kernel/softirq.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  *      linux/kernel/softirq.c
   4  *
   5  *      Copyright (C) 1992 Linus Torvalds
   6  *
   7  *      Rewritten. Old one was good in 2.2, but in 2.3 it was immoral. --ANK (990903)
   8  */
   9
  10 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  11
  12 #include <linux/export.h>
  13 #include <linux/kernel_stat.h>
  14 #include <linux/interrupt.h>
  15 #include <linux/init.h>
  16 #include <linux/mm.h>
  17 #include <linux/notifier.h>
  18 #include <linux/percpu.h>
  19 #include <linux/cpu.h>
  20 #include <linux/freezer.h>
  21 #include <linux/kthread.h>
  22 #include <linux/rcupdate.h>
  23 #include <linux/ftrace.h>
  24 #include <linux/smp.h>
  25 #include <linux/smpboot.h>
  26 #include <linux/tick.h>
  27 #include <linux/irq.h>
  28
  29 #define CREATE_TRACE_POINTS
  30 #include <trace/events/irq.h>
  31
  32 /*
  33    - No shared variables, all the data are CPU local.
  34    - If a softirq needs serialization, let it serialize itself
  35      by its own spinlocks.
  36    - Even if softirq is serialized, only local cpu is marked for
  37      execution. Hence, we get something sort of weak cpu binding.
  38      Though it is still not clear, will it result in better locality
  39      or will not.
  40
  41    Examples:
  42    - NET RX softirq. It is multithreaded and does not require
  43      any global serialization.
  44    - NET TX softirq. It kicks software netdevice queues, hence
  45      it is logically serialized per device, but this serialization
  46      is invisible to common code.
  47    - Tasklets: serialized wrt itself.
  48  */
  49
  50 #ifndef __ARCH_IRQ_STAT
  51 DEFINE_PER_CPU_ALIGNED(irq_cpustat_t, irq_stat);
  52 EXPORT_PER_CPU_SYMBOL(irq_stat);
  53 #endif
  54
  55 static struct softirq_action softirq_vec[NR_SOFTIRQS] __cacheline_aligned_in_smp;
  56
  57 DEFINE_PER_CPU(struct task_struct *, ksoftirqd);
  58
  59 const char * const softirq_to_name[NR_SOFTIRQS] = {
  60         "HI", "TIMER", "NET_TX", "NET_RX", "BLOCK", "IRQ_POLL",
  61         "TASKLET", "SCHED", "HRTIMER", "RCU"
  62 };
  63
  64 /*
  65  * we cannot loop indefinitely here to avoid userspace starvation,
  66  * but we also don't want to introduce a worst case 1/HZ latency
  67  * to the pending events, so lets the scheduler to balance
  68  * the softirq load for us.
  69  */
  70 static void wakeup_softirqd(void)
  71 {
  72         /* Interrupts are disabled: no need to stop preemption */
  73         struct task_struct *tsk = __this_cpu_read(ksoftirqd);
  74
  75         if (tsk && tsk->state != TASK_RUNNING)
  76                 wake_up_process(tsk);
  77 }
  78
  79 /*
  80  * If ksoftirqd is scheduled, we do not want to process pending softirqs
  81  * right now. Let ksoftirqd handle this at its own rate, to get fairness,
  82  * unless we're doing some of the synchronous softirqs.
  83  */
  84 #define SOFTIRQ_NOW_MASK ((1 << HI_SOFTIRQ) | (1 << TASKLET_SOFTIRQ))
  85 static bool ksoftirqd_running(unsigned long pending)
  86 {
  87         struct task_struct *tsk = __this_cpu_read(ksoftirqd);
  88
  89         if (pending & SOFTIRQ_NOW_MASK)
  90                 return false;
  91         return tsk && (tsk->state == TASK_RUNNING) &&
  92                 !__kthread_should_park(tsk);
  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                 lockdep_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                 lockdep_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                 lockdep_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 (lockdep_hardirq_context(current)) {
 228                 in_hardirq = true;
 229                 lockdep_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                 lockdep_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  * irq_enter_rcu - Enter an interrupt context with RCU watching
 344  */
 345 void irq_enter_rcu(void)
 346 {
 347         if (is_idle_task(current) && !in_interrupt()) {
 348                 /*
 349                  * Prevent raise_softirq from needlessly waking up ksoftirqd
 350                  * here, as softirq will be serviced on return from interrupt.
