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Date: Fri, 19 Nov 2021 03:25:29 +0900
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Subject: Re: Handles the masked signal when the thread exits
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To: Florian Weimer <fweimer@redhat.com>
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From: Gibeom Gwon <gb.gwon@stackframe.dev>
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On 11/19/21 03:19, Florian Weimer wrote:
> * Gibeom Gwon:
> 
>> On 11/19/21 03:08, Florian Weimer wrote:
>>> * Gibeom Gwon:
>>>
>>>> I am experiencing strange(unintended?) behavior when using pthread
>>>> with signals. If I set the signal mask with pthread_sigmask() in the
>>>> thread function and the process has pending signal when thread is
>>>> exiting, signal handler executed in thread.
>>>>
>>>> It looks like glibc restores original signal mask at the end of the
>>>> thread. So I suspect this eventually triggers masked signal
>>>> handler. But I'm not sure it is intended or not.
>>> Which glibc version are you using?
> 
>> Ah, I forgot to write the version. I'm using 2.33 and linux
>> distribution is Arch Linux.
> 
> We had some issues with signals and thread exit in 2.34, but they are
> exclusive to that release and cannot happen in 2.33.  I can't reproduce
> the behavior you see with upstream 2.33, either.
> 
> What's your kernel version?  I've tried 5.14.13 and 5.14.17.
> 
> Thanks,
> Florian
> 

Kernel version is 5.15.2.

Well, I accidently dropped sleep.c code sample. Here are the code
samples again. Sorry to bother you.

sleep.c
-------
#include <unistd.h>

int main() {
	sleep(50);
	return 0;
}

signal.c
--------
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>
#include <pthread.h>

pthread_t thread;
pid_t pid;

void stop_threads() {
		pthread_cancel(thread);
		pthread_join(thread, NULL);
}

void sigint_handler(int signum) {
	printf("sigint: %ld\n",pthread_self());
	stop_threads();
	exit(0);
}

void sigchld_handler(int signum) {
	printf("sigchld: %ld\n",pthread_self());
	pid_t pid;
	int status;
	while((pid = waitpid(-1,&status,WNOHANG)) > 0) {}
}

void spawn_sleep() {
	pid = fork();
	if(pid == 0)
		execl("./sleep","sleep",NULL);
}

void* worker(void *arg) {
	sigset_t mask;
	sigfillset(&mask);
	pthread_sigmask(SIG_SETMASK,&mask,NULL);

	pthread_setcancelstate(PTHREAD_CANCEL_DISABLE,NULL);
	int i = 10;
	while(i) {
		printf("worker...\n");
		sleep(1);
		i--;
	}
	pthread_setcancelstate(PTHREAD_CANCEL_ENABLE,NULL);
     	printf("thread close\n");
	pthread_exit(0);
}

void spawn_thread() {
     pthread_create(&thread,NULL,worker,NULL);
     printf("child thread: %ld\n",thread);
}

int main() {
	printf("main thread: %ld\n",pthread_self());
	spawn_sleep();	

	struct sigaction sa1;
	sa1.sa_handler = sigint_handler;
	sigemptyset(&sa1.sa_mask);
	sigaddset(&sa1.sa_mask,SIGCHLD);
	sa1.sa_flags = 0;
	sigaction(SIGINT, &sa1, 0);

	struct sigaction sa2;
	sa2.sa_handler = sigchld_handler;
	sigemptyset(&sa2.sa_mask);
	sa2.sa_flags = 0;
	sigaction(SIGCHLD, &sa2, 0);

	spawn_thread();

	while(1)
		sleep(1);

	return 0;
}


Regards,
Gibeom Gwon