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分类: LINUX

2014-01-04 00:23:57

    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。

    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. manu@manu-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>


  8. static int sig_cnt[NSIG];
  9. static volatile sig_atomic_t get_SIGINT = 0;

  10. void handler(int signo)
  11. {
  12.     if(signo == SIGINT)
  13.         get_SIGINT = 1;
  14.     else
  15.         sig_cnt[signo]++;
  16. }

  17. int main(int argc,char* argv[])
  18. {
  19.     int i = 0;
  20.     sigset_t blockall_mask ;
  21.     sigset_t pending_mask ;
  22.     sigset_t empty_mask ;
  23.     printf("%s:PID is %ld\n",argv[0],getpid());

  24.     
  25.     for(i = 1; i < NSIG; i++)
  26.     {
  27.         if(i == SIGKILL || i == SIGSTOP)
  28.             continue;

  29.         if(signal(i,&handler) == SIG_ERR)
  30.         {
  31.             fprintf(stderr,"signal for signo(%d) failed (%s)\n",i,strerror(errno));
  32. //            return -1;
  33.         }
  34.     }

  35.     if(argc > 1)
  36.     {
  37.         int sleep_time = atoi(argv[1]);
  38.         sigfillset(&blockall_mask);

  39.         if(sigprocmask(SIG_SETMASK,&blockall_mask,NULL) == -1)
  40.         {
  41.             fprintf(stderr,"setprocmask to block all signal failed(%s)\n",strerror(errno));
  42.             return -2;
  43.         }

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

  45.         sleep(sleep_time);
  46.         if(sigpending(&pending_mask) == -1)
  47.         {
  48.             fprintf(stderr,"sigpending failed(%s)\n",strerror(errno));
  49.             return -2;
  50.         }

  51.         for(i = 1 ; i < NSIG ; i++)
  52.         {
  53.             if(sigismember(&pending_mask,i))
  54.             printf("signo(%d) :%s\n",i,strsignal(i));
  55.         }

  56.         sigemptyset(&empty_mask);
  57.         if(sigprocmask(SIG_SETMASK,&empty_mask,NULL) == -1)
  58.         {
  59.             fprintf(stderr,"setprocmask to release all signal failed(%s)\n",strerror(errno));
  60.             return -3;
  61.         }
  62.         
  63.     }

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

  66.     for(i = 1; i < NSIG ; i++)
  67.     {
  68.         if(sig_cnt[i] != 0 )
  69.         {
  70.             printf("%s:signal %d caught %d time%s\n",
  71.                     argv[0],i,sig_cnt[i],(sig_cnt[i] >1)?"s":"");
  72.         }
  73.     }

  74.     return 0;

  75. }
     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. manu@manu-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>

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

  14. int main(int argc,char* argv[])
  15. {
  16.     pid_t pid = -1 ;
  17.     int signo = -1;
  18.     int times = -1;
  19.     int i ;


  20.     if(argc < 4 )
  21.     {
  22.         usage();
  23.         return -1;
  24.     }
  25.     
  26.     pid = atol(argv[1]);
  27.     signo = atoi(argv[2]);
  28.     times = atoi(argv[3]);

  29.     if(pid <= 0 || times < 0 || signo <1 ||signo >=64 ||signo == 32 || signo ==33)
  30.     {
  31.         usage();
  32.         return -1;
  33.     }

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

  35.     for( i = 0 ; i < times ; i++)
  36.     {
  37.         if(kill(pid,signo) == -1)
  38.         {
  39.             fprintf(stderr, "send signo(%d) to pid(%ld) failed,reason(%s)\n",signo,pid,strerror(errno));
  40.             return -2;
  41.         }
  42.     }
  43.     fprintf(stdout,"done\n");
  44.     return 0;

  45. }
     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. manu@manu-hacks:~/code/c/self/signal$ ./signal_receiver &
  2. [1] 23416
  3. manu@manu-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)

  6. manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 23416 10 10000
  7. pid = 23416,signo = 10,times = 10000
  8. done
  9. manu@manu-hacks:~/code/c/self/signal$ sleep 20 ; ./signal_sender 23416 2 1
  10. pid = 23416,signo = 2,times = 1
  11. done
  12. ./signal_receiver:signal 10 caught 2507 times
  13. [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. manu@manu-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. manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 26067 10 10000
  7. pid = 26067,signo = 10,times = 10000
  8. done
  9. manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 26067 36 10000
  10. pid = 26067,signo = 36,times = 10000
  11. done
  12. manu@manu-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. manu@manu-hacks:~/code/c/self/signal$ ./signal_receiver 30 &
  2. [1] 27639
  3. manu@manu-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

