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linux2.4.0版本内核代码fork.c浅显分析

结合fork.c文件分析进程创建的过程

本文为作业任务,只做浅显的分析,为大家提供一个分析的思路,很多细节都没有展示。如果想要更详细的分析请去搜索相关函数代码,博客园内有许多有用的信息供大家学习。

 

int nr_threads; int nr_running;  int max_threads; unsigned long total_forks;    /* Handle normal Linux uptimes. */ int last_pid;  struct task_struct *pidhash[PIDHASH_SZ];

 

文件开头定义了线程数量,进程数量,最大线程数,创建的进程总个数,最新的pid号以及存放pid号的哈希表。

void add_wait_queue(wait_queue_head_t *q, wait_queue_t * wait) {     unsigned long flags;      wq_write_lock_irqsave(&q->lock, flags);     wait->flags = 0;     __add_wait_queue(q, wait);     wq_write_unlock_irqrestore(&q->lock, flags); } void add_wait_queue_exclusive(wait_queue_head_t *q, wait_queue_t * wait) { unsigned long flags;  wq_write_lock_irqsave(&q->lock, flags); wait->flags = WQ_FLAG_EXCLUSIVE; __add_wait_queue_tail(q, wait); wq_write_unlock_irqrestore(&q->lock, flags); } void remove_wait_queue(wait_queue_head_t *q, wait_queue_t * wait) {     unsigned long flags;      wq_write_lock_irqsave(&q->lock, flags);     __remove_wait_queue(q, wait);     wq_write_unlock_irqrestore(&q->lock, flags); }

 

这部分代码与进程的等待队列有关。Linux内核的等待队列是以双循环链表为基础数据结构,与进程调度机制紧密结合,能够用于实现核心的异步事件通知机制。等待队列在include/linux/wait.h中,这是一个通过list_head连接的典型双循环链表,在这个链表中,有两种数据结构:等待队列头(wait_queue_head_t)和等待队列项(wait_queue_t)。等待队列头和等待队列项中都包含一个list_head类型的域作为”连接件”。由于我们只需要对队列进行添加和删除操作,并不会修改其中的对象(等待队列项),因此,我们只需要提供一把保护整个基础设施和所有对象的锁,这把锁保存在等待队列头中,为wq_lock_t类型。在实现中,可以支持读写锁(rwlock)或自旋锁(spinlock)两种类型,通过一个宏定义来切换。如果使用读写锁,将wq_lock_t定义为rwlock_t类型;如果是自旋锁,将wq_lock_t定义为spinlock_t类型。无论哪种情况,分别相应设置wq_read_lock、wq_read_unlock、wq_read_lock_irqsave、wq_read_unlock_irqrestore、wq_write_lock_irq、wq_write_unlock、wq_write_lock_irqsave和wq_write_unlock_irqrestore等宏。在__wait_queue 中定义的WQ_FLAG_EXCLUSIVE表示节点对应的进程对临界资源具有排他性。remove_wait_queue函数用于将等待队列项wait从以q为等待队列头的等待队列中移除

 

void __init fork_init(unsigned long mempages)  {      /*       * The default maximum number of threads is set to a safe       * value: the thread structures can take up at most half       * of memory.       */      max_threads = mempages / (THREAD_SIZE/PAGE_SIZE) / 2;         init_task.rlim[RLIMIT_NPROC].rlim_cur = max_threads/2;      init_task.rlim[RLIMIT_NPROC].rlim_max = max_threads/2;  }

 

 

如注释所说,默认的最大线程数被设置为一个安全值:线程结构最多可以占用一半的内存。__init在include/linux/wait.h中,作用为将带有__init标识符的函数划分到.init.text段中,此段只在启动时做一次初始化载入。

