1.在Linux中轻量级进程就是线程,轻量级进程之间可以共享资源。
进程描述符:task_struct 存放一个进程相关的所有信息。可以说一个进程描述符就代表一个进程。
进程的七种状态:可运行状态、可中断等待状态、不可中断等待状态、暂停状态、跟踪状态、僵死状态、僵死撤销状态
进程标识符(PID)
进程描述符存放在动态内存中，便于内核管理。内核将线程描述符（thread_info）和内核态的进程堆栈一起存放于8K字节的两个页框中。
线程描述符和进程描述符相互关联，即分别存在于对方的字段中。
current宏标识运行在当前CPU上的进程。
进程链表：双向链表，将所有进程的描述符链接起来。进程链表的头是init_task描述符，即所谓的0进程。
Linux为每种进程优先权都建立一个对应的链表。prio_array_t 数据结构存放这些链表。
进程间的关系:
进程描述符中表示进程亲属关系的字段描述:
real_parent、parent、children、sibling
内核为加速从PID导出对应的进程描述符指针引入了4个散列表:PID,TGID,PGID,SID
Linux利用链表来处理冲突的PID:每一个表项由冲突的进程描述符组成的双向链表。
等待队列:当进程需要等待某个事件的发生而需要睡眠时，则当前进程就把自己放进合适的等待队列，等待内核唤醒他们。
两种睡眠进程:互斥进程和非互斥进程
进程资源的限制
2.进程切换
硬件上下文切换，任务状态段的切换
执行进程的切换: http://home.ustc.edu.cn/~hchunhui/linux_sched.html#sec10
保存和加载FPU、MMX、XMM寄存器的值
3.创建进程
创建进程时，子进程共享父进程的大多数资源。
通过写时复制技术允许父子进程读相同的物理页，只有当子进程需要实际写物理页时才给子进程分配新的物理页。
轻量级进程允许父子进程共享在内核的很多数据结构，如页表、打开文件表、信号处理等。
vfork()系统调用创建的子进程共享其父进程的内存地址空间，并且阻塞父进程的执行，直到子进程退出或子进程分离父进程为止。
clone()、fork()、vfork()系统调用
clone():创建轻量级进程。
参数: fn -----子进程的执行函数。该函数返回时子进程终止。函数的返回值代表子进程的退出代码。
arg -----指向传递给fn()函数的参数
flags -----低字节指定子进程结束时发送到父进程的信号代码。
child-stack -----表示把用户态堆栈指针赋给子进程的esp寄存器。调用进程应该为子进程分配新的堆栈
tls -----表示线程局部存储段数据结构的地址
ptid -----表示父进程的用户态变量地址，父进程具有与子进程相同的PID
ctid -----表示子进程的用户态变量地址，该进程具有这一类进程的PID
根据下列Linux源码可以看出clone()、fork()、vfork()、kernel_thread()、do_fork() 函数都是由_do_fork()函数实现的，区别仅在于传递的参数不同。
1 SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp, 2 int __user *, parent_tidptr, 3 int __user *, child_tidptr, 4 unsigned long, tls) 5 6 { 7 return _do_fork(clone_flags, newsp, 0, parent_tidptr, child_tidptr, tls); 8 } 9 /* 10 * Create a kernel thread. 11 */ 12 pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags) 13 { 14 return _do_fork(flags|CLONE_VM|CLONE_UNTRACED, (unsigned long)fn, 15 (unsigned long)arg, NULL, NULL, 0); 16 } 17 18 #ifdef __ARCH_WANT_SYS_FORK 19 SYSCALL_DEFINE0(fork) 20 { 21 #ifdef CONFIG_MMU 22 return _do_fork(SIGCHLD, 0, 0, NULL, NULL, 0); 23 #else 24 /* can not support in nommu mode */ 25 return -EINVAL; 26 #endif 27 } 28 #endif 29 30 #ifdef __ARCH_WANT_SYS_VFORK 31 SYSCALL_DEFINE0(vfork) 32 { 33 return _do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, 0, 34 0, NULL, NULL, 0); 35 } 36 #endif 37 38 39 long do_fork(unsigned long clone_flags, 40 unsigned long stack_start, 41 unsigned long stack_size, 42 int __user *parent_tidptr, 43 int __user *child_tidptr) 44 { 45 return _do_fork(clone_flags, stack_start, stack_size, 46 parent_tidptr, child_tidptr, 0); 47 } 48 49 50 long _do_fork(unsigned long clone_flags, 51 unsigned long stack_start, 52 unsigned long stack_size, 53 int __user *parent_tidptr, 54 int __user *child_tidptr, 55 unsigned long tls) 56 { 57 struct completion vfork; 58 struct pid *pid; 59 struct task_struct *p; 60 int trace = 0; 61 long nr; 62 63 /* 64 * Determine whether and which event to report to ptracer. When 65 * called from kernel_thread or CLONE_UNTRACED is explicitly 66 * requested, no event is reported; otherwise, report if the event 67 * for the type of forking is enabled. 