 351                  */
 352                 local_bh_disable();
 353                 tick_irq_enter();
 354                 _local_bh_enable();
 355         }
 356         __irq_enter();
 357 }
 358
 359 /**
 360  * irq_enter - Enter an interrupt context including RCU update
 361  */
 362 void irq_enter(void)
 363 {
 364         rcu_irq_enter();
 365         irq_enter_rcu();
 366 }
 367
 368 static inline void invoke_softirq(void)
 369 {
 370         if (ksoftirqd_running(local_softirq_pending()))
 371                 return;
 372
 373         if (!force_irqthreads) {
 374 #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
 375                 /*
 376                  * We can safely execute softirq on the current stack if
 377                  * it is the irq stack, because it should be near empty
 378                  * at this stage.
 379                  */
 380                 __do_softirq();
 381 #else
 382                 /*
 383                  * Otherwise, irq_exit() is called on the task stack that can
 384                  * be potentially deep already. So call softirq in its own stack
 385                  * to prevent from any overrun.
 386                  */
 387                 do_softirq_own_stack();
 388 #endif
 389         } else {
 390                 wakeup_softirqd();
 391         }
 392 }
 393
 394 static inline void tick_irq_exit(void)
 395 {
 396 #ifdef CONFIG_NO_HZ_COMMON
 397         int cpu = smp_processor_id();
 398
 399         /* Make sure that timer wheel updates are propagated */
 400         if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) {
 401                 if (!in_irq())
 402                         tick_nohz_irq_exit();
 403         }
 404 #endif
 405 }
 406
 407 static inline void __irq_exit_rcu(void)
 408 {
 409 #ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED
 410         local_irq_disable();
 411 #else
 412         lockdep_assert_irqs_disabled();
 413 #endif
 414         account_irq_exit_time(current);
 415         preempt_count_sub(HARDIRQ_OFFSET);
 416         if (!in_interrupt() && local_softirq_pending())
 417                 invoke_softirq();
 418
 419         tick_irq_exit();
 420 }
 421
 422 /**
 423  * irq_exit_rcu() - Exit an interrupt context without updating RCU
 424  *
 425  * Also processes softirqs if needed and possible.
 426  */
 427 void irq_exit_rcu(void)
 428 {
 429         __irq_exit_rcu();
 430          /* must be last! */
 431         lockdep_hardirq_exit();
 432 }
 433
 434 /**
 435  * irq_exit - Exit an interrupt context, update RCU and lockdep
 436  *
 437  * Also processes softirqs if needed and possible.
 438  */
 439 void irq_exit(void)
 440 {
 441         __irq_exit_rcu();
 442         rcu_irq_exit();
 443          /* must be last! */
 444         lockdep_hardirq_exit();
 445 }
 446
 447 /*
 448  * This function must run with irqs disabled!
 449  */
 450 inline void raise_softirq_irqoff(unsigned int nr)
 451 {
 452         __raise_softirq_irqoff(nr);
 453
 454         /*
 455          * If we're in an interrupt or softirq, we're done
 456          * (this also catches softirq-disabled code). We will
 457          * actually run the softirq once we return from
 458          * the irq or softirq.
 459          *
 460          * Otherwise we wake up ksoftirqd to make sure we
 461          * schedule the softirq soon.
 462          */
 463         if (!in_interrupt())
 464                 wakeup_softirqd();
 465 }
 466
 467 void raise_softirq(unsigned int nr)
 468 {
 469         unsigned long flags;
 470
 471         local_irq_save(flags);
 472         raise_softirq_irqoff(nr);
 473         local_irq_restore(flags);
 474 }
 475
 476 void __raise_softirq_irqoff(unsigned int nr)
 477 {
 478         trace_softirq_raise(nr);
 479         or_softirq_pending(1UL << nr);
 480 }
 481
 482 void open_softirq(int nr, void (*action)(struct softirq_action *))
 483 {
 484         softirq_vec[nr].action = action;
 485 }
 486
 487 /*
 488  * Tasklets
 489  */
 490 struct tasklet_head {
 491         struct tasklet_struct *head;
 492         struct tasklet_struct **tail;
 493 };
 494
 495 static DEFINE_PER_CPU(struct tasklet_head, tasklet_vec);
 496 static DEFINE_PER_CPU(struct tasklet_head, tasklet_hi_vec);
 497
 498 static void __tasklet_schedule_common(struct tasklet_struct *t,
 499                                       struct tasklet_head __percpu *headp,