/* Protects next_safe and last_pid. */  spinlock_t lastpid_lock = SPIN_LOCK_UNLOCKED;     static int get_pid(unsigned long flags)  {      static int next_safe = PID_MAX;      struct task_struct *p;         if (flags & CLONE_PID)          return current->pid;         spin_lock(&lastpid_lock);      if((++last_pid) & 0xffff8000) {          last_pid = 300;      /* Skip daemons etc. */          goto inside;      }      if(last_pid >= next_safe) {  inside:          next_safe = PID_MAX;          read_lock(&tasklist_lock);      repeat:          for_each_task(p) {              if(p->pid == last_pid ||                 p->pgrp == last_pid ||                 p->session == last_pid) {                  if(++last_pid >= next_safe) {                      if(last_pid & 0xffff8000)                          last_pid = 300;                      next_safe = PID_MAX;                  }                  goto repeat;              }              if(p->pid > last_pid && next_safe > p->pid)                  next_safe = p->pid;              if(p->pgrp > last_pid && next_safe > p->pgrp)                  next_safe = p->pgrp;              if(p->session > last_pid && next_safe > p->session)                  next_safe = p->session;          }          read_unlock(&tasklist_lock);      }      spin_unlock(&lastpid_lock);         return last_pid;  }

 

这部分代码用来给进程分配pid,对get_pid函数添加自旋锁保证函数的运行,对tasklist_lock添加读锁,确保pid数据安全。last_pid用于记录上一次分配给进程时的pid值。分配的pid一般而言是last_pid+1,如果超出进程个数的最大值(0xffff8000),那么进程pid值从300开始重新查找未用的。也就是说,一般用户进程的pid值范围[300,ffff8000]。(0~299,留给系统)。变量next_safe的含义是,在[last_pid,next_safe]之间,都是没有使用过的pid,一旦last_pid+1大于了next_safe,也就是说pid值进入了不可靠空间,有可能这个值被使用,这时需要遍历task来确认。这样遍历task找到一个没有用过的pid,同时确定next_safe,以保证next_safe到last_pid的区间中pid是空闲的,这样只要再次分配pid时,其值小于next_safe就可以直接分配,而不需要遍历task来查找空闲的pid。

 

