Re: [PATCH] ring-buffer: Fix polling on trace_pipe

[Martin Lau <kafai@fb.com>]

Yes, I think it should go to both stable and 3.16.

Thanks,
--Martin

On Tue, Jul 15, 2014 at 03:46:12PM -0400, Steven Rostedt wrote:
> On Thu, 10 Jul 2014 15:20:30 -0700
> Martin Lau <kafai@fb.com> wrote:
> 
> > Hi Steve,
> > 
> > Do you have a chance to give it another try?
> > 
> 
> OK, I finally got around to testing it. Yep I see your point. Do you
> think it should go to 3.16 and stable? I can add it to my next git pull
> request.
> 
> Also, I modified your test such that it wouldn't hang on failure, but
> detects that it hung and returns a exit value to add this to my
> testing. That is, it exits with zero on success and non zero on failure.
> 
> -- Steve

> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <sys/types.h>
> #include <sys/wait.h>
> #include <sys/stat.h>
> #include <sys/epoll.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <assert.h>
> #include <signal.h>
> #include <errno.h>
> 
> static const char * debugfs_list[] = {
> 	"/debug/tracing",
> 	"/sys/kernel/debug/tracing",
> 	"/d/tracing",
> 	NULL,
> };
> 
> static const char *debugfs;
> static int markfd;
> static int trace_pipe_fd;
> 
> static void alog(const char *name, int ret)
> {
> 	printf("%d: %s: %d\n", getpid(), name, ret);
> }
> 
> static const char *find_debugfs(void)
> {
> 	struct stat st;
> 	int i;
> 	int r;
> 
> 	for (i = 0; debugfs_list[i]; i++) {
> 		r = stat(debugfs_list[i], &st);
> 		if (r < 0)
> 			continue;
> 		if (S_ISDIR(st.st_mode))
> 			return debugfs_list[i];
> 	}
> 	return NULL;
> }
> 
> static char * make_path(const char *file)
> {
> 	char *path;
> 	int size;
> 
> 	size = strlen(debugfs) + strlen(file) + 2;
> 	path = malloc(size);
> 	if (!path) {
> 		perror("malloc");
> 		exit(-1);
> 	}
> 	sprintf(path, "%s/%s", debugfs, file);
> 	return path;
> }
> 
> static void mark_write(const char *str)
> {
> 	int ret;
> 	ret = write(markfd, str, strlen(str));
> 	alog("write(markfd)", ret);
> }
> 
> static void read_trace_pipe(void)
> {
> 	char buf[1024];
> 	int r;
> 
> 	while ((r = read(trace_pipe_fd, buf, 1024)) > 0)
> 		printf("%.*s", r, buf);
> }
> 
> int main (int argc, char **argv)
> {
> 	struct epoll_event ee;
> 	char *marker;
> 	char *pipe;
> 	int efd;
> 	int ret;
> 	pid_t dwrt_pid;
> 
> 	debugfs = find_debugfs();
> 	if (!debugfs) {
> 		fprintf(stderr, "Could not find debugfs\n");
> 		exit(-1);
> 	}
> 
> 	marker = make_path("trace_marker");
> 	pipe = make_path("trace_pipe");
> 
> 	markfd = open(marker, O_WRONLY);
> 	if (markfd < 0) {
> 		perror("marker");
> 		exit(-1);
> 	}
> 	trace_pipe_fd = open(pipe, O_RDONLY|O_NONBLOCK);
> 	if (trace_pipe_fd < 0) {
> 		perror("trace_pipe");
> 		exit(-1);
> 	}
> 
> 	efd = epoll_create(1);
> 	if (efd < 0) {
> 		perror("epoll_create");
> 		exit(-1);
> 	}
> 
> 	mark_write("some data");
> 	memset(&ee, 0, sizeof(ee));
> 	ee.events = EPOLLIN;
> 	ret = epoll_ctl(efd, EPOLL_CTL_ADD, trace_pipe_fd, &ee);
> 	if (ret < 0) {
> 		perror("epoll_ctl");
> 		exit(-1);
> 	}
> 	alog("waiting data......", 0);
> 	ret = epoll_wait(efd, &ee, 1, -1);
> 	alog("epoll_wait()", ret);
> 	read_trace_pipe();
> 	dwrt_pid = fork();
> 	assert(dwrt_pid != -1);
> 	if (dwrt_pid > 0) {
> 		int status;
> 		sleep(10);
> 		mark_write("more data");
> 		sleep(10);
> 		ret = waitpid(dwrt_pid, &status, WNOHANG);
> 		if (ret != dwrt_pid) {
> 			alog("Poll never finished!", 0);
> 			kill(dwrt_pid, 9);
> 			exit(-1);
> 		}
> 		if (WEXITSTATUS(status) != 0) {
> 			alog("Something failed on polling!",
> 				 WEXITSTATUS(status));
> 			exit(-1);
> 		}
> 	} else {
> 		alog("waiting form more data......", 0);
> 		ret = epoll_wait(efd, &ee, 1, -1);
> 		alog("epoll_wait()", ret);
> 		read_trace_pipe();
> 		if (ret < 0)
> 			exit(errno);
> 	}
> 	exit (0);
> }