SMP read() stopping at memory page boundaries

[Timo Sirainen <tss@iki.fi>]

Tested with various 2.6.x i386/x86-64 SMP kernels and CPUs, for example
2.6.21.3/x86-64.

Process 1:

 - lock file
 - write(4096 + 16 bytes)
 - unlock file

Process 2:

 - lock file
 - read(8192 bytes)
 - unlock file

Sometimes read() returns only 4096 bytes. I'm locking the file, so I
don't think this should ever happen, right?

Attached a test program. Takes from a few seconds to half a minute with
my computer to print "page size cut".

/*
   gcc concurrency2.c -o concurrency -Wall

   start two both a reader and a writer:

   ./concurrency
   ./concurrency 1
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <assert.h>
#include <sys/file.h>

#define MAX_PAGESIZE 8192

int main(int argc, char *argv[])
{
	char buf[MAX_PAGESIZE*2];
	int fd, ret, pagesize;

	memset(buf, 0, sizeof(buf));

	pagesize = getpagesize();
	assert(pagesize <= MAX_PAGESIZE);

	buf[pagesize] = 'h';
	if (argc == 1) {
		printf("writing, page size = %d\n", pagesize);
		for (;;) {
			fd = open("foo", O_RDWR | O_CREAT | O_TRUNC, 0600);
			if (fd == -1) {
				perror("open()");
				return 1;
			}

			if (flock(fd, LOCK_EX) < 0)
				perror("flock()");
			write(fd, buf, pagesize+16);
			if (flock(fd, LOCK_UN) < 0)
				perror("flock()");
			usleep(rand() % 1000);
			close(fd);
		}
	} else {
		printf("reading, page size = %d\n", pagesize);
		fd = open("foo", O_RDWR, 0600);
		if (fd == -1) {
			perror("open()");
			return 1;
		}
		for (;;) {
			usleep(rand() % 1000);
			if (flock(fd, LOCK_SH) < 0)
				perror("flock()");
			lseek(fd, 0, SEEK_SET);
			ret = read(fd, buf, sizeof(buf));
			if (flock(fd, LOCK_UN) < 0)
				perror("flock()");

			if (ret < pagesize) {
				if (ret > 0)
					printf("less than a page: %d\n", ret);
			} else if (ret == pagesize) {
				printf("page size cut\n");
			} else if (buf[pagesize] != 'h') {
				printf("broken data\n");
			}
		}
	}
	return 0;
}

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