Linux signal 那些事儿

Posted sky

tags:

篇首语：本文由小常识网(cha138.com)小编为大家整理，主要介绍了Linux signal 那些事儿相关的知识，希望对你有一定的参考价值。

转自：http://blog.chinaunix.net/uid-24774106-id-4064447.html

上一篇博文，基本算是给glibc的signal函数翻了个身。现在glibc的signal基本修正了传统的UNIX的一些弊端，我们说signal并没有我们想象的那么不堪。但是signal也有不尽人意的地方。比如信号处理期间，我们期望屏蔽某些信号，而不仅仅是屏蔽自身，这时候signal就不行了。信号既然是进程间通信IPC的一种机制，我们期望获取更多的信息，而不仅仅是signo，这时候signal/kill这个机制就基本不行了。
    上面所说的都是signal的一些毛病，但是这些都不是致命的，致命的问题在于老的signal机制的不可靠。信号分成可靠性信号和非可靠性信号，并不是说用sigaction安装，用sigqueue发送的信号就是可靠性性信号，用signal安装，kill/tkill发送的信号就是非可靠性信号。这种理解是错误的。这在Linux环境进程间通信（二）：信号（上）一文中讲的非常清楚了。
    信号值位于[SIGRTMIN,SIGRTMAX] 之间的信号，就是可靠信号，位于[SIGHUP,SIGSYS]之间信号，都是非可靠性信号，与安装函数是signal还是sigaction无关，与发送函数是kill还是sigqueue无关。
技术分享
    1～31之间的所有信号都称为不可靠信号，原因就在于信号不可排队，如果kernel发现同一个信号已经有挂起信号，当前信号就会被丢弃，就好象从来没有被发送过一样，无法引起信号的传递，也无法让进程执行信号处理函数。这种实现的机理，造成了这些信号的不可靠。这正所谓：我本将心向明月，奈何明月照沟渠。
    为了解决这个问题，Linux引入了实时信号,信号值在[32~64]区间内，或者称之为可靠信号。这种信号，kernel不会ignore，哪怕已经有了好多同一个信号，kernel会把新收到信号放入queue之中，等待被传递出去。
    空口说白话，不是我们的风格，我现在用代码证明之。我参考了Linux Programming Interface 一书的例子，写了两个程序，一个是signal_receiver ,一个是signal_sender.
    先看signal_receiver的code：    

  1. [email protected]-hacks:~/code/c/self/signal$ cat signal_receiver.c
  2. #include <stdio.h>
  3. #include <stdlib.h>
  4. #include <unistd.h>
  5. #include <signal.h>
  6. #include <string.h>
  7. #include <errno.h>


  10. static int sig_cnt[NSIG];
  11. static volatile sig_atomic_t get_SIGINT = 0;

  13. void handler(int signo)
  14. {
  15.     if(signo == SIGINT)
  16.         get_SIGINT = 1;
  17.     else
  18.         sig_cnt[signo]++;
  19. }

  21. int main(int argc,char* argv[])
  22. {
  23.     int i = 0;
  24.     sigset_t blockall_mask ;
  25.     sigset_t pending_mask ;
  26.     sigset_t empty_mask ;
  27.     printf("%s:PID is %ld\n",argv[0],getpid());

  29.     
  30.     for(i = 1; i < NSIG; i++)
  31.     {
  32.         if(i == SIGKILL || i == SIGSTOP)
  33.             continue;

  35.         if(signal(i,&handler) == SIG_ERR)
  36.         {
  37.             fprintf(stderr,"signal for signo(%d) failed (%s)\n",i,strerror(errno));
  38. //            return -1;
  39.         }
  40.     }

  42.     if(argc > 1)
  43.     {
  44.         int sleep_time = atoi(argv[1]);
  45.         sigfillset(&blockall_mask);

