SMP read() stopping at memory page boundaries

Timo Sirainen
2007-06-20T15:14:33Z

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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".

[-- Attachment #1.2: concurrency2.c --]
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/*
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;
}