_enable(SOFTIRQ_DISABLE_OFFSET);
 155 }
 156 EXPORT_SYMBOL(_local_bh_enable);
 157
 158 void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 159 {
 160         WARN_ON_ONCE(in_irq());
 161         lockdep_assert_irqs_enabled();
 162 #ifdef CONFIG_TRACE_IRQFLAGS
 163         local_irq_disable();
 164 #endif
 165         /*
 166          * Are softirqs going to be turned on now:
 167          */
 168         if (softirq_count() == SOFTIRQ_DISABLE_OFFSET)
 169                 trace_softirqs_on(ip);
 170         /*
 171          * Keep preemption disabled until we are done with
 172          * softirq processing:
 173          */
 174         preempt_count_sub(cnt - 1);
 175
 176         if (unlikely(!in_interrupt() && local_softirq_pending())) {
 177                 /*
 178                  * Run softirq if any pending. And do it in its own stack
 179                  * as we may be calling this deep in a task call stack already.
 180                  */
 181                 do_softirq();
 182         }
 183
 184         preempt_count_dec();
 185 #ifdef CONFIG_TRACE_IRQFLAGS
 186         local_irq_enable();
 187 #endif
 188         preempt_check_resched();
 189 }
 190 EXPORT_SYMBOL(__local_bh_enable_ip);
 191
 192 /*
 193  * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
 194  * but break the loop if need_resched() is set or after 2 ms.
 195  * The MAX_SOFTIRQ_TIME provides a nice upper bound in most cases, but in
 196  * certain cases, such as stop_machine(), jiffies may cease to
 197  * increment and so we need the MAX_SOFTIRQ_RESTART limit as
 198  * well to make sure we eventually return from this method.
 199  *
 200  * These limits have been established via experimentation.
 201  * The two things to balance is latency against fairness -
 202  * we want to handle softirqs as soon as possible, but they
 203  * should not be able to lock up the box.
 204  */
 205 #define MAX_SOFTIRQ_TIME  msecs_to_jiffies(2)
 206 #define MAX_SOFTIRQ_RESTART 10
 207
 208 #ifdef CONFIG_TRACE_IRQFLAGS
 209 /*
 210  * When we run softirqs from irq_exit() and thus on the hardirq stack we need
 211  * to keep the lockdep irq context tracking as tight as possible in order to
 212  * not miss-qualify lock contexts and miss possible deadlocks.
 213  */
 214
 215 static inline bool lockdep_softirq_start(void)
 216 {
 217         bool in_hardirq = false;
 218
 219         if (trace_hardirq_context(current)) {
 220                 in_hardirq = true;
 221                 trace_hardirq_exit();
 222         }
 223
 224         lockdep_softirq_enter();
 225
 226         return in_hardirq;
 227 }
 228
 229 static inline void lockdep_softirq_end(bool in_hardirq)
 230 {
 231         lockdep_softirq_exit();
 232
 233         if (in_hardirq)
 234                 trace_hardirq_enter();
 235 }
 236 #else
 237 static inline bool lockdep_softirq_start(void) { return false; }
 238 static inline void lockdep_softirq_end(bool in_hardirq) { }
 239 #endif
 240
 241 asmlinkage __visible void __softirq_entry __do_softirq(void)
 242 {
 243         unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
 244         unsigned long old_flags = current->flags;
 245         int max_restart = MAX_SOFTIRQ_RESTART;
 246         struct softirq_action *h;
 247         bool in_hardirq;
 248         __u32 pending;
 249         int softirq_bit;
 250
 251         /*
 252          * Mask out PF_MEMALLOC s current task context is borrowed for the
 253          * softirq. A softirq handled such as network RX might set PF_MEMALLOC
 254          * again if the socket is related to swap
 255          */
 256         current->flags &= ~PF_MEMALLOC;
 257
 258         pending = local_softirq_pending();
 259         account_irq_enter_time(current);
 260
 261         __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
 262         in_hardirq = lockdep_softirq_start();
 263
 264 restart:
 265         /* Reset the pending bitmask before enabling irqs */
 266         set_softirq_pending(0);
 267
 268         local_irq_enable();
 269
 270         h = softirq_vec;
 271
 272         while ((softirq_bit = ffs(pending))) {
 273                 unsigned int vec_nr;
 274                 int prev_count;
 275
 276                 h += softirq_bit - 1;
 277
 278                 vec_nr = h - softirq_vec;
 279                 prev_count = preempt_count();
 280
 281                 kstat_incr_softirqs_this_cpu(vec_nr);
 282
 283                 trace_softirq_entry(vec_nr);
 284                 h->action(h);