68 */ 69 if (!(clone_flags & CLONE_UNTRACED)) { 70 if (clone_flags & CLONE_VFORK) 71 trace = PTRACE_EVENT_VFORK; 72 else if ((clone_flags & CSIGNAL) != SIGCHLD) 73 trace = PTRACE_EVENT_CLONE; 74 else 75 trace = PTRACE_EVENT_FORK; 76 77 if (likely(!ptrace_event_enabled(current, trace))) 78 trace = 0; 79 } 80 81 p = copy_process(clone_flags, stack_start, stack_size, 82 child_tidptr, NULL, trace, tls, NUMA_NO_NODE); 83 add_latent_entropy(); 84 85 if (IS_ERR(p)) 86 return PTR_ERR(p); 87 88 /* 89 * Do this prior waking up the new thread - the thread pointer 90 * might get invalid after that point, if the thread exits quickly. 91 */ 92 trace_sched_process_fork(current, p); 93 94 pid = get_task_pid(p, PIDTYPE_PID); 95 nr = pid_vnr(pid); 96 97 if (clone_flags & CLONE_PARENT_SETTID) 98 put_user(nr, parent_tidptr); 99 100 if (clone_flags & CLONE_VFORK) { 101 p->vfork_done = &vfork; 102 init_completion(&vfork); 103 get_task_struct(p); 104 } 105 106 wake_up_new_task(p); 107 108 /* forking complete and child started to run, tell ptracer */ 109 if (unlikely(trace)) 110 ptrace_event_pid(trace, pid); 111 112 if (clone_flags & CLONE_VFORK) { 113 if (!wait_for_vfork_done(p, &vfork)) 114 ptrace_event_pid(PTRACE_EVENT_VFORK_DONE, pid); 115 } 116 117 put_pid(pid); 118 return nr; 119 }
cpoy_process()函数具体实现如下源代码所示:
1 /* 2 * This creates a new process as a copy of the old one, 3 * but does not actually start it yet. 4 * 5 * It copies the registers, and all the appropriate 6 * parts of the process environment (as per the clone 7 * flags). The actual kick-off is left to the caller. 8 */ 9 static __latent_entropy struct task_struct *copy_process( 10 unsigned long clone_flags, 11 unsigned long stack_start, 12 unsigned long stack_size, 13 int __user *child_tidptr, 14 struct pid *pid, 15 int trace, 16 unsigned long tls, 17 int node) 18 { 19 int retval; 20 struct task_struct *p; 21 22 /* 23 * Don\'t allow sharing the root directory with processes in a different 24 * namespace 25 */ 26 if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS)) 27 return ERR_PTR(-EINVAL); 28 29 if ((clone_flags & (CLONE_NEWUSER|CLONE_FS)) == (CLONE_NEWUSER|CLONE_FS)) 30 return ERR_PTR(-EINVAL); 31 32 /* 33 * Thread groups must share signals as well, and detached threads 34 * can only be started up within the thread group. 35 */ 36 if ((clone_flags & CLONE_THREAD) && !(clone_flags & CLONE_SIGHAND)) 37 return ERR_PTR(-EINVAL); 38 39 /* 40 * Shared signal handlers imply shared VM. By way of the above, 41 * thread groups also imply shared VM. Blocking this case allows 42 * for various simplifications in other code. 43 */ 44 if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM)) 45 return ERR_PTR(-EINVAL); 46 47 /* 48 * Siblings of global init remain as zombies on exit since they are 49 * not reaped by their parent (swapper). To solve this and to avoid 50 * multi-rooted process trees, prevent global and container-inits 51 * from creating siblings. 52 */ 53 if ((clone_flags & CLONE_PARENT) && 54 current->signal->flags & SIGNAL_UNKILLABLE) 55 return ERR_PTR(-EINVAL); 56 57 /* 58 * If the new process will be in a different pid or user namespace 59 * do not allow it to share a thread group with the forking task. 60 */ 61 if (clone_flags & CLONE_THREAD) { 62 if ((clone_flags & (CLONE_NEWUSER | CLONE_NEWPID)) || 63 (task_active_pid_ns(current) != 64 current->nsproxy->pid_ns_for_children)) 65 return ERR_PTR(-EINVAL); 66 } 67 68 retval = -ENOMEM; 69 p = dup_task_struct(current, node); 70 if (!p) 71 goto fork_out; 72 73 /* 74 * This _must_ happen before we call free_task(), i.e. before we jump 75 * to any of the bad_fork_* labels. This is to avoid freeing 76 * p->set_child_tid which is (ab)used as a kthread\'s data pointer for 77 * kernel threads (PF_KTHREAD). 78 */ 79 p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL; 80 /* 81 * Clear TID on mm_release()? 82 */ 83 p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr : NULL; 84 85 ftrace_graph_init_task(p); 86 87 rt_mutex_init_task(p); 88 89 #ifdef CONFIG_PROVE_LOCKING 90 DEBUG_LOCKS_WARN_ON(!p->hardirqs_enabled); 91 DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled); 92 #endif 93 retval = -EAGAIN; 94 if (atomic_read(&p->real_cred->user->processes) >= 95 task_rlimit(p, RLIMIT_NPROC)) { 96 if (p->real_cred->user != INIT_USER && 97 !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) 98 goto bad_fork_free; 99 } 100 current->flags &= ~PF_NPROC_EXCEEDED; 101 102 retval = copy_creds(p, clone_flags); 103 if (retval < 0) 104 goto bad_fork_free; 105 106 /* 107 * If multiple threads are within copy_process(), then this check 108 * triggers too late. This doesn\'t hurt, the check is only there 109 * to stop root fork bombs. 110 */ 111 retval = -EAGAIN; 112 if (nr_threads >= max_threads) 113 goto bad_fork_cleanup_count; 114 115 delayacct_tsk_init(p); /* Must remain after dup_task_struct() */ 116 p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER | PF_IDLE); 117 p->flags |= PF_FORKNOEXEC; 118 INIT_LIST_HEAD(&p->children); 119 INIT_LIST_HEAD(&p->sibling); 120 rcu_copy_process(p); 121 p->vfork_done = NULL; 122 spin_lock_init(&p->alloc_lock); 123 124 init_sigpending(&p->pending); 125 126 p->utime = p->stime = p->gtime = 0; 127 #ifdef CONFIG_ARCH_HAS_SCALED_CPUTIME 128 p->utimescaled = p->stimescaled = 0; 129 #endif 130 prev_cputime_init(&p->prev_cputime); 131 132 #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN 133 seqcount_init(&p->vtime.seqcount); 134 p->vtime.starttime = 0; 135 p->vtime.state = VTIME_INACTIVE; 136 #endif 137 138 #if