 500                                       unsigned int softirq_nr)
 501 {
 502         struct tasklet_head *head;
 503         unsigned long flags;
 504
 505         local_irq_save(flags);
 506         head = this_cpu_ptr(headp);
 507         t->next = NULL;
 508         *head->tail = t;
 509         head->tail = &(t->next);
 510         raise_softirq_irqoff(softirq_nr);
 511         local_irq_restore(flags);
 512 }
 513
 514 void __tasklet_schedule(struct tasklet_struct *t)
 515 {
 516         __tasklet_schedule_common(t, &tasklet_vec,
 517                                   TASKLET_SOFTIRQ);
 518 }
 519 EXPORT_SYMBOL(__tasklet_schedule);
 520
 521 void __tasklet_hi_schedule(struct tasklet_struct *t)
 522 {
 523         __tasklet_schedule_common(t, &tasklet_hi_vec,
 524                                   HI_SOFTIRQ);
 525 }
 526 EXPORT_SYMBOL(__tasklet_hi_schedule);
 527
 528 static void tasklet_action_common(struct softirq_action *a,
 529                                   struct tasklet_head *tl_head,
 530                                   unsigned int softirq_nr)
 531 {
 532         struct tasklet_struct *list;
 533
 534         local_irq_disable();
 535         list = tl_head->head;
 536         tl_head->head = NULL;
 537         tl_head->tail = &tl_head->head;
 538         local_irq_enable();
 539
 540         while (list) {
 541                 struct tasklet_struct *t = list;
 542
 543                 list = list->next;
 544
 545                 if (tasklet_trylock(t)) {
 546                         if (!atomic_read(&t->count)) {
 547                                 if (!test_and_clear_bit(TASKLET_STATE_SCHED,
 548                                                         &t->state))
 549                                         BUG();
 550                                 t->func(t->data);
 551                                 tasklet_unlock(t);
 552                                 continue;
 553                         }
 554                         tasklet_unlock(t);
 555                 }
 556
 557                 local_irq_disable();
 558                 t->next = NULL;
 559                 *tl_head->tail = t;
 560                 tl_head->tail = &t->next;
 561                 __raise_softirq_irqoff(softirq_nr);
 562                 local_irq_enable();
 563         }
 564 }
 565
 566 static __latent_entropy void tasklet_action(struct softirq_action *a)
 567 {
 568         tasklet_action_common(a, this_cpu_ptr(&tasklet_vec), TASKLET_SOFTIRQ);
 569 }
 570
 571 static __latent_entropy void tasklet_hi_action(struct softirq_action *a)
 572 {
 573         tasklet_action_common(a, this_cpu_ptr(&tasklet_hi_vec), HI_SOFTIRQ);
 574 }
 575
 576 void tasklet_init(struct tasklet_struct *t,
 577                   void (*func)(unsigned long), unsigned long data)
 578 {
 579         t->next = NULL;
 580         t->state = 0;
 581         atomic_set(&t->count, 0);
 582         t->func = func;
 583         t->data = data;
 584 }
 585 EXPORT_SYMBOL(tasklet_init);
 586
 587 void tasklet_kill(struct tasklet_struct *t)
 588 {
 589         if (in_interrupt())
 590                 pr_notice("Attempt to kill tasklet from interrupt\n");
 591
 592         while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
 593                 do {
 594                         yield();
 595                 } while (test_bit(TASKLET_STATE_SCHED, &t->state));
 596         }
 597         tasklet_unlock_wait(t);
 598         clear_bit(TASKLET_STATE_SCHED, &t->state);
 599 }
 600 EXPORT_SYMBOL(tasklet_kill);
 601
 602 void __init softirq_init(void)
 603 {
 604         int cpu;
 605
 606         for_each_possible_cpu(cpu) {
 607                 per_cpu(tasklet_vec, cpu).tail =
 608                         &per_cpu(tasklet_vec, cpu).head;
 609                 per_cpu(tasklet_hi_vec, cpu).tail =
 610                         &per_cpu(tasklet_hi_vec, cpu).head;
 611         }
 612
 613         open_softirq(TASKLET_SOFTIRQ, tasklet_action);
 614         open_softirq(HI_SOFTIRQ, tasklet_hi_action);
 615 }
 616
 617 static int ksoftirqd_should_run(unsigned int cpu)
 618 {
 619         return local_softirq_pending();
 620 }
 621
 622 static void run_ksoftirqd(unsigned int cpu)
 623 {
 624         local_irq_disable();
 625         if (local_softirq_pending()) {
 626                 /*
 627                  * We can safely run softirq on inline stack, as we are not deep
 628                  * in the task stack here.
