Question

我在递归调用pthread_create（）时遇到数据争用。我不知道递归是否会导致问题，但是比赛似乎永远不会发生在第一次迭代中，主要是在第二次迭代，很少在第三次迭代。

使用libgc时，存在内存损坏症状，例如分段错误，与数据竞争一致。

以下程序是说明问题的最小示例。我在示例中没有使用libgc，因为只有数据竞争才是这个问题的主题。

使用Helgrind工具运行Valgrind时可以看到数据竞争。报告的问题略有不同，有时甚至没有任何问题。

我正在运行Linux Mint 17.2。 gcc的版本是（Ubuntu 4.8.4-2ubuntu1~14.04）4.8.4。

以下示例'main.c'重现了该问题。它遍历链表，在单独的线程中打印每个元素值：

#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>


typedef struct List {
  int head ;
  struct List* tail ;
} List ;

// create a list element with an integer head and a tail
List* new_list( int head, List* tail ) {
  List* l = (List*)malloc( sizeof( List ) ) ;
  l->head = head ;
  l->tail = tail ;
  return l ;
}


// create a thread and start it
void call( void* (*start_routine)( void* arg ), void* arg ) {
  pthread_t* thread = (pthread_t*)malloc( sizeof( pthread_t ) ) ;

  if ( pthread_create( thread, NULL, start_routine, arg ) ) {
    exit( -1 ) ;
  }

  pthread_detach( *thread ) ;
  return ;
}


void print_list( List* l ) ;

// start routine for thread
void* print_list_start_routine( void* arg ) {

  // verify that the list is not empty ( = NULL )
  // print its head
  // print the rest of it in a new thread
  if ( arg ) {

    List* l = (List*)arg ;

    printf( "%d\n", l->head ) ;

    print_list( l->tail ) ;

  }

  return NULL ;
}

// print elements of a list with one thread for each element printed
// threads are created recursively
void print_list( List* l ) {
  call( print_list_start_routine, (void*)l ) ;
}


int main( int argc, const char* argv[] ) {

  List* l = new_list( 1, new_list( 2, new_list( 3, NULL ) ) ) ;

  print_list( l ) ;  

  // wait for all threads to finnish
  pthread_exit( NULL ) ;