defined(SPLIT_RSS_COUNTING) 139 memset(&p->rss_stat, 0, sizeof(p->rss_stat)); 140 #endif 141 142 p->default_timer_slack_ns = current->timer_slack_ns; 143 144 task_io_accounting_init(&p->ioac); 145 acct_clear_integrals(p); 146 147 posix_cpu_timers_init(p); 148 149 p->start_time = ktime_get_ns(); 150 p->real_start_time = ktime_get_boot_ns(); 151 p->io_context = NULL; 152 p->audit_context = NULL; 153 cgroup_fork(p); 154 #ifdef CONFIG_NUMA 155 p->mempolicy = mpol_dup(p->mempolicy); 156 if (IS_ERR(p->mempolicy)) { 157 retval = PTR_ERR(p->mempolicy); 158 p->mempolicy = NULL; 159 goto bad_fork_cleanup_threadgroup_lock; 160 } 161 #endif 162 #ifdef CONFIG_CPUSETS 163 p->cpuset_mem_spread_rotor = NUMA_NO_NODE; 164 p->cpuset_slab_spread_rotor = NUMA_NO_NODE; 165 seqcount_init(&p->mems_allowed_seq); 166 #endif 167 #ifdef CONFIG_TRACE_IRQFLAGS 168 p->irq_events = 0; 169 p->hardirqs_enabled = 0; 170 p->hardirq_enable_ip = 0; 171 p->hardirq_enable_event = 0; 172 p->hardirq_disable_ip = _THIS_IP_; 173 p->hardirq_disable_event = 0; 174 p->softirqs_enabled = 1; 175 p->softirq_enable_ip = _THIS_IP_; 176 p->softirq_enable_event = 0; 177 p->softirq_disable_ip = 0; 178 p->softirq_disable_event = 0; 179 p->hardirq_context = 0; 180 p->softirq_context = 0; 181 #endif 182 183 p->pagefault_disabled = 0; 184 185 #ifdef CONFIG_LOCKDEP 186 p->lockdep_depth = 0; /* no locks held yet */ 187 p->curr_chain_key = 0; 188 p->lockdep_recursion = 0; 189 lockdep_init_task(p); 190 #endif 191 192 #ifdef CONFIG_DEBUG_MUTEXES 193 p->blocked_on = NULL; /* not blocked yet */ 194 #endif 195 #ifdef CONFIG_BCACHE 196 p->sequential_io = 0; 197 p->sequential_io_avg = 0; 198 #endif 199 200 /* Perform scheduler related setup. Assign this task to a CPU. */ 201 retval = sched_fork(clone_flags, p); 202 if (retval) 203 goto bad_fork_cleanup_policy; 204 205 retval = perf_event_init_task(p); 206 if (retval) 207 goto bad_fork_cleanup_policy; 208 retval = audit_alloc(p); 209 if (retval) 210 goto bad_fork_cleanup_perf; 211 /* copy all the process information */ 212 shm_init_task(p); 213 retval = security_task_alloc(p, clone_flags); 214 if (retval) 215 goto bad_fork_cleanup_audit; 216 retval = copy_semundo(clone_flags, p); 217 if (retval) 218 goto bad_fork_cleanup_security; 219 retval = copy_files(clone_flags, p); 220 if (retval) 221 goto bad_fork_cleanup_semundo; 222 retval = copy_fs(clone_flags, p); 223 if (retval) 224 goto bad_fork_cleanup_files; 225 retval = copy_sighand(clone_flags, p); 226 if (retval) 227 goto bad_fork_cleanup_fs; 228 retval = copy_signal(clone_flags, p); 229 if (retval) 230 goto bad_fork_cleanup_sighand; 231 retval = copy_mm(clone_flags, p); 232 if (retval) 233 goto bad_fork_cleanup_signal; 234 retval = copy_namespaces(clone_flags, p); 235 if (retval) 236 goto bad_fork_cleanup_mm; 237 retval = copy_io(clone_flags, p); 238 if (retval) 