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  * unless we're doing some of the synchronous softirqs.
  84  */
  85 #define SOFTIRQ_NOW_MASK ((1 << HI_SOFTIRQ) | (1 << TASKLET_SOFTIRQ))
  86 static bool ksoftirqd_running(unsigned long pending)
  87 {
  88         struct task_struct *tsk = __this_cpu_read(ksoftirqd);
  89
  90         if (pending & SOFTIRQ_NOW_MASK)
  91                 return false;
  92         return tsk && (tsk->state == TASK_RUNNING);
  93 }
  94
  95 /*
  96  * preempt_count and SOFTIRQ_OFFSET usage:
  97  * - preempt_count is changed by SOFTIRQ_OFFSET on entering or leaving
  98  *   softirq processing.
  99  * - preempt_count is changed by SOFTIRQ_DISABLE_OFFSET (= 2 * SOFTIRQ_OFFSET)
 100  *   on local_bh_disable or local_bh_enable.
 101  * This lets us distinguish between whether we are currently processing
 102  * softirq and whether we just have bh disabled.
 103  */
 104
 105 /*
 106  * This one is for softirq.c-internal use,
 107  * where hardirqs are disabled legitimately:
 108  */
 109 #ifdef CONFIG_TRACE_IRQFLAGS
 110 void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
 111 {
 112         unsigned long flags;
 113
 114         WARN_ON_ONCE(in_irq());
 115
 116         raw_local_irq_save(flags);
 117         /*
 118          * The preempt tracer hooks into preempt_count_add and will break
 119          * lockdep because it calls back into lockdep after SOFTIRQ_OFFSET
 120          * is set and before current->softirq_enabled is cleared.
 121          * We must manually increment preempt_count here and manually
 122          * call the trace_preempt_off later.
 123          */
 124         __preempt_count_add(cnt);
 125         /*
 126          * Were softirqs turned off above:
 127          */
 128         if (softirq_count() == (cnt & SOFTIRQ_MASK))
 129                 trace_softirqs_off(ip);
 130         raw_local_irq_restore(flags);
 131
 132         if (preempt_count() == cnt) {
 133 #ifdef CONFIG_DEBUG_PREEMPT
 134                 current->preempt_disable_ip = get_lock_parent_ip();
 135 #endif
 136                 trace_preempt_off(CALLER_ADDR0, get_lock_parent_ip());
 137         }
 138 }
 139 EXPORT_SYMBOL(__local_bh_disable_ip);
 140 #endif /* CONFIG_TRACE_IRQFLAGS */
 141
 142 static void __local_bh_enable(unsigned int cnt)
 143 {
 144         lockdep_assert_irqs_disabled();
 145
 146         if (preempt_count() == cnt)
 147                 trace_preempt_on(CALLER_ADDR0, get_lock_parent_ip());
 148
 149         if (softirq_count() == (cnt & SOFTIRQ_MASK))
 150                 trace_softirqs_on(_RET_IP_);
 151
 152         __preempt_count_sub(cnt);
 153 }
 154
 155 /*
 156  * Special-case - softirqs can safely be enabled by __do_softirq(),
 157  * without processing still-pending softirqs:
 158  */
 159 void _local_bh_enable(void)
 160 {
 161         WARN_ON_ONCE(in_irq());
 162         __local_bh_enable(SOFTIRQ_DISABLE_OFFSET);
 163 }
 164 EXPORT_SYMBOL(_local_bh_enable);
 165
 166 void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 167 {
 168         WARN_ON_ONCE(in_irq());
 169         lockdep_assert_irqs_enabled();
 170 #ifdef CONFIG_TRACE_IRQFLAGS
 171         local_irq_disable();
 172 #endif
 173         /*
 174          * Are softirqs going to be turned on now:
 175          */
 176         if (softirq_count() == SOFTIRQ_DISABLE_OFFSET)
 177                 trace_softirqs_on(ip);
 178         /*
 179          * Keep preemption disabled until we are done with
 180          * softirq processing:
 181          */
 182         preempt_count_sub(cnt - 1);
 183
 184         if (unlikely(!in_interrupt() && local_softirq_pending())) {
 185                 /*
 186                  * Run softirq if any pending. And do it in its own stack
 187                  * as we may be calling this deep in a task call stack already.