  return 0 ;
}

这是'makefile'：

CC=gcc

a.out: main.o
    $(CC) -pthread main.o

main.o: main.c
    $(CC) -c -g -O0 -std=gnu99 -Wall main.c

clean:
    rm *.o a.out

这是Helgrind最常见的输出。请注意，只有一个数字，1,2和3的行是程序的输出而不是Helgrind：

$ valgrind --tool=helgrind ./a.out 
==13438== Helgrind, a thread error detector
==13438== Copyright (C) 2007-2013, and GNU GPL'd, by OpenWorks LLP et al.
==13438== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
==13438== Command: ./a.out
==13438== 
1
2
==13438== ---Thread-Announcement------------------------------------------
==13438== 
==13438== Thread #3 was created
==13438==    at 0x515543E: clone (clone.S:74)
==13438==    by 0x4E44199: do_clone.constprop.3 (createthread.c:75)
==13438==    by 0x4E458BA: pthread_create@@GLIBC_2.2.5 (createthread.c:245)
==13438==    by 0x4C30C90: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==13438==    by 0x4007EB: call (main.c:25)
==13438==    by 0x400871: print_list (main.c:58)
==13438==    by 0x40084D: print_list_start_routine (main.c:48)
==13438==    by 0x4C30E26: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==13438==    by 0x4E45181: start_thread (pthread_create.c:312)
==13438==    by 0x515547C: clone (clone.S:111)
==13438== 
==13438== ---Thread-Announcement------------------------------------------
==13438== 
==13438== Thread #2 was created
==13438==    at 0x515543E: clone (clone.S:74)
==13438==    by 0x4E44199: do_clone.constprop.3 (createthread.c:75)
==13438==    by 0x4E458BA: pthread_create@@GLIBC_2.2.5 (createthread.c:245)
==13438==    by 0x4C30C90: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==13438==    by 0x4007EB: call (main.c:25)
==13438==    by 0x400871: print_list (main.c:58)
==13438==    by 0x4008BB: main (main.c:66)
==13438== 
==13438== ----------------------------------------------------------------
==13438== 
==13438== Possible data race during write of size 1 at 0x602065F by thread #3
==13438== Locks held: none
==13438==    at 0x4C368F5: mempcpy (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==13438==    by 0x4012CD6: _dl_allocate_tls_init (dl-tls.c:436)
==13438==    by 0x4E45715: pthread_create@@GLIBC_2.2.5 (allocatestack.c:252)
==13438==    by 0x4C30C90: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==13438==    by 0x4007EB: call (main.c:25)
==13438==    by 0x400871: print_list (main.c:58)
==13438==    by 0x40084D: print_list_start_routine (main.c:48)
==13438==    by 0x4C30E26: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==13438==    by 0x4E45181: start_thread (pthread_create.c:312)
==13438==    by 0x515547C: clone (clone.S:111)
==13438== 
==13438== This conflicts with a previous read of size 1 by thread #2
==13438== Locks held: none
==13438==    at 0x51C10B1: res_thread_freeres (in /lib/x86_64-linux-gnu/libc-2.19.so)
==13438==    by 0x51C1061: __libc_thread_freeres (in /lib/x86_64-linux-gnu/libc-2.19.so)
==13438==    by 0x4E45199: start_thread (pthread_create.c:329)
==13438==    by 0x515547C: clone (clone.S:111)
==13438== 
3
==13438== 
==13438== For counts of detected and suppressed errors, rerun with: -v
==13438== Use --history-level=approx or =none to gain increased speed, at
==13438== the cost of reduced accuracy of conflicting-access information
==13438== ERROR SUMMARY: 8 errors from 1 contexts (suppressed: 56 from 48)

正如Pooja Nilangekar所提到的，用pthread_join（）取代pthread_detach（）会消除竞争。但是，分离线程是必需的，因此目标是干净地分离线程。换句话说，在删除竞赛时保留pthread_detach（）。

线程之间似乎有一些无意的共享。意外共享可能与此处讨论的内容有关：http://www.domaigne.com/blog/computing/joinable-and-detached-threads/ 特别是示例中的错误。

我仍然不明白到底发生了什么。

Answer 1

只是一个注释（我没有评论代表），我得到了非常相似的Helgrind输出，而没有递归。我使用lambda生成线程并将其分离。

==9060== Possible data race during write of size 1 at 0x126CE63F by thread #1
==9060== Locks held: none
==9060==    at 0x4C36D85: mempcpy (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==9060==    by 0x4012D66: _dl_allocate_tls_init (dl-tls.c:436)
==9060==    by 0x6B04715: get_cached_stack (allocatestack.c:252)
==9060==    by 0x6B04715: allocate_stack (allocatestack.c:501)
==9060==    by 0x6B04715: pthread_create@@GLIBC_2.2.5 (pthread_create.c:500)
==9060==    by 0x4C30E0D: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==9060==    by 0x6359D23: std::thread::_M_start_thread(std::shared_ptr<std::thread::_Impl_base>, void (*)()) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==9060==    by 0x404075: thread<main()::<lambda()> > (thread:138)
==9060==    by 0x404075: main (test1.cpp:162)
==9060== 
==9060== This conflicts with a previous read of size 8 by thread #2
==9060== Locks held: none
==9060==    at 0x6E83931: res_thread_freeres (in /lib/x86_64-linux-gnu/libc-2.19.so)
==9060==    by 0x6E838E1: __libc_thread_freeres (in /lib/x86_64-linux-gnu/libc-2.19.so)
==9060==    by 0x6B0419B: start_thread (pthread_create.c:329)
==9060==    by 0x6E1803C: clone (clone.S:111)
==9060==  Address 0x126ce63f is not stack'd, malloc'd or on a free list

但是我循环执行此操作，却只报告了一次。这表明TLS机制中可能存在某种触发警报的可能性。

为什么这个递归的pthread_create调用导致数据竞争？

1 个答案: