linux-kernel - Re: [PATCH 1/2] pidfd: verify task is alive when printing fdinfo

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Date:   Tue, 15 Oct 2019 16:56:47 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Christian Kellner <christian@...lner.me>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Aleksa Sarai <cyphar@...har.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        linux-kselftest@...r.kernel.org, Jann Horn <jannh@...gle.com>,
        linux-api@...r.kernel.org
Subject: Re: [PATCH 1/2] pidfd: verify task is alive when printing fdinfo

On Tue, Oct 15, 2019 at 04:43:57PM +0200, Oleg Nesterov wrote:
> On 10/15, Christian Brauner wrote:
> >
> > +static inline bool task_alive(struct pid *pid)
> > +{
> > +	bool alive = true;
> > +
> > +	rcu_read_lock();
> > +	if (!pid_task(pid, PIDTYPE_PID))
> > +		alive = false;
> > +	rcu_read_unlock();
> > +
> > +	return alive;
> > +}
> 
> Well, the usage of rcu_read_lock/unlock looks confusing to me...
> 
> I mean, this helper does not need rcu lock at all. Except
> rcu_dereference_check() will complain.

Yep, I think we have another codepath were the rcu locks might be purely
cosmetic so I thought it's not a big deal (see below).

> 
> 	static inline bool task_alive(struct pid *pid)
> 	{
> 		bool alive;
> 
> 		/* shut up rcu_dereference_check() */
> 		rcu_lock_acquire(&rcu_lock_map);
> 		alive = !!pid_task(pid, PIDTYPE_PID));
> 		rcu_lock_release(&rcu_lock_map);
> 
> 		return alive;
> 	}
> 
> looks more clear imo.
> 
> But in fact I'd suggest to simply use !hlist_empty(&pid->tasks[PIDTYPE_PID])
> in pidfd_show_fdinfo() and do not add a new helper.

Sounds good to me. But can't we then just do something similar just with
!hlist_empty(&pid->tasks[PIDTYPE_TGID])

in v5.4-rc3:kernel/pid.c:pidfd_open():514-517 ?

or would this be problematic because of de_thread()?

Thanks!
Christian