 188                  */
 189                 do_softirq();
 190         }
 191
 192         preempt_count_dec();
 193 #ifdef CONFIG_TRACE_IRQFLAGS
 194         local_irq_enable();
 195 #endif
 196         preempt_check_resched();
 197 }
 198 EXPORT_SYMBOL(__local_bh_enable_ip);
 199
 200 /*
 201  * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
 202  * but break the loop if need_resched() is set or after 2 ms.
 203  * The MAX_SOFTIRQ_TIME provides a nice upper bound in most cases, but in
 204  * certain cases, such as stop_machine(), jiffies may cease to
 205  * increment and so we need the MAX_SOFTIRQ_RESTART limit as
 206  * well to make sure we eventually return from this method.
 207  *
 208  * These limits have been established via experimentation.
 209  * The two things to balance is latency against fairness -
 210  * we want to handle softirqs as soon as possible, but they
 211  * should not be able to lock up the box.
 212  */
 213 #define MAX_SOFTIRQ_TIME  msecs_to_jiffies(2)
 214 #define MAX_SOFTIRQ_RESTART 10
 215
 216 #ifdef CONFIG_TRACE_IRQFLAGS
 217 /*
 218  * When we run softirqs from irq_exit() and thus on the hardirq stack we need
 219  * to keep the lockdep irq context tracking as tight as possible in order to
 220  * not miss-qualify lock contexts and miss possible deadlocks.
 221  */
 222
 223 static inline bool lockdep_softirq_start(void)
 224 {
 225         bool in_hardirq = false;
 226
 227         if (trace_hardirq_context(current)) {
 228                 in_hardirq = true;
 229                 trace_hardirq_exit();
 230         }
 231
 232         lockdep_softirq_enter();
 233
 234         return in_hardirq;
 235 }
 236
 237 static inline void lockdep_softirq_end(bool in_hardirq)
 238 {
 239         lockdep_softirq_exit();
 240
 241         if (in_hardirq)
 242                 trace_hardirq_enter();
 243 }
 244 #else
 245 static inline bool lockdep_softirq_start(void) { return false; }
 246 static inline void lockdep_softirq_end(bool in_hardirq) { }
 247 #endif
 248
 249 asmlinkage __visible void __softirq_entry __do_softirq(void)
 250 {
 251         unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
 252         unsigned long old_flags = current->flags;
 253         int max_restart = MAX_SOFTIRQ_RESTART;
 254         struct softirq_action *h;
 255         bool in_hardirq;
 256         __u32 pending;
 257         int softirq_bit;
 258
 259         /*
 260          * Mask out PF_MEMALLOC as the current task context is borrowed for the
 261          * softirq. A softirq handled, such as network RX, might set PF_MEMALLOC
 262          * again if the socket is related to swapping.
 263          */
 264         current->flags &= ~PF_MEMALLOC;
 265
 266         pending = local_softirq_pending();
 267         account_irq_enter_time(current);
 268
 269         __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
 270         in_hardirq = lockdep_softirq_start();
 271
 272 restart:
 273         /* Reset the pending bitmask before enabling irqs */
 274         set_softirq_pending(0);
 275
 276         local_irq_enable();
 277
 278         h = softirq_vec;
 279
 280         while ((softirq_bit = ffs(pending))) {
 281                 unsigned int vec_nr;
 282                 int prev_count;
 283
 284                 h += softirq_bit - 1;
 285
 286                 vec_nr = h - softirq_vec;
 287                 prev_count = preempt_count();
 288
 289                 kstat_incr_softirqs_this_cpu(vec_nr);
 290
 291                 trace_softirq_entry(vec_nr);
 292                 h->action(h);
 293                 trace_softirq_exit(vec_nr);
 294                 if (unlikely(prev_count != preempt_count())) {
 295                         pr_err("huh, entered softirq %u %s %p with preempt_count %08x, exited with %08x?\n",
 296                                vec_nr, softirq_to_name[vec_nr], h->action,
 297                                prev_count, preempt_count());
 298                         preempt_count_set(prev_count);
 299                 }
 300                 h++;
 301                 pending >>= softirq_bit;
 302         }
 303
 304         if (__this_cpu_read(ksoftirqd) == current)
 305                 rcu_softirq_qs();
 306         local_irq_disable();
 307
 308         pending = local_softirq_pending();
 309         if (pending) {
 310                 if (time_before(jiffies, end) && !need_resched() &&
 311                     --max_restart)
 312                         goto restart;
 313
 314                 wakeup_softirqd();
 315         }
 316
 317         lockdep_softirq_end(in_hardirq);
 318         account_irq_exit_time(current);
 319         __local_bh_enable(SOFTIRQ_OFFSET);
 320         WARN_ON_ONCE(in_interrupt());
 321         current_restore_flags(old_flags, PF_MEMALLOC);
 322 }
 323
 324 asmlinkage __visible void do_softirq(void)
 325 {
 326         __u32 pending;
 327         unsigned long flags;
 328
 329         if (in_interrupt())
 330                 return;
 331
 332         local_irq_save(flags);
 333
 334         pending = local_softirq_pending();
 335
 336         if (pending && !ksoftirqd_running(pending))
 337                 do_softirq_own_stack();
 338
 339         local_irq_restore(flags);
 340 }
 341
 342 /*
 343  * Enter an interrupt context.
