Date: Fri, 6 Oct 2000 06:41:42 -0700 (PDT) From: wilco.oelen@cmg.nl To: freebsd-gnats-submit@FreeBSD.org Subject: kern/21783: When msgrcv() blocks, it blocks ALL threads in multi-threaded application Message-ID: <20001006134142.41BDB37B672@hub.freebsd.org>
next in thread | raw e-mail | index | archive | help
>Number: 21783
>Category: kern
>Synopsis: When msgrcv() blocks, it blocks ALL threads in multi-threaded application
>Confidential: no
>Severity: critical
>Priority: high
>Responsible: freebsd-bugs
>State: open
>Quarter:
>Keywords:
>Date-Required:
>Class: sw-bug
>Submitter-Id: current-users
>Arrival-Date: Fri Oct 06 06:50:01 PDT 2000
>Closed-Date:
>Last-Modified:
>Originator: Wilco Oelen
>Release: 3.5.1
>Organization:
CMG
>Environment:
FreeBSD bsd1.cmg.nl 3.5.1-RELEASE FreeBSD 3.5.1-RELEASE #1: Thu Oct 5 16:16:53 GMT 2000 root@bsd1:/usr/src/sys/compile/LOCAL i386
>Description:
When an application uses multiple threads (created by pthread_create()), and one of the threads performs a blocking msgrcv() call, then all threads are blocked, not just the thread which does the msgrcv() call.
>How-To-Repeat:
We have created a little test program, which creates a thread, in which a little loop just prints a line of text every 200 milliseconds. The main thread reads from a message queue, which is created by the program, just before the thread is created.
The program can be compiled using the command
cc -D_THREAD_SAFE -o tst tst.c -pthread
Here it is assumed that the program source is called tst.c.
When the program is started, without arguments, then you see that it prints a line of text every 200 milliseconds. After 5 seconds this stops. The program creates a message queue with key 12345678. When this key is removed (using ipcrm -q <ID>, where <ID> is the queue id for the queue with KEY==12345678), then the program appears to continue and it mixes messages about read errors every second through the output.
We expect the program to print messages every 200 milliseconds, also if the message queue is not yet broken and the main thread is still waiting in msgrcv().
Here follows the C-program:
/**************** START OF PROGRAM **********************/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
static void *thread_func(void *dummy);
int main(void)
{
int msqid_in;
key_t key = (key_t)12345678;
pthread_t thr1;
char msg[100];
/* Create a message queue. */
msqid_in = msgget(key, 0644 | IPC_CREAT);
/* Create a new thread. */
pthread_create(&thr1, NULL, thread_func, NULL);
/* Sleep a while, in the meantime the other thread runs. */
sleep(5);
while (1)
{
int status;
/* Read from the message queue. This read will block. */
/* We expect this thread to block, but the other thread */
/* should continue, however, it doesn't. */
status = msgrcv(msqid_in,
(struct msgbuf *)&msg, 10, 0, MSG_NOERROR);
if (status == -1)
{
/* This code is reached if one removes the message */
/* queue, while this program is running. If this */
/* code is reached, then the other thread also */
/* runs again. */
printf("ERROR receiving message\n");
sleep(1);
}
else
{
printf("Received message\n");
}
}
return 0;
}
static void *thread_func(void *dummy)
{
while (1)
{
usleep(200000);
printf("Thread function\n");
}
return NULL;
}
/********************* END OF PROGRAM **************************/
The problem also exists for programs, where the msgrcv() call is not in the main thread but in another thread. If a program is created with 3 or 4 threads, then also ALL threads block, if the msgrcv() call blocks.
We compared the behaviour with LINUX and Digital OSF4.0f. On these platforms the other threads continue if one of them blocks in a msgrcv() call.
>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted:
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-bugs" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20001006134142.41BDB37B672>