239 goto bad_fork_cleanup_namespaces; 240 retval = copy_thread_tls(clone_flags, stack_start, stack_size, p, tls); 241 if (retval) 242 goto bad_fork_cleanup_io; 243 244 if (pid != &init_struct_pid) { 245 pid = alloc_pid(p->nsproxy->pid_ns_for_children); 246 if (IS_ERR(pid)) { 247 retval = PTR_ERR(pid); 248 goto bad_fork_cleanup_thread; 249 } 250 } 251 252 #ifdef CONFIG_BLOCK 253 p->plug = NULL; 254 #endif 255 #ifdef CONFIG_FUTEX 256 p->robust_list = NULL; 257 #ifdef CONFIG_COMPAT 258 p->compat_robust_list = NULL; 259 #endif 260 INIT_LIST_HEAD(&p->pi_state_list); 261 p->pi_state_cache = NULL; 262 #endif 263 /* 264 * sigaltstack should be cleared when sharing the same VM 265 */ 266 if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM) 267 sas_ss_reset(p); 268 269 /* 270 * Syscall tracing and stepping should be turned off in the 271 * child regardless of CLONE_PTRACE. 272 */ 273 user_disable_single_step(p); 274 clear_tsk_thread_flag(p, TIF_SYSCALL_TRACE); 275 #ifdef TIF_SYSCALL_EMU 276 clear_tsk_thread_flag(p, TIF_SYSCALL_EMU); 277 #endif 278 clear_all_latency_tracing(p); 279 280 /* ok, now we should be set up.. */ 281 p->pid = pid_nr(pid); 282 if (clone_flags & CLONE_THREAD) { 283 p->exit_signal = -1; 284 p->group_leader = current->group_leader; 285 p->tgid = current->tgid; 286 } else { 287 if (clone_flags & CLONE_PARENT) 288 p->exit_signal = current->group_leader->exit_signal; 289 else 290 p->exit_signal = (clone_flags & CSIGNAL); 291 p->group_leader = p; 292 p->tgid = p->pid; 293 } 294 295 p->nr_dirtied = 0; 296 p->nr_dirtied_pause = 128 >> (PAGE_SHIFT - 10); 297 p->dirty_paused_when = 0; 298 299 p->pdeath_signal = 0; 300 INIT_LIST_HEAD(&p->thread_group); 301 p->task_works = NULL; 302 303 cgroup_threadgroup_change_begin(current); 304 /* 305 * Ensure that the cgroup subsystem policies allow the new process to be 306 * forked. It should be noted the the new process\'s css_set can be changed 307 * between here and cgroup_post_fork() if an organisation operation is in 308 * progress. 309 */ 310 retval = cgroup_can_fork(p); 311 if (retval) 312 goto bad_fork_free_pid; 313 314 /* 315 * Make it visible to the rest of the system, but dont wake it up yet. 316 * Need tasklist lock for parent etc handling! 317 */ 318 write_lock_irq(&tasklist_lock); 319 320 /* CLONE_PARENT re-uses the old parent */ 321 if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) { 322 p->real_parent = current->real_parent; 323 p->parent_exec_id = current->parent_exec_id; 324 } else { 325 p->real_parent = current; 326 p->parent_exec_id = current->self_exec_id; 327 } 328 329 klp_copy_process(p); 330 331 spin_lock(¤t->sighand->siglock); 332 333 /* 334 * Copy seccomp details explicitly here, in case they were changed 335 * before holding sighand lock. 336 */ 337 copy_seccomp(p); 338 339 /* 340 * Process group and session signals need to be delivered to just the 341 * parent before the fork or both the parent and the child after the 342 * fork. Restart if a signal comes in before we add the new process to 343 * it\'s process group. 