 285                 trace_softirq_exit(vec_nr);
 286                 if (unlikely(prev_count != preempt_count())) {
 287                         pr_err("huh, entered softirq %u %s %p with preempt_count %08x, exited with %08x?\n",
 288                                vec_nr, softirq_to_name[vec_nr], h->action,
 289                                prev_count, preempt_count());
 290                         preempt_count_set(prev_count);
 291                 }
 292                 h++;
 293                 pending >>= softirq_bit;
 294         }
 295
 296         rcu_bh_qs();
 297         local_irq_disable();
 298
 299         pending = local_softirq_pending();
 300         if (pending) {
 301                 if (time_before(jiffies, end) && !need_resched() &&
 302                     --max_restart)
 303                         goto restart;
 304
 305                 wakeup_softirqd();
 306         }
 307
 308         lockdep_softirq_end(in_hardirq);
 309         account_irq_exit_time(current);
 310         __local_bh_enable(SOFTIRQ_OFFSET);
 311         WARN_ON_ONCE(in_interrupt());
 312         current_restore_flags(old_flags, PF_MEMALLOC);
 313 }
 314
 315 asmlinkage __visible void do_softirq(void)
 316 {
 317         __u32 pending;
 318         unsigned long flags;
 319
 320         if (in_interrupt())
 321                 return;
 322
 323         local_irq_save(flags);
 324
 325         pending = local_softirq_pending();
 326
 327         if (pending && !ksoftirqd_running())
 328                 do_softirq_own_stack();
 329
 330         local_irq_restore(flags);
 331 }
 332
 333 /*
 334  * Enter an interrupt context.
 335  */
 336 void irq_enter(void)
 337 {
 338         rcu_irq_enter();
 339         if (is_idle_task(current) && !in_interrupt()) {
 340                 /*
 341                  * Prevent raise_softirq from needlessly waking up ksoftirqd
 342                  * here, as softirq will be serviced on return from interrupt.
 343                  */
 344                 local_bh_disable();
 345                 tick_irq_enter();
 346                 _local_bh_enable();
 347         }
 348
 349         __irq_enter();
 350 }
 351
 352 static inline void invoke_softirq(void)
 353 {
 354         if (ksoftirqd_running())
 355                 return;
 356
 357         if (!force_irqthreads) {
 358 #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
 359                 /*
 360                  * We can safely execute softirq on the current stack if
 361                  * it is the irq stack, because it should be near empty
 362                  * at this stage.
 363                  */
 364                 __do_softirq();
 365 #else
 366                 /*
 367                  * Otherwise, irq_exit() is called on the task stack that can
 368                  * be potentially deep already. So call softirq in its own stack
 369                  * to prevent from any overrun.
 370                  */
 371                 do_softirq_own_stack();
 372 #endif
 373         } else {
 374                 wakeup_softirqd();
 375         }
 376 }
 377
 378 static inline void tick_irq_exit(void)
 379 {
 380 #ifdef CONFIG_NO_HZ_COMMON
 381         int cpu = smp_processor_id();
 382
 383         /* Make sure that timer wheel updates are propagated */
 384         if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) {
 385                 if (!in_interrupt())
 386                         tick_nohz_irq_exit();
 387         }
 388 #endif
 389 }
 390
 391 /*
 392  * Exit an interrupt context. Process softirqs if needed and possible:
 393  */
 394 void irq_exit(void)
 395 {
 396 #ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED
 397         local_irq_disable();
 398 #else
 399         lockdep_assert_irqs_disabled();
 400 #endif
 401         account_irq_exit_time(current);
 402         preempt_count_sub(HARDIRQ_OFFSET);
 403         if (!in_interrupt() && local_softirq_pending())
 404                 invoke_softirq();
 405
 406         tick_irq_exit();
 407         rcu_irq_exit();
 408         trace_hardirq_exit(); /* must be last! */
 409 }
 410
 411 /*
 412  * This function must run with irqs disabled!
 413  */
 414 inline void raise_softirq_irqoff(unsigned int nr)
 415 {
 416         __raise_softirq_irqoff(nr);
 417
 418         /*
 419          * If we're in an interrupt or softirq, we're done
 420          * (this also catches softirq-disabled code). We will
 421          * actually run the softirq once we return from
 422          * the irq or softirq.