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

  16. manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 27639 2 1
  17. pid = 27639,signo = 2,times = 1
  18. done
  19. ./signal_receiver:signal 10 caught 1 time
  20. ./signal_receiver:signal 36 caught 10000 times
  21. [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. manu@manu-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. manu@manu-hacks:~/code/c/self/signal$ ./signal_receiver 30 &
  2. [1] 16488
  3. manu@manu-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

  7. manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 16488 36 1000000
  8. pid = 16488,signo = 36,times = 1000000
  9. done
  10. manu@manu-hacks:~/code/c/self/signal$ signo(36) :Real-time signal 2

  11. manu@manu-hacks:~/code/c/self/signal$ ./signal_sender 16488 2 1
  12. pid = 16488,signo = 2,times = 1
  13. done
  14. ./signal_receiver:signal 36 caught 15408 times
  15. [1]+ Done ./signal_receiver 30
      内核是怎么做到的?
    
    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;

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

  15.        ...
  16. }

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

  27. struct sigpending {
  28.     struct list_head list;
  29.     sigset_t signal;
  30. };

  31. #define _NSIG        64

  32. #ifdef __i386__
  33. # define _NSIG_BPW    32
  34. #else
  35. # define _NSIG_BPW    64
  36. #endif

  37. #define _NSIG_WORDS    (_NSIG / _NSIG_BPW)

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

  39. typedef struct {
  40.     unsigned long sig[_NSIG_WORDS];
  41. } 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。
      
    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;

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

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

  13.     pending = group ? &t->signal->shared_pending : &t->pending;
  14.     /*
  15.      * Short-circuit ignored signals and support queuing
  16.      * exactly one non-rt signal, so that we can get more
  17.      * detailed information about the cause of the signal.
  18.      */
  19.     result = TRACE_SIGNAL_ALREADY_PENDING;
  20.     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
  21.         goto ret;

  22.     result = TRACE_SIGNAL_DELIVERED;
  23.     /*
  24.      * fast-pathed signals for kernel-internal things like SIGSTOP
  25.      * or SIGKILL.
  26.      */
  27.     if (info == SEND_SIG_FORCED)
  28.         goto out_set;

  29.     /*
  30.      * Real-time signals must be queued if sent by sigqueue, or
  31.      * some other real-time mechanism. It is implementation
  32.      * defined whether kill() does so. We attempt to do so, on
  33.      * the principle of least surprise, but since kill is not
  34.      * allowed to fail with EAGAIN when low on memory we just
  35.      * make sure at least one signal gets delivered and don't
  36.      * pass on the info struct.
  37.      */
  38.     if (sig < SIGRTMIN)
  39.         override_rlimit = (is_si_special(info) || info->si_code >= 0);
  40.     else
  41.         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.         }

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

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

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

  54. static inline int legacy_queue(struct sigpending *signals, int sig)
  55. {
  56.     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;

  6.     /*
  7.      * Protect access to @t credentials. This can go away when all
  8.      * callers hold rcu read lock.
  9.      */
  10.     rcu_read_lock();
  11.     user = get_uid(__task_cred(t)->user);
  12.     atomic_inc(&user->sigpending);    //计数器+1
  13.     rcu_read_unlock();

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

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

  29.     return q;
  30. }
     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:
    


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  

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给主人留下些什么吧!~~

mazinkaiser19912016-04-29 16:55:03

博主您好,最近读了您的文章对我启发很大。但我在阅读过程中有一些不懂的地方,忘您不吝赐教。您在第一篇文章中使用了一个实例来说明glibc的信号机制不会屏蔽正在处理的信号,根据那个实验SIGINT信号好像是被保存了起来,待程序准备好后再进行下一次处理。但在您第二篇文章中对于非实时信号,若当前有信号挂起则相同信号会被丢弃,所以按照这个说法后来的SIGINT信号应该被丢弃。所以我想问您的是glibc并没有严格实现这一语义?还是我什么地方理解有问题?

pppStar2015-12-21 16:19:21

写的很好

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