From mboxrd@z Thu Jan  1 00:00:00 1970
From: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH 1/2] pid: add pidfd_open()
Date: Wed, 15 May 2019 17:35:15 +0200
Message-ID: <20190515153515.GA20783@redhat.com>
References: <20190515100400.3450-1-christian@brauner.io>
 <20190515143857.GB18892@redhat.com>
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To: Christian Brauner <christian@brauner.io>
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List-Id: linux-api@vger.kernel.org

On 05/15, Oleg Nesterov wrote:
>
> On 05/15, Christian Brauner wrote:
> >
> > +SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
> > +{
> > +	int fd, ret;
> > +	struct pid *p;
> > +	struct task_struct *tsk;
> > +
> > +	if (flags)
> > +		return -EINVAL;
> > +
> > +	if (pid <= 0)
> > +		return -EINVAL;
> > +
> > +	p = find_get_pid(pid);
> > +	if (!p)
> > +		return -ESRCH;
> > +
> > +	rcu_read_lock();
> > +	tsk = pid_task(p, PIDTYPE_PID);
>
> You do not need find_get_pid() before rcu_lock and put_pid() at the end.
> You can just do find_vpid() under rcu_read_lock().

Ah, sorry. Somehow I forgot you need to call pidfd_create(pid), you can't
do this under rcu_read_lock().

So I was wrong, you can't avoid get/put_pid.

Oleg.