 344  */
 345 void irq_enter(void)
 346 {
 347         rcu_irq_enter();
 348         if (is_idle_task(current) && !in_interrupt()) {
 349                 /*
 350                  * Prevent raise_softirq from needlessly waking up ksoftirqd
 351                  * here, as softirq will be serviced on return from interrupt.
 352                  */
 353                 local_bh_disable();
 354                 tick_irq_enter();
 355                 _local_bh_enable();
 356         }
 357
 358         __irq_enter();
 359 }
 360
 361 static inline void invoke_softirq(void)
 362 {
 363         if (ksoftirqd_running(local_softirq_pending()))
 364                 return;
 365
 366         if (!force_irqthreads) {
 367 #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
 368                 /*
 369                  * We can safely execute softirq on the current stack if
 370                  * it is the irq stack, because it should be near empty
 371                  * at this stage.
 372                  */
 373                 __do_softirq();
 374 #else
 375                 /*
 376                  * Otherwise, irq_exit() is called on the task stack that can
 377                  * be potentially deep already. So call softirq in its own stack
 378                  * to prevent from any overrun.
 379                  */
 380                 do_softirq_own_stack();
 381 #endif
 382         } else {
 383                 wakeup_softirqd();
 384         }
 385 }
 386
 387 static inline void tick_irq_exit(void)
 388 {
 389 #ifdef CONFIG_NO_HZ_COMMON
 390         int cpu = smp_processor_id();
 391
 392         /* Make sure that timer wheel updates are propagated */
 393         if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) {
 394                 if (!in_irq())
 395                         tick_nohz_irq_exit();
 396         }
 397 #endif
 398 }
 399
 400 /*
 401  * Exit an interrupt context. Process softirqs if needed and possible:
 402  */
 403 void irq_exit(void)
 404 {
 405 #ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED
 406         local_irq_disable();
 407 #else
 408         lockdep_assert_irqs_disabled();
 409 #endif
 410         account_irq_exit_time(current);
 411         preempt_count_sub(HARDIRQ_OFFSET);
 412         if (!in_interrupt() && local_softirq_pending())
 413                 invoke_softirq();
 414
 415         tick_irq_exit();
 416         rcu_irq_exit();
 417         trace_hardirq_exit(); /* must be last! */
 418 }
 419
 420 /*
 421  * This function must run with irqs disabled!
 422  */
 423 inline void raise_softirq_irqoff(unsigned int nr)
 424 {
 425         __raise_softirq_irqoff(nr);
 426
 427         /*
 428          * If we're in an interrupt or softirq, we're done
 429          * (this also catches softirq-disabled code). We will
 430          * actually run the softirq once we return from
 431          * the irq or softirq.
 432          *
 433          * Otherwise we wake up ksoftirqd to make sure we
 434          * schedule the softirq soon.
