NPTL进程创建流程

pid_t__libc_fork (void){  pid_t pid;  struct used_handler  {    struct fork_handler *handler;    struct used_handler *next;  } *allp = NULL;  /* Run all the registered preparation handlers.  In reverse order.     While doing this we build up a list of all the entries.  */  struct fork_handler *runp;  while ((runp = __fork_handlers) != NULL)    {      /* Make sure we read from the current RUNP pointer.  */      atomic_full_barrier ();      unsigned int oldval = runp->refcntr;      if (oldval == 0)/* This means some other thread removed the list just after   the pointer has been loaded.  Try again.  Either the list   is empty or we can retry it.  */continue;      /* Bump the reference counter.  */      if (atomic_compare_and_exchange_bool_acq (&__fork_handlers->refcntr,oldval + 1, oldval))/* The value changed, try again.  */continue;      /* We bumped the reference counter for the first entry in the list.  That means that none of the following entries will just go away.  The unloading code works in the order of the list. While executing the registered handlers we are building a list of all the entries so that we can go backward later on.  */      while (1){  /* Execute the handler if there is one.  */  if (runp->prepare_handler != NULL)    runp->prepare_handler ();  /* Create a new element for the list.  */  struct used_handler *newp    = (struct used_handler *) alloca (sizeof (*newp));  newp->handler = runp;  newp->next = allp;  allp = newp;  /* Advance to the next handler.  */  runp = runp->next;  if (runp == NULL)    break;  /* Bump the reference counter for the next entry.  */  atomic_increment (&runp->refcntr);}      /* We are done.  */      break;    }  _IO_list_lock ();#ifndef NDEBUG  pid_t ppid = THREAD_GETMEM (THREAD_SELF, tid);#endif  /* We need to prevent the getpid() code to update the PID field so     that, if a signal arrives in the child very early and the signal     handler uses getpid(), the value returned is correct.  */  pid_t parentpid = THREAD_GETMEM (THREAD_SELF, pid);  THREAD_SETMEM (THREAD_SELF, pid, -parentpid);#ifdef ARCH_FORK  pid = ARCH_FORK ();#else# error "ARCH_FORK must be defined so that the CLONE_SETTID flag is used"  pid = INLINE_SYSCALL (fork, 0);#endif  if (pid == 0)    {      struct pthread *self = THREAD_SELF;      assert (THREAD_GETMEM (self, tid) != ppid);      /* See __pthread_once.  */      if (__fork_generation_pointer != NULL)*__fork_generation_pointer += __PTHREAD_ONCE_FORK_GEN_INCR;      /* Adjust the PID field for the new process.  */      THREAD_SETMEM (self, pid, THREAD_GETMEM (self, tid));#if HP_TIMING_AVAIL      /* The CPU clock of the thread and process have to be set to zero.  */      hp_timing_t now;      HP_TIMING_NOW (now);      THREAD_SETMEM (self, cpuclock_offset, now);      GL(dl_cpuclock_offset) = now;#endif#ifdef __NR_set_robust_list      /* Initialize the robust mutex list which has been reset during the fork.  We do not check for errors since if it fails here it failed at process start as well and noone could have used robust mutexes.  We also do not have to set self->robust_head.futex_offset since we inherit the correct value from the parent.  */# ifdef SHARED      if (__builtin_expect (__libc_pthread_functions_init, 0))PTHFCT_CALL (ptr_set_robust, (self));# else      extern __typeof (__nptl_set_robust) __nptl_set_robust__attribute__((weak));      if (__builtin_expect (__nptl_set_robust != NULL, 0))__nptl_set_robust (self);# endif#endif      /* Reset the file list.  These are recursive mutexes.  */      fresetlockfiles ();      /* Reset locks in the I/O code.  */      _IO_list_resetlock ();      /* Reset the lock the dynamic loader uses to protect its data.  */      __rtld_lock_initialize (GL(dl_load_lock));      /* Run the handlers registered for the child.  */      while (allp != NULL){  if (allp->handler->child_handler != NULL)    allp->handler->child_handler ();  /* Note that we do not have to wake any possible waiter.     This is the only thread in the new process.  The count     may have been bumped up by other threads doing a fork.     We reset it to 1, to avoid waiting for non-existing     thread(s) to release the count.  */  allp->handler->refcntr = 1;  /* XXX We could at this point look through the object pool     and mark all objects not on the __fork_handlers list as     unused.  This is necessary in case the fork() happened     while another thread called dlclose() and that call had     to create a new list.  */  allp = allp->next;}      /* Initialize the fork lock.  */      __fork_lock = LLL_LOCK_INITIALIZER;    }  else    {      assert (THREAD_GETMEM (THREAD_SELF, tid) == ppid);      /* Restore the PID value.  */      THREAD_SETMEM (THREAD_SELF, pid, parentpid);      /* We execute this even if the 'fork' call failed.  */      _IO_list_unlock ();      /* Run the handlers registered for the parent.  */      while (allp != NULL){  if (allp->handler->parent_handler != NULL)    allp->handler->parent_handler ();  if (atomic_decrement_and_test (&allp->handler->refcntr)      && allp->handler->need_signal)    lll_futex_wake (&allp->handler->refcntr, 1, LLL_PRIVATE);  allp = allp->next;}    }  return pid;}