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Subjectthe "read" syscall sees partial effects of the "write" syscall
Hi

I'd like to ask about this problem: when we write to a file, the kernel
takes the write inode lock. When we read from a file, no lock is taken -
thus the read syscall can read data that are halfway modified by the write
syscall.

The standard specifies the effects of the write syscall are atomic - see
this:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_09_07

> 2.9.7 Thread Interactions with Regular File Operations
>
> All of the following functions shall be atomic with respect to each
> other in the effects specified in POSIX.1-2017 when they operate on
> regular files or symbolic links:
>
> chmod() fchownat() lseek() readv() unlink()
> chown() fcntl() lstat() pwrite() unlinkat()
> close() fstat() open() rename() utime()
> creat() fstatat() openat() renameat() utimensat()
> dup2() ftruncate() pread() stat() utimes()
> fchmod() lchown() read() symlink() write()
> fchmodat() link() readlink() symlinkat() writev()
> fchown() linkat() readlinkat() truncate()
>
> If two threads each call one of these functions, each call shall either
> see all of the specified effects of the other call, or none of them. The
> requirement on the close() function shall also apply whenever a file
> descriptor is successfully closed, however caused (for example, as a
> consequence of calling close(), calling dup2(), or of process
> termination).

Should the read call take the read inode lock to make it atomic w.r.t. the
write syscall? (I know - taking the read lock causes big performance hit
due to cache line bouncing)

I've created this program to test it - it has two threads, one writing and
the other reading and verifying. When I run it on OpenBSD or FreeBSD, it
passes, on Linux it fails with "we read modified bytes".

Mikulas



#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <pthread.h>

#define L 65536

static int h;
static pthread_barrier_t barrier;
static pthread_t thr;

static char rpattern[L];
static char wpattern[L];

static void *reader(__attribute__((unused)) void *ptr)
{
while (1) {
int r;
size_t i;
r = pthread_barrier_wait(&barrier);
if (r > 0) fprintf(stderr, "pthread_barrier_wait: %s\n", strerror(r)), exit(1);
r = pread(h, rpattern, L, 0);
if (r != L) perror("pread"), exit(1);
for (i = 0; i < L; i++) {
if (rpattern[i] != rpattern[0])
fprintf(stderr, "we read modified bytes\n"), exit(1);
}
}
return NULL;
}

int main(__attribute__((unused)) int argc, char *argv[])
{
int r;
h = open(argv[1], O_RDWR | O_CREAT | O_TRUNC, 0644);
if (h < 0) perror("open"), exit(1);
r = pwrite(h, wpattern, L, 0);
if (r != L) perror("pwrite"), exit(1);
r = pthread_barrier_init(&barrier, NULL, 2);
if (r) fprintf(stderr, "pthread_barrier_init: %s\n", strerror(r)), exit(1);
r = pthread_create(&thr, NULL, reader, NULL);
if (r) fprintf(stderr, "pthread_create: %s\n", strerror(r)), exit(1);
while (1) {
size_t i;
for (i = 0; i < L; i++)
wpattern[i]++;
r = pthread_barrier_wait(&barrier);
if (r > 0) fprintf(stderr, "pthread_barrier_wait: %s\n", strerror(r)), exit(1);
r = pwrite(h, wpattern, L, 0);
if (r != L) perror("pwrite"), exit(1);
}
}

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