 435          */
 436         if (!in_interrupt())
 437                 wakeup_softirqd();
 438 }
 439
 440 void raise_softirq(unsigned int nr)
 441 {
 442         unsigned long flags;
 443
 444         local_irq_save(flags);
 445         raise_softirq_irqoff(nr);
 446         local_irq_restore(flags);
 447 }
 448
 449 void __raise_softirq_irqoff(unsigned int nr)
 450 {
 451         trace_softirq_raise(nr);
 452         or_softirq_pending(1UL << nr);
 453 }
 454
 455 void open_softirq(int nr, void (*action)(struct softirq_action *))
 456 {
 457         softirq_vec[nr].action = action;
 458 }
 459
 460 /*
 461  * Tasklets
 462  */
 463 struct tasklet_head {
 464         struct tasklet_struct *head;
 465         struct tasklet_struct **tail;
 466 };
 467
 468 static DEFINE_PER_CPU(struct tasklet_head, tasklet_vec);
 469 static DEFINE_PER_CPU(struct tasklet_head, tasklet_hi_vec);
 470
 471 static void __tasklet_schedule_common(struct tasklet_struct *t,
 472                                       struct tasklet_head __percpu *headp,
 473                                       unsigned int softirq_nr)
 474 {
 475         struct tasklet_head *head;
 476         unsigned long flags;
 477
 478         local_irq_save(flags);
 479         head = this_cpu_ptr(headp);
 480         t->next = NULL;
 481         *head->tail = t;
 482         head->tail = &(t->next);
 483         raise_softirq_irqoff(softirq_nr);
 484         local_irq_restore(flags);
 485 }
 486
 487 void __tasklet_schedule(struct tasklet_struct *t)
 488 {
 489         __tasklet_schedule_common(t, &tasklet_vec,
 490                                   TASKLET_SOFTIRQ);
 491 }
 492 EXPORT_SYMBOL(__tasklet_schedule);
 493
 494 void __tasklet_hi_schedule(struct tasklet_struct *t)
 495 {
 496         __tasklet_schedule_common(t, &tasklet_hi_vec,
 497                                   HI_SOFTIRQ);
 498 }
 499 EXPORT_SYMBOL(__tasklet_hi_schedule);
 500
 501 static void tasklet_action_common(struct softirq_action *a,
 502                                   struct tasklet_head *tl_head,
 503                                   unsigned int softirq_nr)
 504 {
 505         struct tasklet_struct *list;
 506
 507         local_irq_disable();
 508         list = tl_head->head;
 509         tl_head->head = NULL;
 510         tl_head->tail = &tl_head->head;
 511         local_irq_enable();
 512
 513         while (list) {
 514                 struct tasklet_struct *t = list;
 515
 516                 list = list->next;
 517
 518                 if (tasklet_trylock(t)) {
 519                         if (!atomic_read(&t->count)) {
 520                                 if (!test_and_clear_bit(TASKLET_STATE_SCHED,
 521                                                         &t->state))
 522                                         BUG();
 523                                 t->func(t->data);
 524                                 tasklet_unlock(t);
 525                                 continue;
 526                         }
 527                         tasklet_unlock(t);
 528                 }
 529
 530                 local_irq_disable();
 531                 t->next = NULL;
 532                 *tl_head->tail = t;
 533                 tl_head->tail = &t->next;
 534                 __raise_softirq_irqoff(softirq_nr);
 535                 local_irq_enable();
 536         }
 537 }
 538
 539 static __latent_entropy void tasklet_action(struct softirq_action *a)
 540 {
 541         tasklet_action_common(a, this_cpu_ptr(&tasklet_vec), TASKLET_SOFTIRQ);
 542 }
 543
 544 static __latent_entropy void tasklet_hi_action(struct softirq_action *a)
 545 {
 546         tasklet_action_common(a, this_cpu_ptr(&tasklet_hi_vec), HI_SOFTIRQ);
 547 }
 548
 549 void tasklet_init(struct tasklet_struct *t,
 550                   void (*func)(unsigned long), unsigned long data)
 551 {
 552         t->next = NULL;
 553         t->state = 0;
 554         atomic_set(&t->count, 0);
 555         t->func = func;
 556         t->data = data;
 557 }
 558 EXPORT_SYMBOL(tasklet_init);
 559
 560 void tasklet_kill(struct tasklet_struct *t)
 561 {
 562         if (in_interrupt())
 563                 pr_notice("Attempt to kill tasklet from interrupt\n");
 564
 565         while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
 566                 do {
 567                         yield();
 568                 } while (test_bit(TASKLET_STATE_SCHED, &t->state));
 569         }
 570         tasklet_unlock_wait(t);
 571         clear_bit(TASKLET_STATE_SCHED, &t->state);
 572 }
 573 EXPORT_SYMBOL(tasklet_kill);
 574
 575 /*
 576  * tasklet_hrtimer
 577  */
 578
 579 /*
 580  * The trampoline is called when the hrtimer expires. It schedules a tasklet
 581  * to run __tasklet_hrtimer_trampoline() which in turn will call the intended
 582  * hrtimer callback, but from softirq context.
