Date: Sat, 27 Nov 2004 08:30:00 +0800 From: David Xu <davidxu@freebsd.org> To: Alexander Bubnov <ab_fatal@mail.ru> Cc: freebsd-threads@freebsd.org Subject: Re: deadlock Message-ID: <41A7CA88.60505@freebsd.org> In-Reply-To: <41A72410.9080500@mail.ru> References: <41A72410.9080500@mail.ru>
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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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