Re: [PATCH 1/7] Freezer: Read PF_BORROWED_MM in a nonracy way

[Oleg Nesterov <oleg@tv-sign.ru>]

On 05/11, Linus Torvalds wrote:
>
> On Sat, 12 May 2007, Oleg Nesterov wrote:
> > 
> > without task_lock() we can see "p->mm != NULL" but not PF_BORROWED_MM.
> 
> Let me explain it one more time:
>  - shouldn't the *caller* protect this?
>
> Afaik, there's two situations:
>  - either things don't change (in which case you don't need locking at 
>    all, since things are statically one way or the other)
>  - or things change (in which case the caller can't rely on the return 
>    value anyway, since they might change *after* you release the lock)

things change, ->mm is not stable if the kernel thread does use_mm/unuse_mm.

However, the return value == 0 does not change in that particular case,
exactly because is_user_space() takes task_lock().

Oleg.