 583  */
 584 static enum hrtimer_restart __hrtimer_tasklet_trampoline(struct hrtimer *timer)
 585 {
 586         struct tasklet_hrtimer *ttimer =
 587                 container_of(timer, struct tasklet_hrtimer, timer);
 588
 589         tasklet_hi_schedule(&ttimer->tasklet);
 590         return HRTIMER_NORESTART;
 591 }
 592
 593 /*
 594  * Helper function which calls the hrtimer callback from
 595  * tasklet/softirq context
 596  */
 597 static void __tasklet_hrtimer_trampoline(unsigned long data)
 598 {
 599         struct tasklet_hrtimer *ttimer = (void *)data;
 600         enum hrtimer_restart restart;
 601
 602         restart = ttimer->function(&ttimer->timer);
 603         if (restart != HRTIMER_NORESTART)
 604                 hrtimer_restart(&ttimer->timer);
 605 }
 606
 607 /**
 608  * tasklet_hrtimer_init - Init a tasklet/hrtimer combo for softirq callbacks
 609  * @ttimer:      tasklet_hrtimer which is initialized
 610  * @function:    hrtimer callback function which gets called from softirq context
 611  * @which_clock: clock id (CLOCK_MONOTONIC/CLOCK_REALTIME)
 612  * @mode:        hrtimer mode (HRTIMER_MODE_ABS/HRTIMER_MODE_REL)
 613  */
 614 void tasklet_hrtimer_init(struct tasklet_hrtimer *ttimer,
 615                           enum hrtimer_restart (*function)(struct hrtimer *),
 616                           clockid_t which_clock, enum hrtimer_mode mode)
 617 {
 618         hrtimer_init(&ttimer->timer, which_clock, mode);
 619         ttimer->timer.function = __hrtimer_tasklet_trampoline;
 620         tasklet_init(&ttimer->tasklet, __tasklet_hrtimer_trampoline,
 621                      (unsigned long)ttimer);
 622         ttimer->function = function;
 623 }
 624 EXPORT_SYMBOL_GPL(tasklet_hrtimer_init);
 625
 626 void __init softirq_init(void)
 627 {
 628         int cpu;
 629
 630         for_each_possible_cpu(cpu) {
 631                 per_cpu(tasklet_vec, cpu).tail =
 632                         &per_cpu(tasklet_vec, cpu).head;
 633                 per_cpu(tasklet_hi_vec, cpu).tail =
 634                         &per_cpu(tasklet_hi_vec, cpu).head;
 635         }
 636
 637         open_softirq(TASKLET_SOFTIRQ, tasklet_action);
 638         open_softirq(HI_SOFTIRQ, tasklet_hi_action);
 639 }
 640
 641 static int ksoftirqd_should_run(unsigned int cpu)
 642 {
 643         return local_softirq_pending();
 644 }
 645
 646 static void run_ksoftirqd(unsigned int cpu)
 647 {
 648         local_irq_disable();
 649         if (local_softirq_pending()) {
 650                 /*
 651                  * We can safely run softirq on inline stack, as we are not deep
 652                  * in the task stack here.
 653                  */
 654                 __do_softirq();
 655                 local_irq_enable();
 656                 cond_resched();
 657                 return;
 658         }
 659         local_irq_enable();
 660 }
 661
 662 #ifdef CONFIG_HOTPLUG_CPU
 663 /*
 664  * tasklet_kill_immediate is called to remove a tasklet which can already be
 665  * scheduled for execution on @cpu.