  47.         if(sigprocmask(SIG_SETMASK,&blockall_mask,NULL) == -1)
  48.         {
  49.             fprintf(stderr,"setprocmask to block all signal failed(%s)\n",strerror(errno));
  50.             return -2;
  51.         }

  53.         printf("I will sleep %d second\n",sleep_time);

  55.         sleep(sleep_time);
  56.         if(sigpending(&pending_mask) == -1)
  57.         {
  58.             fprintf(stderr,"sigpending failed(%s)\n",strerror(errno));
  59.             return -2;
  60.         }

  62.         for(i = 1 ; i < NSIG ; i++)
  63.         {
  64.             if(sigismember(&pending_mask,i))
  65.             printf("signo(%d) :%s\n",i,strsignal(i));
  66.         }

  68.         sigemptyset(&empty_mask);
  69.         if(sigprocmask(SIG_SETMASK,&empty_mask,NULL) == -1)
  70.         {
  71.             fprintf(stderr,"setprocmask to release all signal failed(%s)\n",strerror(errno));
  72.             return -3;
  73.         }
  74.         
  75.     }

  77.     while(!get_SIGINT)
  78.         continue ; //why not use pause ? I will explain later

  80.     for(i = 1; i < NSIG ; i++)
  81.     {
  82.         if(sig_cnt[i] != 0 )
  83.         {
  84.             printf("%s:signal %d caught %d time%s\n",
  85.                     argv[0],i,sig_cnt[i],(sig_cnt[i] >1)?"s":"");
  86.         }
  87.     }

  89.     return 0;

  91. }

     因为我们知道，SIGKILL和SIGSTOP这两个信号是不能够定制自己的信号处理函数的，当然也不能block，原因很简单，OS或者说root才是final boss，必须有稳定终结进程的办法。假如所有的信号，进程都能ignore，OS如何终结进程？
    这个signal_receiver会等待所有的信号，接收到某信号后，该信号的捕捉到的次数++，SIGINT会终结进程，进程退出前，会打印信号的捕捉统计。
    如果进程有参数，表示sleep时间，signal_receiver会先屏蔽所有信号（当然，SIGKILL和SIGSTOP并不能被真正屏蔽）。然后sleep 一段时间后，取消信号屏蔽。我们可以想象，在信号屏蔽期间，我们收到的信号，都会在kernel记录下来，但是并不能delivery，这种信号称之挂起信号。如果在sleep期间或者说信号屏蔽期间，我收到SIGUSR1 这个信号1次和10000次，对内核来说，都是没差别的，因为后面的9999次都会被ignore掉。SIGUSR1属于不可靠信号，位图表示有没有挂起信号，有的话，直接ignore，没有的话，则记录在kernel。
    然后我们看下，signal_sender： 

  1. [email protected]-hacks:~/code/c/self/signal$ cat signal_sender.c
  2. #include <stdio.h>
  3. #include <stdlib.h>
  4. #include <getopt.h>
  5. #include <signal.h>
  6. #include <string.h>
  7. #include <errno.h>

  9. void usage()
  10. {
  11.     fprintf(stderr,"USAGE:\n");
  12.     fprintf(stderr,"--------------------------------\n");
  13.     fprintf(stderr,"signal_sender pid signo times\n");
  14. }

  16. int main(int argc,char* argv[])
  17. {
  18.     pid_t pid = -1 ;
  19.     int signo = -1;
  20.     int times = -1;
  21.     int i ;


  24.     if(argc < 4 )
  25.     {
  26.         usage();
  27.         return -1;
  28.     }
  29.     
  30.     pid = atol(argv[1]);
  31.     signo = atoi(argv[2]);
  32.     times = atoi(argv[3]);

  34.     if(pid <= 0 || times < 0 || signo <1 ||signo >=64 ||signo == 32 || signo ==33)
  35.     {
  36.         usage();
  37.         return -1;
  38.     }

  40.     printf("pid = %ld,signo = %d,times = %d\n",pid,signo,times);

