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

From: Christian Brauner <christian.brauner@ubuntu.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: linux-kernel@vger.kernel.org, Shuah Khan <shuah@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>, Michal Hocko <mhocko@suse.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Elena Reshetova <elena.reshetova@intel.com>,
	Christian Kellner <christian@kellner.me>,
	Roman Gushchin <guro@fb.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Aleksa Sarai <cyphar@cyphar.com>,
	"Dmitry V. Levin" <ldv@altlinux.org>,
	linux-kselftest@vger.kernel.org, Jann Horn <jannh@google.com>,
	linux-api@vger.kernel.org
Subject: Re: [PATCH 1/2] pidfd: verify task is alive when printing fdinfo
Date: Tue, 15 Oct 2019 16:56:47 +0200	[thread overview]
Message-ID: <20191015145646.72eqrw6j52ehvfn2@wittgenstein> (raw)
In-Reply-To: <20191015144356.GA16978@redhat.com>

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