Re: [PATCH] kernel/fork: stop playing lockless games for exe_file replacement

[Oleg Nesterov <oleg@redhat.com>]

On 08/13, Mateusz Guzik wrote:
>
> fe69d560b5bd ("kernel/fork: always deny write access to current MM
> exe_file") added another lock trip to synchronize the state of exe_file
> against fork, further defeating the point of xchg.
>
> As such I think the atomic here only adds complexity for no benefit.
>
> Just write-lock around the replacement.

Well, I tend to agree but can't really comment because I forgot everything
about these code paths.

But I have to admit that I don't understand the code in replace_mm_exe_file()
without this patch...

	old_exe_file = xchg(&mm->exe_file, new_exe_file);
	if (old_exe_file) {
		/*
		 * Don't race with dup_mmap() getting the file and disallowing
		 * write access while someone might open the file writable.
		 */
		mmap_read_lock(mm);
		allow_write_access(old_exe_file);
		fput(old_exe_file);
		mmap_read_unlock(mm);
	}

Can someone please explain me which exactly race this mmap_read_lock() tries
to avoid and how ?

Oleg.