Re: [PATCH] unserialized task->files changing (v2)

[Kirill Korotaev <dev@sw.ru>]

Eric,

> Sorry but there is something I dont understand. You ignored my point.
Sorry, I missed it thinking that you are talking about another thing...
Pavel described the race in more details and why barrier doesn't help.
Hope, it became more clear now.

> +void reset_files_struct(struct task_struct *tsk, struct files_struct *files)
> +{
> +       struct files_struct *old;
> +
> +       old = tsk->files;
> +       task_lock(tsk);
> +       tsk->files = files;
> +       task_unlock(tsk);
> +       put_files_struct(old);
> +}
> 
> Its seems very strange to protect tsk->files = files with a 
> task_lock()/task_unlock(). What is it supposed to guard against ???
> 
> If this patch corrects the 'bug', then a simpler fix would  be to use a memory 
> barrier between "tsk->files = files" and "put_files_struct(old);" 
> 
> No need to perform 2 atomics ops on the task lock.
> 
> old = tsk->files;
> tsk->files = files;
> smp_mb();
> put_files_struct(old);
> 
> That would be enough to guard against proc code (because this code only needs 
> to read tsk->files of course)
> 
> The same remark can be said for __exit_files() from kernel/exit.c
> 
> If this task_lock()/task_unlock() patch is really needed, then a comment in 
> the source would be very fair.

Kirill