 629                  */
 630                 __do_softirq();
 631                 local_irq_enable();
 632                 cond_resched();
 633                 return;
 634         }
 635         local_irq_enable();
 636 }
 637
 638 #ifdef CONFIG_HOTPLUG_CPU
 639 /*
 640  * tasklet_kill_immediate is called to remove a tasklet which can already be
 641  * scheduled for execution on @cpu.
 642  *
 643  * Unlike tasklet_kill, this function removes the tasklet
 644  * _immediately_, even if the tasklet is in TASKLET_STATE_SCHED state.
 645  *
 646  * When this function is called, @cpu must be in the CPU_DEAD state.
 647  */
 648 void tasklet_kill_immediate(struct tasklet_struct *t, unsigned int cpu)
 649 {
 650         struct tasklet_struct **i;
 651
 652         BUG_ON(cpu_online(cpu));
 653         BUG_ON(test_bit(TASKLET_STATE_RUN, &t->state));
 654
 655         if (!test_bit(TASKLET_STATE_SCHED, &t->state))
 656                 return;
 657
 658         /* CPU is dead, so no lock needed. */
 659         for (i = &per_cpu(tasklet_vec, cpu).head; *i; i = &(*i)->next) {
 660                 if (*i == t) {
 661                         *i = t->next;
 662                         /* If this was the tail element, move the tail ptr */
 663                         if (*i == NULL)
 664                                 per_cpu(tasklet_vec, cpu).tail = i;
 665                         return;
 666                 }
 667         }
 668         BUG();
 669 }
 670
 671 static int takeover_tasklets(unsigned int cpu)
 672 {
 673         /* CPU is dead, so no lock needed. */
 674         local_irq_disable();
 675
 676         /* Find end, append list for that CPU. */
 677         if (&per_cpu(tasklet_vec, cpu).head != per_cpu(tasklet_vec, cpu).tail) {
 678                 *__this_cpu_read(tasklet_vec.tail) = per_cpu(tasklet_vec, cpu).head;
 679                 __this_cpu_write(tasklet_vec.tail, per_cpu(tasklet_vec, cpu).tail);
 680                 per_cpu(tasklet_vec, cpu).head = NULL;
 681                 per_cpu(tasklet_vec, cpu).tail = &per_cpu(tasklet_vec, cpu).head;
 682         }
 683         raise_softirq_irqoff(TASKLET_SOFTIRQ);
 684
 685         if (&per_cpu(tasklet_hi_vec, cpu).head != per_cpu(tasklet_hi_vec, cpu).tail) {
 686                 *__this_cpu_read(tasklet_hi_vec.tail) = per_cpu(tasklet_hi_vec, cpu).head;
 687                 __this_cpu_write(tasklet_hi_vec.tail, per_cpu(tasklet_hi_vec, cpu).tail);
 688                 per_cpu(tasklet_hi_vec, cpu).head = NULL;
 689                 per_cpu(tasklet_hi_vec, cpu).tail = &per_cpu(tasklet_hi_vec, cpu).head;
 690         }
 691         raise_softirq_irqoff(HI_SOFTIRQ);
 692
 693         local_irq_enable();
 694         return 0;
 695 }
 696 #else
 697 #define takeover_tasklets       NULL
 698 #endif /* CONFIG_HOTPLUG_CPU */
 699
 700 static struct smp_hotplug_thread softirq_threads = {
 701         .store                  = &ksoftirqd,
 702         .thread_should_run      = ksoftirqd_should_run,
 703         .thread_fn              = run_ksoftirqd,
 704         .thread_comm            = "ksoftirqd/%u",
 705 };
 706
 707 static __init int spawn_ksoftirqd(void)
 708 {
 709         cpuhp_setup_state_nocalls(CPUHP_SOFTIRQ_DEAD, "softirq:dead", NULL,
 710                                   takeover_tasklets);
 711         BUG_ON(smpboot_register_percpu_thread(&softirq_threads));
 712
 713         return 0;
 714 }
 715 early_initcall(spawn_ksoftirqd);
 716
 717 /*
 718  * [ These __weak aliases are kept in a separate compilation unit, so that
 719  *   GCC does not inline them incorrectly. ]
 720  */
 721
 722 int __init __weak early_irq_init(void)
 723 {
 724         return 0;
 725 }
 726
 727 int __init __weak arch_probe_nr_irqs(void)
 728 {
 729         return NR_IRQS_LEGACY;
 730 }
 731
 732 int __init __weak arch_early_irq_init(void)
 733 {
 734         return 0;
 735 }
 736
 737 unsigned int __weak arch_dynirq_lower_bound(unsigned int from)
 738 {
 739         return from;
 740 }