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Date: Sat, 14 Feb 2015 21:57:45 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Darren Hart <darren@dvhart.com>, Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jerome Marchand <jmarchan@redhat.com>,
        Larry Woodman <lwoodman@redhat.com>, Mateusz Guzik <mguzik@redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] futex: check PF_KTHREAD rather than !p->mm to
	filter out kthreads
Message-ID: <20150214205745.GA15695@redhat.com>
References: <20150202140515.GA26398@redhat.com> <20150202140536.GA26406@redhat.com> <1423936866.2046.88.camel@stgolabs.net>
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On 02/14, Davidlohr Bueso wrote:
>
> On Mon, 2015-02-02 at 15:05 +0100, Oleg Nesterov wrote:
> > --- a/kernel/futex.c
> > +++ b/kernel/futex.c
> > @@ -900,7 +900,7 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,
> >  	if (!p)
> >  		return -ESRCH;
> >
> > -	if (!p->mm) {
> > +	if (unlikely(p->flags & PF_KTHREAD)) {
> >  		put_task_struct(p);
> >  		return -EPERM;
> >  	}
>
> Futexes aren't the only naughty checkers,

Yes, this is the common mistake,

> a quick search shows that, at
> least, the oom killer and proc have this same problem.

oom looks fine, note the PF_KTHREAD check in oom_unkillable(). It check ->mm
for another reason, to figure out if this task has passed exit_mm() or not.

proc looks fine too at first glance... we do not care if this mm was adopted
via use_mm() or not.

> Should we make
> this generic and update accordingly? ie:
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 8db31ef..b0d37d6 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1991,6 +1991,11 @@ extern void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut,
>  #define tsk_used_math(p) ((p)->flags & PF_USED_MATH)
>  #define used_math() tsk_used_math(current)
>
> +static inline bool task_is_kthread(struct task_struct *task)
> +{
> +	return task->flags & PF_KTHREAD;
> +}

Perhaps... but PF_KTHREAD looks self-documented too ;)

Oleg.