static inline int dup_mmap(struct mm_struct * mm)  {      struct vm_area_struct * mpnt, *tmp, **pprev;      int retval;         flush_cache_mm(current->mm);      mm->locked_vm = 0;      mm->mmap = NULL;      mm->mmap_avl = NULL;      mm->mmap_cache = NULL;      mm->map_count = 0;      mm->cpu_vm_mask = 0;      mm->swap_cnt = 0;      mm->swap_address = 0;      pprev = &mm->mmap;      for (mpnt = current->mm->mmap ; mpnt ; mpnt = mpnt->vm_next) {          struct file *file;             retval = -ENOMEM;          if(mpnt->vm_flags & VM_DONTCOPY)              continue;          tmp = kmem_cache_alloc(vm_area_cachep, SLAB_KERNEL);          if (!tmp)              goto fail_nomem;          *tmp = *mpnt;          tmp->vm_flags &= ~VM_LOCKED;          tmp->vm_mm = mm;          mm->map_count++;          tmp->vm_next = NULL;          file = tmp->vm_file;          if (file) {              struct inode *inode = file->f_dentry->d_inode;              get_file(file);              if (tmp->vm_flags & VM_DENYWRITE)                  atomic_dec(&inode->i_writecount);                     /* insert tmp into the share list, just after mpnt */              spin_lock(&inode->i_mapping->i_shared_lock);              if((tmp->vm_next_share = mpnt->vm_next_share) != NULL)                  mpnt->vm_next_share->vm_pprev_share =                      &tmp->vm_next_share;              mpnt->vm_next_share = tmp;              tmp->vm_pprev_share = &mpnt->vm_next_share;              spin_unlock(&inode->i_mapping->i_shared_lock);          }             /* Copy the pages, but defer checking for errors */          retval = copy_page_range(mm, current->mm, tmp);          if (!retval && tmp->vm_ops && tmp->vm_ops->open)              tmp->vm_ops->open(tmp);             /*           * Link in the new vma even if an error occurred,           * so that exit_mmap() can clean up the mess.           */          *pprev = tmp;          pprev = &tmp->vm_next;             if (retval)              goto fail_nomem;      }      retval = 0;      if (mm->map_count >= AVL_MIN_MAP_COUNT)          build_mmap_avl(mm);     fail_nomem:      flush_tlb_mm(current->mm);      return retval;  }     spinlock_t mmlist_lock __cacheline_aligned = SPIN_LOCK_UNLOCKED;     #define allocate_mm() (kmem_cache_alloc(mm_cachep, SLAB_KERNEL))  #define free_mm(mm)  (kmem_cache_free(mm_cachep, (mm)))     static struct mm_struct * mm_init(struct mm_struct * mm)  {      atomic_set(&mm->mm_users, 1);      atomic_set(&mm->mm_count, 1);      init_MUTEX(&mm->mmap_sem);      mm->page_table_lock = SPIN_LOCK_UNLOCKED;      mm->pgd = pgd_alloc();      if (mm->pgd)          return mm;      free_mm(mm);      return NULL;  }          /*   * Allocate and initialize an mm_struct.   */  struct mm_struct * mm_alloc(void)  {      struct mm_struct * mm;         mm = allocate_mm();      if (mm) {          memset(mm, 0, sizeof(*mm));          return mm_init(mm);      }      return NULL;  }     /*   * Called when the last reference to the mm   * is dropped: either by a lazy thread or by   * mmput. Free the page directory and the mm.   */  inline void __mmdrop(struct mm_struct *mm)  {      if (mm == &init_mm) BUG();      pgd_free(mm->pgd);      destroy_context(mm);      free_mm(mm);  }     /*   * Decrement the use count and release all resources for an mm.   */  void mmput(struct mm_struct *mm)  {      if (atomic_dec_and_lock(&mm->mm_users, &mmlist_lock)) {          list_del(&mm->mmlist);          spin_unlock(&mmlist_lock);          exit_mmap(mm);          mmdrop(mm);      }  }     void mm_release(void)  {      struct task_struct *tsk = current;         /* notify parent sleeping on vfork() */      if (tsk->flags & PF_VFORK) {          tsk->flags &= ~PF_VFORK;          up(tsk->p_opptr->vfork_sem);      }  }

 

这部分代码为内存管理部分,代码中的注释向我们大致说明了本段代码的功能。

Linux内核通过一个被称为进程描述符的task_struct结构体来管理进程,这个结构体包含了一个进程所需的所有信息。它定义在include/linux/sched.h文件中。每一个进程都会有自己独立的mm_struct,这样每一个进程都会有自己独立的地址空间,这样才能互不干扰。在地址空间中,mmap为地址空间的内存区域(用vm_area_struct结构来表示)链表,表示起来更加方便。mm_struct的结构描述了进程的用户空间的结构,定义了用户空间的段分布:数据段,代码段,堆栈段。其中pgd_t是该进程用户空间地址映射到物理地址时使用vm_area_struct是进程用户空间已映射到物理空间的虚拟地址区间,定义在/include/linux/mm.h。mmap是该空间区块组成的链表。vm_flag是描述对虚拟区间的操作的标志。

 

 

