Re: [PATCH] tty: fix stall caused by missing memory barrier indrivers/tty/n_tty.c

[Kosuke Tatsukawa]

This is a reproducer for the stall problem in drivers/tty/n_tty.c

To reproduce the problem, save the program below as pty.c, compile it,
and run it in parallel.

# cc -o pty pty.c
# for i in {1..16}; do ./pty& done; wait

The problem can be reproduced on a multi-socket server with recent CPUs.
The program always stalled during the first run when I used a server
with the following CPU.
  Intel(R) Xeon(R) CPU E5-2698 v3 @ 2.30GHz
  2-sockets x 16-cores x 2-threads

Best regards.
---
Kosuke TATSUKAWA  | 3rd IT Platform Department
                  | IT Platform Division, NEC Corporation
                  | tatsu@xxxxxxxxxxxxx

--------------------------------< pty.c >--------------------------------
#define _XOPEN_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <sys/wait.h>
#include <sys/time.h>

#define COUNT	300000
#define SIZE	1

char	data[SIZE];

int
main(
	int	argc,
	char	**argv)
{
	int		i, fdp, fdt, status;
	char		*slavename;
	struct termios	t;
	pid_t		pid;
	struct timeval	tv1, tv2;

	if ((fdp = open("/dev/ptmx", 2)) < 0) {
		perror("ptmx open");
		exit(1);
	}
	grantpt(fdp);
	unlockpt(fdp);
	slavename = ptsname(fdp);
	if ((fdt = open(slavename, 2)) < 0) {
		perror("pty open");
		close(fdp);
		exit(1);
	}
	tcgetattr(fdt, &t);
	t.c_lflag &= ~ECHO;
	t.c_oflag &= ~OPOST;
	t.c_lflag &= ~ICANON;
	t.c_cc[VMIN] = 1;
	t.c_cc[VTIME] = 0;
	tcsetattr(fdt, TCSANOW, &t);

	tcgetattr(fdp, &t);
	t.c_lflag &= ~ECHO;
	t.c_oflag &= ~OPOST;
	t.c_lflag &= ~ICANON;
	t.c_cc[VMIN] = 1;
	t.c_cc[VTIME] = 0;
	tcsetattr(fdp, TCSANOW, &t);

	memset(data, 'A', SIZE);

	if ((pid = fork()) < 0) {
		perror("fork");
		close(fdt);
		close(fdp);
		exit(1);
	}
	if (pid == 0) {		/* Child */
		sleep(1);
		for (i = 0; i < COUNT; i++) {
			if (write(fdt, data, SIZE) != SIZE) {
				perror("write fdt");
				exit(1);
			}
			if (read(fdt, data, SIZE) != SIZE) {
				perror("read fdt");
				exit(1);
			}
		}
		exit(0);
	} else {		/* Parent */
		for (i = 0; i < COUNT; i++) {
			if (i == 1)
				gettimeofday(&tv1, NULL);
			if (read(fdp, data, SIZE) != SIZE) {
				perror("read fdp");
				exit(1);
			}
			if (write(fdp, data, SIZE) != SIZE) {
				perror("write fdp");
				exit(1);
			}
		}
	}
	gettimeofday(&tv2, NULL);
#if 0
	printf("%d\n", (tv2.tv_sec - tv1.tv_sec) * 1000000 +
		tv2.tv_usec - tv1.tv_usec);
#endif
	wait(&status);
	exit(0);
}
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