344 * A fatal signal pending means that current will exit, so the new 345 * thread can\'t slip out of an OOM kill (or normal SIGKILL). 346 */ 347 recalc_sigpending(); 348 if (signal_pending(current)) { 349 retval = -ERESTARTNOINTR; 350 goto bad_fork_cancel_cgroup; 351 } 352 if (unlikely(!(ns_of_pid(pid)->pid_allocated & PIDNS_ADDING))) { 353 retval = -ENOMEM; 354 goto bad_fork_cancel_cgroup; 355 } 356 357 if (likely(p->pid)) { 358 ptrace_init_task(p, (clone_flags & CLONE_PTRACE) || trace); 359 360 init_task_pid(p, PIDTYPE_PID, pid); 361 if (thread_group_leader(p)) { 362 init_task_pid(p, PIDTYPE_PGID, task_pgrp(current)); 363 init_task_pid(p, PIDTYPE_SID, task_session(current)); 364 365 if (is_child_reaper(pid)) { 366 ns_of_pid(pid)->child_reaper = p; 367 p->signal->flags |= SIGNAL_UNKILLABLE; 368 } 369 370 p->signal->leader_pid = pid; 371 p->signal->tty = tty_kref_get(current->signal->tty); 372 /* 373 * Inherit has_child_subreaper flag under the same 374 * tasklist_lock with adding child to the process tree 375 * for propagate_has_child_subreaper optimization. 376 */ 377 p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper || 378 p->real_parent->signal->is_child_subreaper; 379 list_add_tail(&p->sibling, &p->real_parent->children); 380 list_add_tail_rcu(&p->tasks, &init_task.tasks); 381 attach_pid(p, PIDTYPE_PGID); 382 attach_pid(p, PIDTYPE_SID); 383 __this_cpu_inc(process_counts); 384 } else { 385 current->signal->nr_threads++; 386 atomic_inc(¤t->signal->live); 387 atomic_inc(¤t->signal->sigcnt); 388 list_add_tail_rcu(&p->thread_group, 389 &p->group_leader->thread_group); 390 list_add_tail_rcu(&p->thread_node, 391 &p->signal->thread_head); 392 } 393 attach_pid(p, PIDTYPE_PID); 394 nr_threads++; 395 } 396 397 total_forks++; 398 spin_unlock(¤t->sighand->siglock); 399 syscall_tracepoint_update(p); 400 write_unlock_irq(&tasklist_lock); 401 402 proc_fork_connector(p); 403 cgroup_post_fork(p); 404 cgroup_threadgroup_change_end(current); 405 perf_event_fork(p); 406 407 trace_task_newtask(p, clone_flags); 408 uprobe_copy_process(p, clone_flags); 409 410 return p; 411 412 bad_fork_cancel_cgroup: 413 spin_unlock(¤t->sighand->siglock); 414 write_unlock_irq(&tasklist_lock); 415 cgroup_cancel_fork(p); 416 bad_fork_free_pid: 417 cgroup_threadgroup_change_end(current); 418 if (pid != &init_struct_pid) 419 free_pid(pid); 420 bad_fork_cleanup_thread: 421 exit_thread(p); 422 bad_fork_cleanup_io: 423 if (p->io_context) 424 exit_io_context(p); 425 bad_fork_cleanup_namespaces: 426 exit_task_namespaces(p); 427 bad_fork_cleanup_mm: 428 if (p->mm) 429 mmput(p->mm); 430 bad_fork_cleanup_signal: 431 if (!