 423          *
 424          * Otherwise we wake up ksoftirqd to make sure we
 425          * schedule the softirq soon.
 426          */
 427         if (!in_interrupt())
 428                 wakeup_softirqd();
 429 }
 430
 431 void raise_softirq(unsigned int nr)
 432 {
 433         unsigned long flags;
 434
 435         local_irq_save(flags);
 436         raise_softirq_irqoff(nr);
 437         local_irq_restore(flags);
 438 }
 439
 440 void __raise_softirq_irqoff(unsigned int nr)
 441 {
 442         trace_softirq_raise(nr);
 443         or_softirq_pending(1UL << nr);
 444 }
 445
 446 void open_softirq(int nr, void (*action)(struct softirq_action *))
 447 {
 448         softirq_vec[nr].action = action;
 449 }
 450
 451 /*
 452  * Tasklets
 453  */
 454 struct tasklet_head {
 455         struct tasklet_struct *head;
 456         struct tasklet_struct **tail;
 457 };
 458
 459 static DEFINE_PER_CPU(struct tasklet_head, tasklet_vec);
 460 static DEFINE_PER_CPU(struct tasklet_head, tasklet_hi_vec);
 461
 462 static void __tasklet_schedule_common(struct tasklet_struct *t,
 463                                       struct tasklet_head __percpu *headp,
 464                                       unsigned int softirq_nr)
 465 {
 466         struct tasklet_head *head;
 467         unsigned long flags;
 468
 469         local_irq_save(flags);
 470         head = this_cpu_ptr(headp);
 471         t->next = NULL;
 472         *head->tail = t;
 473         head->tail = &(t->next);
 474         raise_softirq_irqoff(softirq_nr);
 475         local_irq_restore(flags);
 476 }
 477
 478 void __tasklet_schedule(struct tasklet_struct *t)
 479 {
 480         __tasklet_schedule_common(t, &tasklet_vec,
 481                                   TASKLET_SOFTIRQ);
 482 }
 483 EXPORT_SYMBOL(__tasklet_schedule);
 484
 485 void __tasklet_hi_schedule(struct tasklet_struct *t)
 486 {
 487         __tasklet_schedule_common(t, &tasklet_hi_vec,
 488                                   HI_SOFTIRQ);
 489 }
 490 EXPORT_SYMBOL(__tasklet_hi_schedule);
 491
 492 static void tasklet_action_common(struct softirq_action *a,
 493                                   struct tasklet_head *tl_head,
 494                                   unsigned int softirq_nr)
 495 {
 496         struct tasklet_struct *list;
 497
 498         local_irq_disable();
 499         list = tl_head->head;
 500         tl_head->head = NULL;
 501         tl_head->tail = &tl_head->head;
 502         local_irq_enable();
 503
 504         while (list) {
 505                 struct tasklet_struct *t = list;
 506
 507                 list = list->next;
 508
 509                 if (tasklet_trylock(t)) {
 510                         if (!atomic_read(&t->count)) {
 511                                 if (!test_and_clear_bit(TASKLET_STATE_SCHED,
 512                                                         &t->state))
 513                                         BUG();
 514                                 t->func(t->data);
 515                                 tasklet_unlock(t);
 516                                 continue;
 517                         }
 518                         tasklet_unlock(t);
 519                 }
 520
 521                 local_irq_disable();
 522                 t->next = NULL;
 523                 *tl_head->tail = t;
 524                 tl_head->tail = &t->next;
 525                 __raise_softirq_irqoff(softirq_nr);
 526                 local_irq_enable();
 527         }
 528 }
 529
 530 static __latent_entropy void tasklet_action(struct softirq_action *a)
 531 {
 532         tasklet_action_common(a, this_cpu_ptr(&tasklet_vec), TASKLET_SOFTIRQ);
 533 }
 534
 535 static __latent_entropy void tasklet_hi_action(struct softirq_action *a)
 536 {
 537         tasklet_action_common(a, this_cpu_ptr(&tasklet_hi_vec), HI_SOFTIRQ);
 538 }
 539
 540 void tasklet_init(struct tasklet_struct *t,
 541                   void (*func)(unsigned long), unsigned long data)
 542 {
 543         t->next = NULL;
 544         t->state = 0;
 545         atomic_set(&t->count, 0);
 546         t->func = func;
 547         t->data = data;
 548 }
 549 EXPORT_SYMBOL(tasklet_init);
 550
 551 void tasklet_kill(struct tasklet_struct *t)
 552 {
 553         if (in_interrupt())
 554                 pr_notice("Attempt to kill tasklet from interrupt\n");
 555
 556         while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
 557                 do {
 558                         yield();
 559                 } while (test_bit(TASKLET_STATE_SCHED, &t->state));
 560         }
 561         tasklet_unlock_wait(t);
 562         clear_bit(TASKLET_STATE_SCHED, &t->state);
 563 }
 564 EXPORT_SYMBOL(tasklet_kill);
 565
 566 /*
 567  * tasklet_hrtimer
 568  */
 569
 570 /*
 571  * The trampoline is called when the hrtimer expires. It schedules a tasklet
 572  * to run __tasklet_hrtimer_trampoline() which in turn will call the intended
 573  * hrtimer callback, but from softirq context.
