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From: Salman Qazi on 20 May 2010 16:40
One of our internal workloads ran into a problem with waitpid. A
simple repro case is as follows:


#include <sys/types.h>
#include <sys/wait.h>
#include <sys/time.h>
#include <signal.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <assert.h>
#include <sched.h>

#define NUM_CPUS 4

void *thread_code(void *args)
{
int j;
int pid2;
for (j = 0; j < 1000; j++) {
pid2 = fork();
if (pid2 == 0)
while(1) { sleep(1000); }
}

while (1) {
int status;
if (waitpid(-1, &status, WNOHANG)) {
printf("! %d\n", errno);
}

}
exit(0);

}

/*
* non-blocking waitpids in tight loop, with many children to go through,
* done on multiple thread, so that they can "pass the torch" to eachother
* and eliminate the window that a writer has to get in.
*
* This maximizes the holding of the tasklist_lock in read mode, starving
* any attempts to take the lock in the write mode.
*/
int main(int argc, char **argv)
{
int i;
pthread_attr_t attr;
pthread_t threads[NUM_CPUS];
for (i = 0; i < NUM_CPUS; i++) {
assert(!pthread_attr_init(&attr));
assert(!pthread_create(&threads[i], &attr, thread_code));
}
while(1) { sleep(1000);}
return 0;
}


Basically, it is possibly for readers to continuously hold
tasklist_lock (theoretically forever, as they pass from one to other),
preventing the writer from taking that lock. This typically causes a
lockup on a CPU where a task is attempting to do a fork() or exit(),
resulting in the NMI watchdog firing.

Yes, WNOHANG is being used. And I agree that this is an inefficient
use of wait(). However, I think it should be possible to produce the
same effect without WNOHANG on sufficiently large number of threads:
by having it so that at least one thread always has the reader lock.

I think the most direct approach to the problem is to have the
readers-writer locks be writer biased (i.e. as soon as a writer
contends, we do not permit any new readers). However all suggestions
are welcome.
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