  42.     for( i = 0 ; i < times ; i++)
  43.     {
  44.         if(kill(pid,signo) == -1)
  45.         {
  46.             fprintf(stderr, "send signo(%d) to pid(%ld) failed,reason(%s)\n",signo,pid,strerror(errno));
  47.             return -2;
  48.         }
  49.     }
  50.     fprintf(stdout,"done\n");
  51.     return 0;

  53. }

     signal_sender需要三个参数，pid signo times，就是向拿个进程发送什么信号多少次的意思。如 signal_sender 1234 10 10000,含义是向pid=1234的 进程发送10号信号（SIGUSR1），连续发送10000次。
    有这两个进程，我们就可以实验了  。 

  1. [email protected]-hacks:~/code/c/self/signal$ ./signal_receiver &
  2. [1] 23416
  3. [email protected]-hacks:~/code/c/self/signal$ ./signal_receiver:PID is 23416
  4. signal for signo(32) failed (Invalid argument)
  5. signal for signo(33) failed (Invalid argument)

  7. [email protected]-hacks:~/code/c/self/signal$ ./signal_sender 23416 10 10000
  8. pid = 23416,signo = 10,times = 10000
  9. done
  10. [email protected]-hacks:~/code/c/self/signal$ sleep 20 ; ./signal_sender 23416 2 1
  11. pid = 23416,signo = 2,times = 1
  12. done
  13. ./signal_receiver:signal 10 caught 2507 times
  14. [1]+ Done ./signal_receiver

    signal_receiver等待signal的来临，singal_sender向其发送SIGUSR1 10000次，然后sleep 20秒，确保sig_receiver处理完成。但是我们发现，其实一共才caught信号SIGUSR1  2507次，7000多次的发送都丢失了，所以我们称SIGUSR1 是非可靠信号，存在丢信号的问题。
    俗话说不怕不识货，就怕货比货 ，我们让可靠信号参战，看下效果：

  1. [email protected]-hacks:~/code/c/self/signal$ ./signal_receiver &
  2. [1] 26067
  3. ./signal_receiver:PID is 26067
  4. signal for signo(32) failed (Invalid argument)
  5. signal for signo(33) failed (Invalid argument)
  6. [email protected]-hacks:~/code/c/self/signal$ ./signal_sender 26067 10 10000
  7. pid = 26067,signo = 10,times = 10000
  8. done
  9. [email protected]-hacks:~/code/c/self/signal$ ./signal_sender 26067 36 10000
  10. pid = 26067,signo = 36,times = 10000
  11. done
  12. [email protected]-hacks:~/code/c/self/signal$ ./signal_sender 26067 2 1
  13. pid = 26067,signo = 2,times = 1
  14. done
  15. ./signal_receiver:signal 10 caught 2879 times
  16. ./signal_receiver:signal 36 caught 10000 times
  17. [1]+ Done ./signal_receiver

    可靠性信号36，发送10000次，signal_receiver全部收到，不可靠性信号10，共收到2879次。这个数字是不可预期的，取决于内核进程的调度。
    这个如果还不够直观，我们在比较一次，让signal_receiver先屏蔽所有信号一段时间，如30s，然后解除屏蔽。

  1. [email protected]-hacks:~/code/c/self/signal$ ./signal_receiver 30 &
  2. [1] 27639
  3. [email protected]-hacks:~/code/c/self/signal$ ./signal_receiver:PID is 27639
  4. signal for signo(32) failed (Invalid argument)
  5. signal for signo(33) failed (Invalid argument)
  6. I will sleep 30 second

  8. [email protected]-hacks:~/code/c/self/signal$ ./signal_sender 27639 10 10000
  9. pid = 27639,signo = 10,times = 10000
  10. done
  11. [email protected]-hacks:~/code/c/self/signal$ ./signal_sender 27639 36 10000
  12. pid = 27639,signo = 36,times = 10000
  13. done
  14. [email protected]-hacks:~/code/c/self/signal$
  15. [email protected]-hacks:~/code/c/self/signal$ signo(10) :User defined signal 1
  16. signo(36) :Real-time signal 2