 574  */
 575 static enum hrtimer_restart __hrtimer_tasklet_trampoline(struct hrtimer *timer)
 576 {
 577         struct tasklet_hrtimer *ttimer =
 578                 container_of(timer, struct tasklet_hrtimer, timer);
 579
 580         tasklet_hi_schedule(&ttimer->tasklet);
 581         return HRTIMER_NORESTART;
 582 }
 583
 584 /*
 585  * Helper function which calls the hrtimer callback from
 586  * tasklet/softirq context
 587  */
 588 static void __tasklet_hrtimer_trampoline(unsigned long data)
 589 {
 590         struct tasklet_hrtimer *ttimer = (void *)data;
 591         enum hrtimer_restart restart;
 592
 593         restart = ttimer->function(&ttimer->timer);
 594         if (restart != HRTIMER_NORESTART)
 595                 hrtimer_restart(&ttimer->timer);
 596 }
 597
 598 /**
 599  * tasklet_hrtimer_init - Init a tasklet/hrtimer combo for softirq callbacks
 600  * @ttimer:      tasklet_hrtimer which is initialized
 601  * @function:    hrtimer callback function which gets called from softirq context
 602  * @which_clock: clock id (CLOCK_MONOTONIC/CLOCK_REALTIME)
 603  * @mode:        hrtimer mode (HRTIMER_MODE_ABS/HRTIMER_MODE_REL)
 604  */
 605 void tasklet_hrtimer_init(struct tasklet_hrtimer *ttimer,
 606                           enum hrtimer_restart (*function)(struct hrtimer *),
 607                           clockid_t which_clock, enum hrtimer_mode mode)
 608 {
 609         hrtimer_init(&ttimer->timer, which_clock, mode);
 610         ttimer->timer.function = __hrtimer_tasklet_trampoline;
 611         tasklet_init(&ttimer->tasklet, __tasklet_hrtimer_trampoline,
 612                      (unsigned long)ttimer);
 613         ttimer->function = function;
 614 }
 615 EXPORT_SYMBOL_GPL(tasklet_hrtimer_init);
 616
 617 void __init softirq_init(void)
 618 {
 619         int cpu;
 620
 621         for_each_possible_cpu(cpu) {
 622                 per_cpu(tasklet_vec, cpu).tail =
 623                         &per_cpu(tasklet_vec, cpu).head;
 624                 per_cpu(tasklet_hi_vec, cpu).tail =
 625                         &per_cpu(tasklet_hi_vec, cpu).head;
 626         }
 627
 628         open_softirq(TASKLET_SOFTIRQ, tasklet_action);
 629         open_softirq(HI_SOFTIRQ, tasklet_hi_action);
 630 }
 631
 632 static int ksoftirqd_should_run(unsigned int cpu)
 633 {
 634         return local_softirq_pending();
 635 }
 636
 637 static void run_ksoftirqd(unsigned int cpu)
 638 {
 639         local_irq_disable();
 640         if (local_softirq_pending()) {
 641                 /*
 642                  * We can safely run softirq on inline stack, as we are not deep
 643                  * in the task stack here.
 644                  */
 645                 __do_softirq();
 646                 local_irq_enable();
 647                 cond_resched();
 648                 return;
 649         }
 650         local_irq_enable();
 651 }
 652
 653 #ifdef CONFIG_HOTPLUG_CPU
 654 /*
 655  * tasklet_kill_immediate is called to remove a tasklet which can already be
 656  * scheduled for execution on @cpu.
