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