  18. [email protected]-hacks:~/code/c/self/signal$ ./signal_sender 27639 2 1
  19. pid = 27639,signo = 2,times = 1
  20. done
  21. ./signal_receiver:signal 10 caught 1 time
  22. ./signal_receiver:signal 36 caught 10000 times
  23. [1]+ Done ./signal_receiver 30

      这个比较反差比较大，不可靠signal10 共收到1次，可靠性信号36 共caught到10000次。原因就在于sigprocmask将所有的信号都屏蔽了，造成所有的信号都不能delivery。对1～31的信号，内核发现已经有相应的挂起信号，则ignore到新来的信号。但是可靠性信号则不同，会添加队列中去，尽管已经有了相同的信号。需要注意的是，signal pending有上限，并不能无限制的发：

  1. [email protected]-hacks:~/code/c/self/signal$ ulimit -a
  2. core file size (blocks, -c) 0
  3. data seg size (kbytes, -d) unlimited
  4. scheduling priority (-e) 0
  5. file size (blocks, -f) unlimited
  6. pending signals (-i) 15408
  7. max locked memory (kbytes, -l) 64
  8. max memory size (kbytes, -m) unlimited
  9. open files (-n) 1024
  10. pipe size (512 bytes, -p) 8
  11. POSIX message queues (bytes, -q) 819200
  12. real-time priority (-r) 0
  13. stack size (kbytes, -s) 8192
  14. cpu time (seconds, -t) unlimited
  15. max user processes (-u) 15408
  16. virtual memory (kbytes, -v) unlimited
  17. file locks (-x) unlimited

    我发送100万，最终会收到15408个可靠信号：

  1. [email protected]-hacks:~/code/c/self/signal$ ./signal_receiver 30 &
  2. [1] 16488
  3. [email protected]-hacks:~/code/c/self/signal$ ./signal_receiver:PID is 16488
  4. signal for signo(32) failed (Invalid argument)
  5. signal for signo(33) failed (Invalid argument)
  6. I will sleep 30 second

  8. [email protected]-hacks:~/code/c/self/signal$ ./signal_sender 16488 36 1000000
  9. pid = 16488,signo = 36,times = 1000000
  10. done
  11. [email protected]-hacks:~/code/c/self/signal$ signo(36) :Real-time signal 2

  13. [email protected]-hacks:~/code/c/self/signal$ ./signal_sender 16488 2 1
  14. pid = 16488,signo = 2,times = 1
  15. done
  16. ./signal_receiver:signal 36 caught 15408 times
  17. [1]+ Done ./signal_receiver 30

      内核是怎么做到的？
    技术分享
    上图是内核中signal相关的数据结构。其中task_struct中有sigpending类型的成员变量pending
    

  1. struct task_struct {
  2.     volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */
  3.     void *stack;
  4.     atomic_t usage;
  5.     unsigned int flags; /* per process flags, defined below */
  6.     unsigned int ptrace;
  7.         ...
  8.         ...
  9. /* signal handlers */
  10.     struct signal_struct *signal;
  11.     struct sighand_struct *sighand;

  13.     sigset_t blocked, real_blocked;
  14.     sigset_t saved_sigmask; /* restored if set_restore_sigmask() was used */
  15.     struct sigpending pending;

  17.        ...
  18. }

  20. struct signal_struct {
  21.       atomic_t     sigcnt;
  22.       atomic_t     live;
  23.       int     nr_threads;
  24.       ...
  25.       ...
  26.       /* shared signal handling: */
  27.       struct sigpending    shared_pending;
  28.       ...
  29. }

  31. struct sigpending {
  32.     struct list_head list;
  33.     sigset_t signal;
  34. };