 657  *
 658  * Unlike tasklet_kill, this function removes the tasklet
 659  * _immediately_, even if the tasklet is in TASKLET_STATE_SCHED state.
 660  *
 661  * When this function is called, @cpu must be in the CPU_DEAD state.
 662  */
 663 void tasklet_kill_immediate(struct tasklet_struct *t, unsigned int cpu)
 664 {
 665         struct tasklet_struct **i;
 666
 667         BUG_ON(cpu_online(cpu));
 668         BUG_ON(test_bit(TASKLET_STATE_RUN, &t->state));
 669
 670         if (!test_bit(TASKLET_STATE_SCHED, &t->state))
 671                 return;
 672
 673         /* CPU is dead, so no lock needed. */
 674         for (i = &per_cpu(tasklet_vec, cpu).head; *i; i = &(*i)->next) {
 675                 if (*i == t) {
 676                         *i = t->next;
 677                         /* If this was the tail element, move the tail ptr */
 678                         if (*i == NULL)
 679                                 per_cpu(tasklet_vec, cpu).tail = i;
 680                         return;
 681                 }
 682         }
 683         BUG();
 684 }
 685
 686 static int takeover_tasklets(unsigned int cpu)
 687 {
 688         /* CPU is dead, so no lock needed. */
 689         local_irq_disable();
 690
 691         /* Find end, append list for that CPU. */
 692         if (&per_cpu(tasklet_vec, cpu).head != per_cpu(tasklet_vec, cpu).tail) {
 693                 *__this_cpu_read(tasklet_vec.tail) = per_cpu(tasklet_vec, cpu).head;
 694                 this_cpu_write(tasklet_vec.tail, per_cpu(tasklet_vec, cpu).tail);
 695                 per_cpu(tasklet_vec, cpu).head = NULL;
 696                 per_cpu(tasklet_vec, cpu).tail = &per_cpu(tasklet_vec, cpu).head;
 697         }
 698         raise_softirq_irqoff(TASKLET_SOFTIRQ);
 699
 700         if (&per_cpu(tasklet_hi_vec, cpu).head != per_cpu(tasklet_hi_vec, cpu).tail) {
 701                 *__this_cpu_read(tasklet_hi_vec.tail) = per_cpu(tasklet_hi_vec, cpu).head;
 702                 __this_cpu_write(tasklet_hi_vec.tail, per_cpu(tasklet_hi_vec, cpu).tail);
 703                 per_cpu(tasklet_hi_vec, cpu).head = NULL;
 704                 per_cpu(tasklet_hi_vec, cpu).tail = &per_cpu(tasklet_hi_vec, cpu).head;
 705         }
 706         raise_softirq_irqoff(HI_SOFTIRQ);
 707
 708         local_irq_enable();
 709         return 0;
 710 }
 711 #else
 712 #define takeover_tasklets       NULL
 713 #endif /* CONFIG_HOTPLUG_CPU */
 714
 715 static struct smp_hotplug_thread softirq_threads = {
 716         .store                  = &ksoftirqd,
 717         .thread_should_run      = ksoftirqd_should_run,
 718         .thread_fn              = run_ksoftirqd,
 719         .thread_comm            = "ksoftirqd/%u",
 720 };
 721
 722 static __init int spawn_ksoftirqd(void)
 723 {
 724         cpuhp_setup_state_nocalls(CPUHP_SOFTIRQ_DEAD, "softirq:dead", NULL,
 725                                   takeover_tasklets);
 726         BUG_ON(smpboot_register_percpu_thread(&softirq_threads));
 727
 728         return 0;
 729 }
 730 early_initcall(spawn_ksoftirqd);
 731
 732 /*
 733  * [ These __weak aliases are kept in a separate compilation unit, so that
 734  *   GCC does not inline them incorrectly. ]
 735  */
 736
 737 int __init __weak early_irq_init(void)
 738 {
 739         return 0;
 740 }
 741
 742 int __init __weak arch_probe_nr_irqs(void)
 743 {
 744         return NR_IRQS_LEGACY;
 745 }
 746
 747 int __init __weak arch_early_irq_init(void)
 748 {
 749         return 0;
 750 }
 751
 752 unsigned int __weak arch_dynirq_lower_bound(unsigned int from)
 753 {
 754         return from;
 755 }