(clone_flags & CLONE_THREAD)) 432 free_signal_struct(p->signal); 433 bad_fork_cleanup_sighand: 434 __cleanup_sighand(p->sighand); 435 bad_fork_cleanup_fs: 436 exit_fs(p); /* blocking */ 437 bad_fork_cleanup_files: 438 exit_files(p); /* blocking */ 439 bad_fork_cleanup_semundo: 440 exit_sem(p); 441 bad_fork_cleanup_security: 442 security_task_free(p); 443 bad_fork_cleanup_audit: 444 audit_free(p); 445 bad_fork_cleanup_perf: 446 perf_event_free_task(p); 447 bad_fork_cleanup_policy: 448 lockdep_free_task(p); 449 #ifdef CONFIG_NUMA 450 mpol_put(p->mempolicy); 451 bad_fork_cleanup_threadgroup_lock: 452 #endif 453 delayacct_tsk_free(p); 454 bad_fork_cleanup_count: 455 atomic_dec(&p->cred->user->processes); 456 exit_creds(p); 457 bad_fork_free: 458 p->state = TASK_DEAD; 459 put_task_stack(p); 460 free_task(p); 461 fork_out: 462 return ERR_PTR(retval); 463 }
4.内核线程
内核线程与普通进程的区别:
内核线程只运行在内核态，普通进程既可以运行在内核态也可以运行在用户态。
内核线程只运行在内核态，只使用大于PAGE_OFFSET(即3GB~4GB之间)的线性地址空间，普通进程可以用4GB地址空间
创建内核线程:调用kernel_thread()函数,该函数本质也是调用_do_fork()函数。_do_fork(flags|CLONE_VM|CLONE_UNTRACED,0,pregs,0,NULL,NULL)
进程0--所有进程的祖先,idle进程 进程1--由进程0创建的内核线程执行init()函数
其他内核线程:keventd、kapmd、kswapd、pdflush、kblockd、ksoftirqd
5.撤销进程
进程终止:
系统调用:exit_group() -----> do_group_exit() :终止整个线程组，也是C库函数exit()调用的系统调用
系统调用:exit() -----> do_exit() :终止某一个线程，不管该线程所属线程组中的其他线程。
6.进程删除
子进程结束后，父进程必须调用wait()族函数来回收子进程的系统资源。然后内核才会丢弃子进程的进程描述符。
孤儿进程:父进程先于子进程退出,则子进程成为孤儿进程。内核会强制将孤儿进程设为init进程的子进程来解决孤儿问题。
release_task()函数:
该函数从僵死进程的描述符中分离出最后的数据结构，对僵死进程的处理有两种方式:
1.如果父进程不需要接收子进程的信号，则直接调用do_exit()
2.如果已经给父进程发送了一个信号，则调用wait4()或waitpid()系统调用
1 void release_task(struct task_struct *p) 2 { 3 struct task_struct *leader; 4 int zap_leader; 5 repeat: 6 /* don\'t need to get the RCU readlock here - the process is dead and 7 * can\'t be modifying its own credentials. But shut RCU-lockdep up */ 8 rcu_read_lock(); 9 atomic_dec(&__task_cred(p)->user->processes); 10 rcu_read_unlock(); 11 12 proc_flush_task(p); 13 14 write_lock_irq(&tasklist_lock); 15 ptrace_release_task(p); 16 __exit_signal(p); 17 18 /* 19 * If we are the last non-leader member of the thread 20 * group, and the leader is zombie, then notify the 21 * group leader\'s parent process. (if it wants notification.) 22 */ 23 zap_leader = 0; 24 leader = p->group_leader; 25 if (leader != p && thread_group_empty(leader) 26 && leader->exit_state == EXIT_ZOMBIE) { 27 /* 28 * If we were the last child thread and the leader has 29 * exited already, and the leader\'s parent ignores SIGCHLD, 30 * then we are the one who should release the leader. 31 */ 32 zap_leader = do_notify_parent(leader, leader->exit_signal); 33 if (zap_leader) 34 leader->exit_state = EXIT_DEAD; 35 } 36 37 write_unlock_irq(&tasklist_lock); 38 release_thread(p); 39 call_rcu(&p->rcu, delayed_put_task_struct); 40 41 p = leader; 42 if (unlikely(zap_leader)) 43 goto repeat; 44 }