  36. #define _NSIG        64

  38. #ifdef __i386__
  39. # define _NSIG_BPW    32
  40. #else
  41. # define _NSIG_BPW    64
  42. #endif

  44. #define _NSIG_WORDS    (_NSIG / _NSIG_BPW)

  46. typedef unsigned long old_sigset_t;        /* at least 32 bits */

  48. typedef struct {
  49.     unsigned long sig[_NSIG_WORDS];
  50. } sigset_t;

      task_struct中的pending，和signal->shared_pending都是记录挂起信号的数据结构，读到此处，你可能会迷惑，为何有两个这样的结构。这牵扯到thread与信号的一些问题，我们此处简化，就认为是一个就好，后面讲述线程与信号关系的时候，再展开。
    技术分享  
    我们看到了，kill也好，tkill也罢，最终都走到了_send_signal.当然了kill系统调用根据pid的情况会分成多个分支如pid >0 pid = 0 pid=-1;pid < 0&pid !=-1,总之了，我的图只绘制了pid >0 的分支。tkill也有类似情况。
    那么kernel是怎么做到的非可靠信号和可靠信号的的这些差别的呢？

  1. static int __send_signal(int sig, struct siginfo *info, struct task_struct *t,
  2.             int group, int from_ancestor_ns)
  3. {
  4.     struct sigpending *pending;
  5.     struct sigqueue *q;
  6.     int override_rlimit;
  7.     int ret = 0, result;

  9.     assert_spin_locked(&t->sighand->siglock);

  11.     result = TRACE_SIGNAL_IGNORED;
  12.     if (!prepare_signal(sig, t,
  13.             from_ancestor_ns || (info == SEND_SIG_FORCED)))
  14.         goto ret;

  16.     pending = group ? &t->signal->shared_pending : &t->pending;
  17.     /*
  18.      * Short-circuit ignored signals and support queuing
  19.      * exactly one non-rt signal, so that we can get more
  20.      * detailed information about the cause of the signal.
  21.      */
  22.     result = TRACE_SIGNAL_ALREADY_PENDING;
  23.     if (legacy_queue(pending, sig)) //如果是低于32的信号，并且已经在pending中出现了的信号，就直接返回了，ignore
  24.         goto ret;

  26.     result = TRACE_SIGNAL_DELIVERED;
  27.     /*
  28.      * fast-pathed signals for kernel-internal things like SIGSTOP
  29.      * or SIGKILL.
  30.      */
  31.     if (info == SEND_SIG_FORCED)
  32.         goto out_set;

  34.     /*
  35.      * Real-time signals must be queued if sent by sigqueue, or
  36.      * some other real-time mechanism. It is implementation
  37.      * defined whether kill() does so. We attempt to do so, on
  38.      * the principle of least surprise, but since kill is not
  39.      * allowed to fail with EAGAIN when low on memory we just
  40.      * make sure at least one signal gets delivered and don‘t
  41.      * pass on the info struct.
  42.      */
  43.     if (sig < SIGRTMIN)
  44.         override_rlimit = (is_si_special(info) || info->si_code >= 0);
  45.     else
  46.         override_rlimit = 0;
      //分配sigqueue结构，并且链入到相应的pending。
  1.     q = __sigqueue_alloc(sig, t, GFP_ATOMIC | __GFP_NOTRACK_FALSE_POSITIVE,
  2.         override_rlimit);
  3.     if (q) {
  4.         list_add_tail(&q->list, &pending->list);
  5.         switch ((unsigned long) info) {
  6.         case (unsigned long) SEND_SIG_NOINFO:
  7.             q->info.si_signo = sig;
  8.             q->info.si_errno = 0;
  9.             q->info.si_code = SI_USER;
  10.             q->info.si_pid = task_tgid_nr_ns(current,
  11.                             task_active_pid_ns(t));
  12.             q->info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
  13.             break;
  14.         case (unsigned long) SEND_SIG_PRIV:
  15.             q->info.si_signo = sig;
  16.             q->info.si_errno = 0;
  17.             q->info.si_code = SI_KERNEL;
  18.             q->info.si_pid = 0;
  19.             q->info.si_uid = 0;
  20.             break;
  21.         default:
  22.             copy_siginfo(&q->info, info);
  23.             if (from_ancestor_ns)
  24.                 q->info.si_pid = 0;
  25.             break;
  26.         }