static int copy_mm(unsigned long clone_flags, struct task_struct * tsk)  {      struct mm_struct * mm, *oldmm;      int retval;         tsk->min_flt = tsk->maj_flt = 0;      tsk->cmin_flt = tsk->cmaj_flt = 0;      tsk->nswap = tsk->cnswap = 0;         tsk->mm = NULL;      tsk->active_mm = NULL;         /*       * Are we cloning a kernel thread?       *       * We need to steal a active VM for that..       */      oldmm = current->mm;      if (!oldmm)          return 0;         if (clone_flags & CLONE_VM) {          atomic_inc(&oldmm->mm_users);          mm = oldmm;          goto good_mm;      }         retval = -ENOMEM;      mm = allocate_mm();      if (!mm)          goto fail_nomem;         /* Copy the current MM stuff.. */      memcpy(mm, oldmm, sizeof(*mm));      if (!mm_init(mm))          goto fail_nomem;         down(&oldmm->mmap_sem);      retval = dup_mmap(mm);      up(&oldmm->mmap_sem);         /*       * Add it to the mmlist after the parent.       *       * Doing it this way means that we can order       * the list, and fork() won't mess up the       * ordering significantly.       */      spin_lock(&mmlist_lock);      list_add(&mm->mmlist, &oldmm->mmlist);      spin_unlock(&mmlist_lock);         if (retval)          goto free_pt;         /*       * child gets a private LDT (if there was an LDT in the parent)       */      copy_segments(tsk, mm);         if (init_new_context(tsk,mm))          goto free_pt;     good_mm:      tsk->mm = mm;      tsk->active_mm = mm;      return 0;     free_pt:      mmput(mm);  fail_nomem:      return retval;  }     static inline struct fs_struct *__copy_fs_struct(struct fs_struct *old)  {      struct fs_struct *fs = kmem_cache_alloc(fs_cachep, GFP_KERNEL);      /* We don't need to lock fs - think why ;-) */      if (fs) {          atomic_set(&fs->count, 1);          fs->lock = RW_LOCK_UNLOCKED;          fs->umask = old->umask;          read_lock(&old->lock);          fs->rootmnt = mntget(old->rootmnt);          fs->root = dget(old->root);          fs->pwdmnt = mntget(old->pwdmnt);          fs->pwd = dget(old->pwd);          if (old->altroot) {              fs->altrootmnt = mntget(old->altrootmnt);              fs->altroot = dget(old->altroot);          } else {              fs->altrootmnt = NULL;              fs->altroot = NULL;          }            read_unlock(&old->lock);      }      return fs;  }     struct fs_struct *copy_fs_struct(struct fs_struct *old)  {      return __copy_fs_struct(old);  }     static inline int copy_fs(unsigned long clone_flags, struct task_struct * tsk)  {      if (clone_flags & CLONE_FS) {          atomic_inc(&current->fs->count);          return 0;      }      tsk->fs = __copy_fs_struct(current->fs);      if (!tsk->fs)          return -1;      return 0;  }     static int count_open_files(struct files_struct *files, int size)  {      int i;           /* Find the last open fd */      for (i = size/(8*sizeof(long)); i > 0; ) {          if (files->open_fds->fds_bits[--i])              break;      }      i = (i+1) * 8 * sizeof(long);      return i;  }     static int copy_files(unsigned long clone_flags, struct task_struct * tsk)  {      struct files_struct *oldf, *newf;      struct file **old_fds, **new_fds;      int open_files, nfds, size, i, error = 0;         /*       * A background process may not have any files ...       */      oldf = current->files;      if (!oldf)          goto out;         if (clone_flags & CLONE_FILES) {          atomic_inc(&oldf->count);          goto out;      }         tsk->files = NULL;      error = -ENOMEM;      newf = kmem_cache_alloc(files_cachep, SLAB_KERNEL);      if (!newf)          goto out;         atomic_set(&newf->count, 1);         newf->file_lock      = RW_LOCK_UNLOCKED;      newf->next_fd     = 0;      newf->max_fds     = NR_OPEN_DEFAULT;      newf->max_fdset      = __FD_SETSIZE;      newf->close_on_exec = &newf->close_on_exec_init;      newf->open_fds       = &newf->open_fds_init;      newf->fd     = &newf->fd_array[0];         /* We don't yet have the oldf readlock, but even if the old             fdset gets grown now, we'll only copy up to "size" fds */      size = oldf->max_fdset;      if (size > __FD_SETSIZE) {          newf->max_fdset = 0;          write_lock(&newf->file_lock);          error = expand_fdset(newf, size);          write_unlock(&newf->file_lock);          if (error)              goto out_release;      }      read_lock(&oldf->file_lock);         open_files = count_open_files(oldf, size);         /*       * Check whether we need to allocate a larger fd array.       * Note: we're not a clone task, so the open count won't       * change.       */      nfds = NR_OPEN_DEFAULT;      if (open_files > nfds) {          read_unlock(&oldf->file_lock);          newf->max_fds = 0;          write_lock(&newf->file_lock);          error = expand_fd_array(newf, open_files);          write_unlock(&newf->file_lock);          if (error)              goto out_release;          nfds = newf->max_fds;          read_lock(&oldf->file_lock);      }         old_fds = oldf->fd;      new_fds = newf->fd;         memcpy(newf->open_fds->fds_bits, oldf->open_fds->fds_bits, open_files/8);      memcpy(newf->close_on_exec->fds_bits, oldf->close_on_exec->fds_bits, open_files/8);         for (i = open_files; i != 0; i--) {          struct file *f = *old_fds++;          if (f)              get_file(f);          *new_fds++ = f;      }      read_unlock(&oldf->file_lock);         /* compute the remainder to be cleared */      size = (newf->max_fds - open_files) * sizeof(struct file *);         /* This is long word aligned thus could use a optimized version */      memset(new_fds, 0, size);         if (newf->max_fdset > open_files) {          int left = (newf->max_fdset-open_files)/8;          int start = open_files / (8 * sizeof(unsigned long));                   memset(&newf->open_fds->fds_bits[start], 0, left);          memset(&newf->close_on_exec->fds_bits[start], 0, left);      }         tsk->files = newf;      error = 0;  out:      return error;     out_release:      free_fdset (newf->close_on_exec, newf->max_fdset);      free_fdset (newf->open_fds, newf->max_fdset);      kmem_cache_free(files_cachep, newf);      goto out;  }     static inline int copy_sighand(unsigned long clone_flags, struct task_struct * tsk)  {      struct signal_struct *sig;         if (clone_flags & CLONE_SIGHAND) {          atomic_inc(&current->sig->count);          return 0;      }      sig = kmem_cache_alloc(sigact_cachep, GFP_KERNEL);      tsk->sig = sig;      if (!sig)          return -1;      spin_lock_init(&sig->siglock);      atomic_set(&sig->count, 1);      memcpy(tsk->sig->action, current->sig->action, sizeof(tsk->sig->action));      return 0;  }     static inline void copy_flags(unsigned long clone_flags, struct task_struct *p)  {      unsigned long new_flags = p->flags;         new_flags &= ~(PF_SUPERPRIV | PF_USEDFPU | PF_VFORK);      new_flags |= PF_FORKNOEXEC;      if (!(clone_flags & CLONE_PTRACE))          p->ptrace = 0;      if (clone_flags & CLONE_VFORK)          new_flags |= PF_VFORK;      p->flags = new_flags;  }

 

父进程中在调用fork()派生新进程,实际上相当于创建了进程的一个拷贝;复制出来的子进程有自己的 task_struct结构和系统空间堆栈,但与父进程共享其他所有的资源。Linux为此提供了两个系统调用,一个是fork(),另一个是clone()。我们现在主要讨论fork()。fork()是全部复制,父进程所需的资源全部通过数据结构的复制传递给子进程,而完成这一操作的函数定义就是上方所写的代码段。调用fork时,内核会在copy_mm函数中处理子进程的mm_struct,在copy_files函数中处理拷贝父进程打开的文件的相关事宜,在copy_fs中记录进程所在文件系统的根目录和当前目录信息, copy_sighand中复制进程对信号的处理方式。

/*   *  Ok, this is the main fork-routine. It copies the system process   * information (task[nr]) and sets up the necessary registers. It also   * copies the data segment in its entirety.  The "stack_start" and   * "stack_top" arguments are simply passed along to the platform   * specific copy_thread() routine.  Most platforms ignore stack_top.   * For an example that's using stack_top, see   * arch/ia64/kernel/process.c.   */  int do_fork(unsigned long clone_flags, unsigned long stack_start,struct pt_regs *regs, unsigned long stack_size)  {      int retval = -ENOMEM;      struct task_struct *p;      DECLARE_MUTEX_LOCKED(sem);         if (clone_flags & CLONE_PID) {          /* This is only allowed from the boot up thread */          if (current->pid)              return -EPERM;      }           current->vfork_sem = &sem;         p = alloc_task_struct();      if (!p)          goto fork_out;         *p = *current;         retval = -EAGAIN;      if (atomic_read(&p->user->processes) >= p->rlim[RLIMIT_NPROC].rlim_cur)          goto bad_fork_free;      atomic_inc(&p->user->__count);      atomic_inc(&p->user->processes);         /*       * Counter increases are protected by       * the kernel lock so nr_threads can't       * increase under us (but it may decrease).       */      if (nr_threads >= max_threads)          goto bad_fork_cleanup_count;           get_exec_domain(p->exec_domain);         if (p->binfmt && p->binfmt->module)          __MOD_INC_USE_COUNT(p->binfmt->module);         p->did_exec = 0;      p->swappable = 0;      p->state = TASK_UNINTERRUPTIBLE;         copy_flags(clone_flags, p);      p->pid = get_pid(clone_flags);         p->run_list.next = NULL;      p->run_list.prev = NULL;         if ((clone_flags & CLONE_VFORK) || !(clone_flags & CLONE_PARENT)) {          p->p_opptr = current;          if (!(p->ptrace & PT_PTRACED))              p->p_pptr = current;      }      p->p_cptr = NULL;      init_waitqueue_head(&p->wait_chldexit);      p->vfork_sem = NULL;      spin_lock_init(&p->alloc_lock);         p->sigpending = 0;      init_sigpending(&p->pending);         p->it_real_value = p->it_virt_value = p->it_prof_value = 0;      p->it_real_incr = p->it_virt_incr = p->it_prof_incr = 0;      init_timer(&p->real_timer);      p->real_timer.data = (unsigned long) p;         p->leader = 0;       /* session leadership doesn't inherit */      p->tty_old_pgrp = 0;      p->times.tms_utime = p->times.tms_stime = 0;      p->times.tms_cutime = p->times.tms_cstime = 0;  #ifdef CONFIG_SMP      {          int i;          p->has_cpu = 0;          p->processor = current->processor;          /* ?? should we just memset this ?? */          for(i = 0; i < smp_num_cpus; i++)              p->per_cpu_utime[i] = p->per_cpu_stime[i] = 0;          spin_lock_init(&p->sigmask_lock);      }  #endif      p->lock_depth = -1;       /* -1 = no lock */      p->start_time = jiffies;         retval = -ENOMEM;      /* copy all the process information */      if (copy_files(clone_flags, p))          goto bad_fork_cleanup;      if (copy_fs(clone_flags, p))          goto bad_fork_cleanup_files;      if (copy_sighand(clone_flags, p))          goto bad_fork_cleanup_fs;      if (copy_mm(clone_flags, p))          goto bad_fork_cleanup_sighand;      retval = copy_thread(0, clone_flags, stack_start, stack_size, p, regs);      if (retval)          goto bad_fork_cleanup_sighand;      p->semundo = NULL;           /* Our parent execution domain becomes current domain         These must match for thread signalling to apply */              p->parent_exec_id = p->self_exec_id;         /* ok, now we should be set up.. */      p->swappable = 1;      p->exit_signal = clone_flags & CSIGNAL;      p->pdeath_signal = 0;         /*       * "share" dynamic priority between parent and child, thus the       * total amount of dynamic priorities in the system doesnt change,       * more scheduling fairness. This is only important in the first       * timeslice, on the long run the scheduling behaviour is unchanged.       */      p->counter = (current->counter + 1) >> 1;      current->counter >>= 1;      if (!current->counter)          current->need_resched = 1;         /*       * Ok, add it to the run-queues and make it       * visible to the rest of the system.       *       * Let it rip!       */      retval = p->pid;      p->tgid = retval;      INIT_LIST_HEAD(&p->thread_group);      write_lock_irq(&tasklist_lock);      if (clone_flags & CLONE_THREAD) {          p->tgid = current->tgid;          list_add(&p->thread_group, &current->thread_group);      }      SET_LINKS(p);      hash_pid(p);      nr_threads++;      write_unlock_irq(&tasklist_lock);         if (p->ptrace & PT_PTRACED)          send_sig(SIGSTOP, p, 1);         wake_up_process(p);      /* do this last */      ++total_forks;     fork_out:      if ((clone_flags & CLONE_VFORK) && (retval > 0))          down(&sem);      return retval;     bad_fork_cleanup_sighand:      exit_sighand(p);  bad_fork_cleanup_fs:      exit_fs(p); /* blocking */  bad_fork_cleanup_files:      exit_files(p); /* blocking */  bad_fork_cleanup:      put_exec_domain(p->exec_domain);      if (p->binfmt && p->binfmt->module)          __MOD_DEC_USE_COUNT(p->binfmt->module);  bad_fork_cleanup_count:      atomic_dec(&p->user->processes);      free_uid(p->user);  bad_fork_free:      free_task_struct(p);      goto fork_out;  }

