linux-kernel - Futex wait is not interrupted by signals if no timeout is provided

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Message-ID: <88516d10-bd53-9725-77f5-2772a6c784@elronnd.net>
Date:   Fri, 12 Aug 2022 18:14:48 -0700 (PDT)
From:   Elijah Stone <elronnd-linux@...onnd.net>
To:     linux-kernel@...r.kernel.org
Subject: Futex wait is not interrupted by signals if no timeout is provided

I've observed a strange behaviour of futexes that I believe to be a bug.  If a 
signal is delivered while a futex wait is in progress, the wait will be 
interrupted, but only if that wait had a timeout.  I expect it to be 
interrupted regardless of whether a timeout is provided.  The following 
runnable snippet demonstrates this:

void handler(int) { printf("SIGINT\n"); }
int main() {
 	signal(SIGINT,handler);
 	int i=2;
 	printf("1\n");
 	syscall(SYS_futex, &i, FUTEX_WAIT, 2, &(struct timespec){.tv_sec=1000});
 	printf("2\n");
 	syscall(SYS_futex, &i, FUTEX_WAIT, 2, NULL);
 	printf("3\n");
}

Pressing ^C once causes the first wait to terminate, and '2' to be printed. 
But repeatedly pressing it after that does not allow the program to make 
progress.