  28.         userns_fixup_signal_uid(&q->info, t);

  30.     } else if (!is_si_special(info)) {
  31.         if (sig >= SIGRTMIN && info->si_code != SI_USER) {
  32.             /*
  33.              * Queue overflow, abort. We may abort if the
  34.              * signal was rt and sent by user using something
  35.              * other than kill().
  36.              */
  37.             result = TRACE_SIGNAL_OVERFLOW_FAIL;
  38.             ret = -EAGAIN;
  39.             goto ret;
  40.         } else {
  41.             /*
  42.              * This is a silent loss of information. We still
  43.              * send the signal, but the *info bits are lost.
  44.              */
  45.             result = TRACE_SIGNAL_LOSE_INFO;
  46.         }
  47.     }

  49. out_set:
  50.     signalfd_notify(t, sig);
  51.     sigaddset(&pending->signal, sig);  //加入位图
  52.     complete_signal(sig, t, group);
  53. ret:
  54.     trace_signal_generate(sig, info, t, group, result);
  55.     return ret;
  56. }

  58. static inline int legacy_queue(struct sigpending *signals, int sig)
  59. {
  60.     return (sig < SIGRTMIN) && sigismember(&signals->signal, sig); //是不可靠信号，并且该信号已经存在挂起信号，

    那么15408的限制是在哪里呢？在__sigqueue_alloc 里面。

  1. static struct sigqueue *
  2. __sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags, int override_rlimit)
  3. {
  4.     struct sigqueue *q = NULL;
  5.     struct user_struct *user;

  7.     /*
  8.      * Protect access to @t credentials. This can go away when all
  9.      * callers hold rcu read lock.
  10.      */
  11.     rcu_read_lock();
  12.     user = get_uid(__task_cred(t)->user);
  13.     atomic_inc(&user->sigpending);    //计数器+1
  14.     rcu_read_unlock();

  16.     if (override_rlimit ||
  17.      atomic_read(&user->sigpending) <=
  18.             task_rlimit(t, RLIMIT_SIGPENDING)) {
  19.         q = kmem_cache_alloc(sigqueue_cachep, flags);
  20.     } else {
  21.         print_dropped_signal(sig);
  22.     }

  24.     if (unlikely(q == NULL)) {
  25.         atomic_dec(&user->sigpending);
  26.         free_uid(user);
  27.     } else {
  28.         INIT_LIST_HEAD(&q->list);
  29.         q->flags = 0;
  30.         q->user = user;
  31.     }

  33.     return q;
  34. }

     我们看到，legacy_queue就是用来判断是否是非可靠信号（signo低于32），并且相同signo值已经存在在挂起信号之中，如果是，直接返回。
     而对于可靠信号，会分配一个sigqueue的结构，然后讲sigqueue链入到sigpending结构的中链表中。从而就不会丢失信号。当然对pending信号的总数作了限制，限制最多不可超过15408.当然了这个值是可以修改的：
    技术分享


参考文献：
1 Linux programming interface
2 深入理解linux内核
3 linux kernel 3.8.0 内核源码 

以上是关于Linux signal 那些事儿的主要内容，如果未能解决你的问题，请参考以下文章

Linux signal那些事儿

Linux signal 那些事儿信号的deliver顺序

Linux压缩那些事儿

有关linux的那些事儿

Linux压缩那些事儿

Linux中软硬链接那些事儿