From owner-freebsd-current  Mon Oct 19 13:46:27 1998
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	Mon, 19 Oct 1998 16:45:32 -0400 (EDT)
Date: Mon, 19 Oct 1998 16:45:32 -0400 (EDT)
Message-Id: <199810192045.QAA21744@highwind.com>
From: HighWind Software Information <info@highwind.com>
To: eischen@vigrid.com
CC: current@FreeBSD.ORG, bright@hotjobs.com
In-reply-to: <199810191926.PAA10944@pcnet1.pcnet.com> (message from Daniel
	Eischen on Mon, 19 Oct 1998 15:26:27 -0400 (EDT))
Subject: Re: Another Serious libc_r problem
Sender: owner-freebsd-current@FreeBSD.ORG
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X-Loop: FreeBSD.ORG


   I believe your test program is in error.  Your threads are
   not ever going to awake from the pthread_cond_wait() statement
   because the mutex cannot be acquired.  Your main thread locks
   the mutex and never unlocks it.

I claim that doesn't matter. WHY should the CPU spin if I signal
twice on a lock I have? That is PERFECTLY valid.

Let's say I want to say that TWO events are ready to be handled.
That is perfectly fine.

---

Simply to placate your claim, here is another version that locks and
unlocks the mutex in proper fashion. I simply had to insert an
additional "sleep" to get the bug to appear.

In both cases, spinning the CPU at 100% and allowing the program to 
hang and make no forward progress is a SERIOUS bug.

I claim that this new version is 100% valid. Does proper locking, and
also works on other O/S's. On FreeBSD it does not work at all.

-Rob


-------

/* Illustration of FreeBSD pthread_cond_wait() bug
   
   This program sets up a conditional wait and fires off a dozen threads
   that simply wait for the condition. Once the threads are started, 
   the main thread loops signalling the condition once a second.

   Normally, this should result in "Signalling" and "Got Condition"
   being printed once a second. However, because of some bugs in
   FreeBSD, the pthread_cond_wait() spins the CPU and no progress is
   made.

   g++ -o condWaitBug -D_REENTRANT -D_THREAD_SAFE -g -Wall condWaitBug.C -pthread

*/

#include <assert.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

pthread_mutex_t lock;
pthread_cond_t condition;

static void *condThread(void *)
{
    // Wait until we are signalled, then print.
    while (true) {
	// Be sure to do proper locking and unlocking
	assert(!::pthread_mutex_lock(&lock));	
	assert(!::pthread_cond_wait(&condition, &lock));
	::printf("Got Condition!\n");
	assert(!::pthread_mutex_unlock(&lock));	
    }
}

int main(int, char **)
{
    // Initialize Lock
    pthread_mutexattr_t lock_attr;
    assert(!::pthread_mutexattr_init(&lock_attr));
    assert(!::pthread_mutex_init(&lock, &lock_attr));
    assert(!::pthread_mutexattr_destroy(&lock_attr));

    // Initialize Condition
    pthread_condattr_t cond_attr;
    assert(!::pthread_condattr_init(&cond_attr));
    assert(!::pthread_cond_init(&condition, &cond_attr));
    assert(!::pthread_condattr_destroy(&cond_attr));

    // Spawn off a dozen threads to get signalled
    for (int j = 0; j < 12; ++j) {
	pthread_t tid;
	pthread_attr_t attr;
	assert(!::pthread_attr_init(&attr));
	assert(!::pthread_create(&tid, &attr, condThread, 0));
	assert(!::pthread_attr_destroy(&attr));
    }

    // Sleep for 3 seconds to make sure the threads started up.
    timeval timeout;
    timeout.tv_sec = 3;
    timeout.tv_usec = 0;
    ::select(0, 0, 0, 0, &timeout);

    for (int k = 0; k < 60; ++k) {
	// Signal while locked
	assert(!::pthread_mutex_lock(&lock));	
	::printf("Signalling\n");
	assert(!::pthread_cond_signal(&condition));

	// Sleep for 1 second
	timeout.tv_sec = 1;
	timeout.tv_usec = 0;
	::select(0, 0, 0, 0, &timeout);

	::printf("Signalled again\n");
	assert(!::pthread_cond_signal(&condition));

	assert(!::pthread_mutex_unlock(&lock));	

	// Sleep for 1 second
	timeout.tv_sec = 1;
	timeout.tv_usec = 0;
	::select(0, 0, 0, 0, &timeout);
    }

    return EXIT_SUCCESS;
}

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