 

如开头注释第一句所说,这部分代码是fork.c中最主要的函数。

do_fork首先进行一些参数及权限的检查,仅允许从线程启动。之后进行内存的分配,复制父进程的task_struct。判断进程数量,将从父进程中继承的task_struct初始化,获取新的pid,分配CPU,解锁后设定运行时间。将子进程的pid放入pidhash表中,就可以唤醒子进程了。代码中间部分有设置进程判断,若发现非法进程会直接清理掉。清理函数在代码尾部定义。

 

/* SLAB cache for signal_struct structures (tsk->sig) */  kmem_cache_t *sigact_cachep;     /* SLAB cache for files_struct structures (tsk->files) */  kmem_cache_t *files_cachep;     /* SLAB cache for fs_struct structures (tsk->fs) */  kmem_cache_t *fs_cachep;     /* SLAB cache for vm_area_struct structures */  kmem_cache_t *vm_area_cachep;     /* SLAB cache for mm_struct structures (tsk->mm) */  kmem_cache_t *mm_cachep;     void __init proc_caches_init(void)  {      sigact_cachep = kmem_cache_create("signal_act",              sizeof(struct signal_struct), 0,              SLAB_HWCACHE_ALIGN, NULL, NULL);      if (!sigact_cachep)          panic("Cannot create signal action SLAB cache");         files_cachep = kmem_cache_create("files_cache",               sizeof(struct files_struct), 0,               SLAB_HWCACHE_ALIGN, NULL, NULL);      if (!files_cachep)          panic("Cannot create files SLAB cache");         fs_cachep = kmem_cache_create("fs_cache",               sizeof(struct fs_struct), 0,               SLAB_HWCACHE_ALIGN, NULL, NULL);      if (!fs_cachep)          panic("Cannot create fs_struct SLAB cache");         vm_area_cachep = kmem_cache_create("vm_area_struct",              sizeof(struct vm_area_struct), 0,              SLAB_HWCACHE_ALIGN, NULL, NULL);      if(!vm_area_cachep)          panic("vma_init: Cannot alloc vm_area_struct SLAB cache");         mm_cachep = kmem_cache_create("mm_struct",              sizeof(struct mm_struct), 0,              SLAB_HWCACHE_ALIGN, NULL, NULL);      if(!mm_cachep)          panic("vma_init: Cannot alloc mm_struct SLAB cache");  }

 

最后这部分代码作用是处理进程的缓存,为proc文件系统创建高速缓冲。

 

 

从文件开头的宏定义,到等待队列的处理,到线程数的安全处理,到pid的分配,到进程的内存管理,到父进程复制出子进程。fork()函数中对进程的创建大致是以上步骤。主要在于copy部分对task_struct复制和复制后的初始化。

 

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未经允许不得转载:张拓的天空 » linux2.4.0版本内核代码fork.c浅显分析
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