 666  *
 667  * Unlike tasklet_kill, this function removes the tasklet
 668  * _immediately_, even if the tasklet is in TASKLET_STATE_SCHED state.
 669  *
 670  * When this function is called, @cpu must be in the CPU_DEAD state.
 671  */
 672 void tasklet_kill_immediate(struct tasklet_struct *t, unsigned int cpu)
 673 {
 674         struct tasklet_struct **i;
 675
 676         BUG_ON(cpu_online(cpu));
 677         BUG_ON(test_bit(TASKLET_STATE_RUN, &t->state));
 678
 679         if (!test_bit(TASKLET_STATE_SCHED, &t->state))
 680                 return;
 681
 682         /* CPU is dead, so no lock needed. */
 683         for (i = &per_cpu(tasklet_vec, cpu).head; *i; i = &(*i)->next) {
 684                 if (*i == t) {
 685                         *i = t->next;
 686                         /* If this was the tail element, move the tail ptr */
 687                         if (*i == NULL)
 688                                 per_cpu(tasklet_vec, cpu).tail = i;
 689                         return;
 690                 }
 691         }
 692         BUG();
 693 }
 694
 695 static int takeover_tasklets(unsigned int cpu)
 696 {
 697         /* CPU is dead, so no lock needed. */
 698         local_irq_disable();
 699
 700         /* Find end, append list for that CPU. */
 701         if (&per_cpu(tasklet_vec, cpu).head != per_cpu(tasklet_vec, cpu).tail) {
 702                 *__this_cpu_read(tasklet_vec.tail) = per_cpu(tasklet_vec, cpu).head;
 703                 this_cpu_write(tasklet_vec.tail, per_cpu(tasklet_vec, cpu).tail);
 704                 per_cpu(tasklet_vec, cpu).head = NULL;
 705                 per_cpu(tasklet_vec, cpu).tail = &per_cpu(tasklet_vec, cpu).head;
 706         }
 707         raise_softirq_irqoff(TASKLET_SOFTIRQ);
 708
 709         if (&per_cpu(tasklet_hi_vec, cpu).head != per_cpu(tasklet_hi_vec, cpu).tail) {
 710                 *__this_cpu_read(tasklet_hi_vec.tail) = per_cpu(tasklet_hi_vec, cpu).head;
 711                 __this_cpu_write(tasklet_hi_vec.tail, per_cpu(tasklet_hi_vec, cpu).tail);
 712                 per_cpu(tasklet_hi_vec, cpu).head = NULL;
 713                 per_cpu(tasklet_hi_vec, cpu).tail = &per_cpu(tasklet_hi_vec, cpu).head;
 714         }
 715         raise_softirq_irqoff(HI_SOFTIRQ);
 716
 717         local_irq_enable();
 718         return 0;
 719 }
 720 #else
 721 #define takeover_tasklets       NULL
 722 #endif /* CONFIG_HOTPLUG_CPU */
 723
 724 static struct smp_hotplug_thread softirq_threads = {
 725         .store                  = &ksoftirqd,
 726         .thread_should_run      = ksoftirqd_should_run,
 727         .thread_fn              = run_ksoftirqd,
 728         .thread_comm            = "ksoftirqd/%u",
 729 };
 730
 731 static __init int spawn_ksoftirqd(void)
 732 {
 733         cpuhp_setup_state_nocalls(CPUHP_SOFTIRQ_DEAD, "softirq:dead", NULL,
 734                                   takeover_tasklets);
 735         BUG_ON(smpboot_register_percpu_thread(&softirq_threads));
 736
 737         return 0;
 738 }
 739 early_initcall(spawn_ksoftirqd);
 740
 741 /*
 742  * [ These __weak aliases are kept in a separate compilation unit, so that
 743  *   GCC does not inline them incorrectly. ]
 744  */
 745
 746 int __init __weak early_irq_init(void)
 747 {
 748         return 0;
 749 }
 750
 751 int __init __weak arch_probe_nr_irqs(void)
 752 {
 753         return NR_IRQS_LEGACY;
 754 }
 755
 756 int __init __weak arch_early_irq_init(void)
 757 {
 758         return 0;
 759 }
 760
 761 unsigned int __weak arch_dynirq_lower_bound(unsigned int from)
 762 {
 763         return from;
 764 }