deadlock

[David Xu]

Your program has obvious race condition:
main: create thread A:

LOOP:

A: wait on Start
main: signal Start
main: wait on Restart
A: resumed from Start
A: Signal Restart
main: resumed from Restart
main: Create thread B
goto loop

This demo shows FreeBSD has better throughput then others.

David Xu

Alexander Bubnov wrote:

> /*
> Hello!
> Could you tell me why this exemple is deadlocked under FreeBSD, not 
> Linux?
> (FreeBSD 4.10 and 5.2 , I have not tried under other versions)
>
> a string that includes this problem marked, see below the main function
>
> */
> #include<sys/types.h>
> #include<sys/ipc.h>
> #include<sys/sem.h>
> #include<errno.h>
> #include<pthread.h>
> #include<stdlib.h>
> #include<stdio.h>
>
> int Exit=0;/* uses in hand(), changes in main() */
> int ReStart,Start;/* semaphores ID which returns semget() */
> /*
>    Start semphore controls starting threads,
>    EXETHREADS - how many threads can start in one time
>       ReStart semaphore for wait when the starting threads display a 
> string
> */
> struct sembuf dec={0},inc={0},op={0};/* they initilize in the main 
> function*/
> void*hand(void*p){
>    do{
>        semop(Start,&dec,1);
>        if(Exit)break;
>        puts(p);
>        semop(ReStart,&inc,1);
>    }while(1);
>    fprintf(stderr,"bye, %s!\n",p);
>    return NULL;
> }
>
> #define MAXTHREADS 5
> #define EXETHREADS 2 /* number of threads which is executed in one time*/
> #define ITERAT 2 /* number of iterations, see the main function */
>
> int main(void){
>    char*s[MAXTHREADS]={"Mike","Leo","Don","Raph","Splinter"};
>    int i;
>    pthread_t thread[MAXTHREADS];
>    /*  to initlize semaphores operations */
>    dec.sem_op=-1,inc.sem_op=1,op.sem_op=EXETHREADS;
>       if( -1==(Start=semget('A',1,0666|IPC_CREAT)) ||
>            -1==(ReStart=semget('B',1,0666|IPC_CREAT))
>        ){perror("semget");return EXIT_FAILURE;}
>       /* to initilize semaphores value */
>    semctl(Start,0,SETVAL,0);
>    semctl(ReStart,0,SETVAL,0);
>    #ifndef linux    /* may be this string is not needed, I do not know*/
>    if(pthread_setconcurrency(MAXTHREADS+1))perror("concurrency");
> #endif
>
>    /* to create the threads */
>    for(i=0;i<MAXTHREADS;++i)
>        pthread_create(thread+i,NULL,hand,s[i]);
>          for(i=0;i<ITERAT;++i){
>        puts("____START____");
>        semop(Start,&op,1);
>        /* to wait EXETHREADS threads */
>        op.sem_op*=-1;
>        /*
>            !!! this problem is here !!!
>                       if I insert 'pause()' or 'while(1)' instend of 
> 'semop(ReStart,&op,1)'
>            that only one thread is executed two time, why?
>                       this code does not help too (but I can see names 
> that is printed by hand())
>            op.sem_flg=IPC_NOWAIT
>            while(-1==semop(ReStart,&op,1) && EAGAIN);
>                   */
>        semop(ReStart,&op,1);
>        op.sem_op*=-1;
>               puts("____FINISH____");
>    }
>          /* to wait the threads to exit*/
>    semctl(Start,0,SETVAL,MAXTHREADS);
>    Exit=1;
>    for(i=0;i<MAXTHREADS;++i)
>        pthread_join(thread[i],NULL);
>   
>    if(    -1==semctl(ReStart,0,IPC_RMID) ||
>            -1==semctl(Start,0,IPC_RMID)
>        ) perror("semdel");
>           return EXIT_SUCCESS;
> }
>
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