From: Joel Fernandes <joel@joelfernandes.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: linux-kernel@vger.kernel.org, luto@amacapital.net,
rostedt@goodmis.org, dancol@google.com, christian@brauner.io,
jannh@google.com, surenb@google.com,
torvalds@linux-foundation.org,
Alexey Dobriyan <adobriyan@gmail.com>,
Al Viro <viro@zeniv.linux.org.uk>,
Andrei Vagin <avagin@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Arnd Bergmann <arnd@arndb.de>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Kees Cook <keescook@chromium.org>,
linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org,
Michal Hocko <mhocko@suse.com>, Nadav Amit <namit@vmware.com>,
Serge Hallyn <serge@hallyn.com>, Shuah Khan <shuah@kernel.org>,
Stephen Rothwell <sfr@canb.auug.org.au>,
Taehee Yoo <ap420073@gmail.com>, Tejun Heo <tj@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
kernel-team@android.com, Tycho Andersen <tycho@tycho.ws>
Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd
Date: Tue, 16 Apr 2019 15:20:51 -0400 [thread overview]
Message-ID: <20190416192051.GA184889@google.com> (raw)
In-Reply-To: <20190416120430.GA15437@redhat.com>
On Tue, Apr 16, 2019 at 02:04:31PM +0200, Oleg Nesterov wrote:
> On 04/11, Joel Fernandes (Google) wrote:
> >
> > +static unsigned int proc_tgid_base_poll(struct file *file, struct poll_table_struct *pts)
> > +{
> > + int poll_flags = 0;
> > + struct task_struct *task;
> > + struct pid *pid;
> > +
> > + task = get_proc_task(file->f_path.dentry->d_inode);
> > +
> > + WARN_ON_ONCE(task && !thread_group_leader(task));
> > +
> > + /*
> > + * tasklist_lock must be held because to avoid racing with
> > + * changes in exit_state and wake up. Basically to avoid:
> > + *
> > + * P0: read exit_state = 0
> > + * P1: write exit_state = EXIT_DEAD
> > + * P1: Do a wake up - wq is empty, so do nothing
> > + * P0: Queue for polling - wait forever.
> > + */
> > + read_lock(&tasklist_lock);
> > + if (!task)
> > + poll_flags = POLLIN | POLLRDNORM | POLLERR;
> > + else if (task->exit_state == EXIT_DEAD)
> > + poll_flags = POLLIN | POLLRDNORM;
> > + else if (task->exit_state == EXIT_ZOMBIE && thread_group_empty(task))
> > + poll_flags = POLLIN | POLLRDNORM;
> > +
> > + if (!poll_flags) {
> > + pid = proc_pid(file->f_path.dentry->d_inode);
> > + poll_wait(file, &pid->wait_pidfd, pts);
> > + }
>
> can't understand...
>
> Could you explain when it should return POLLIN? When the whole process exits?
It returns POLLIN when the task is dead or doesn't exist anymore, or when it
is in a zombie state and there's no other thread in the thread group.
> Then all you need is
>
> !task || task->exit_state && thread_group_empty(task)
Yes this works as well, all the tests pass with your suggestion so I'll
change it to that. Although I will the be giving up returing EPOLLERR if the
task_struct doesn't exit. We don't need that, but I thought it was cool to
return it anyway.
> Please do not use EXIT_DEAD/EXIT_ZOMBIE. And ->wait_pidfd should probably
> live in task->signal_struct.
About wait_pidfd living in signal_struct, that wont work since the waitqueue
has to survive for the duration of the poll system call. Linus also confirmed
this: https://lore.kernel.org/patchwork/patch/1060650/#1257371
Also the waitqueue living in struct pid solves the de_thread() issue I
mentioned later in the following thread and in the commit message:
https://lore.kernel.org/patchwork/comment/1257175/
thanks,
- Joel
next prev parent reply other threads:[~2019-04-16 19:20 UTC|newest]
Thread overview: 66+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-11 17:50 [PATCH RFC 1/2] Add polling support to pidfd Joel Fernandes (Google)
2019-04-11 17:50 ` [PATCH RFC 2/2] Add selftests for pidfd polling Joel Fernandes (Google)
2019-04-12 14:51 ` Tycho Andersen
2019-04-11 20:00 ` [PATCH RFC 1/2] Add polling support to pidfd Joel Fernandes
2019-04-11 20:02 ` Christian Brauner
2019-04-11 20:20 ` Joel Fernandes
2019-04-12 21:32 ` Andy Lutomirski
2019-04-13 0:09 ` Joel Fernandes
[not found] ` <CAKOZuetX4jMPDtDqAvGgSNo4BHf9BOnu79ufEiULfM5X5nDyyQ@mail.gmail.com>
2019-04-13 0:56 ` Daniel Colascione
2019-04-14 18:19 ` Linus Torvalds
2019-04-16 12:04 ` Oleg Nesterov
2019-04-16 12:43 ` Oleg Nesterov
2019-04-16 19:20 ` Joel Fernandes [this message]
2019-04-16 19:32 ` Joel Fernandes
2019-04-17 13:09 ` Oleg Nesterov
2019-04-18 17:23 ` Jann Horn
2019-04-18 17:26 ` Christian Brauner
2019-04-18 17:53 ` Daniel Colascione
2019-04-19 19:02 ` Joel Fernandes
2019-04-19 19:18 ` Christian Brauner
2019-04-19 19:22 ` Christian Brauner
2019-04-19 19:42 ` Christian Brauner
2019-04-19 19:49 ` Joel Fernandes
2019-04-19 20:01 ` Christian Brauner
2019-04-19 21:13 ` Joel Fernandes
2019-04-19 20:34 ` Daniel Colascione
2019-04-19 20:57 ` Christian Brauner
2019-04-19 21:20 ` Joel Fernandes
2019-04-19 21:24 ` Daniel Colascione
2019-04-19 21:45 ` Joel Fernandes
2019-04-19 22:08 ` Daniel Colascione
2019-04-19 22:17 ` Christian Brauner
2019-04-19 22:37 ` Daniel Colascione
2019-04-24 8:04 ` Enrico Weigelt, metux IT consult
2019-04-19 21:59 ` Christian Brauner
2019-04-20 11:51 ` Oleg Nesterov
2019-04-20 12:26 ` Oleg Nesterov
2019-04-20 12:35 ` Christian Brauner
2019-04-19 23:11 ` Linus Torvalds
2019-04-19 23:20 ` Christian Brauner
2019-04-19 23:32 ` Linus Torvalds
2019-04-19 23:36 ` Daniel Colascione
2019-04-20 0:46 ` Joel Fernandes
2019-04-19 21:21 ` Daniel Colascione
2019-04-19 21:48 ` Christian Brauner
2019-04-19 22:02 ` Christian Brauner
2019-04-19 22:46 ` Daniel Colascione
2019-04-19 23:12 ` Christian Brauner
2019-04-19 23:46 ` Daniel Colascione
2019-04-20 0:17 ` Christian Brauner
2019-04-24 9:05 ` Enrico Weigelt, metux IT consult
2019-04-24 9:03 ` Enrico Weigelt, metux IT consult
2019-04-19 22:35 ` Daniel Colascione
2019-04-19 23:02 ` Christian Brauner
2019-04-19 23:29 ` Daniel Colascione
2019-04-20 0:02 ` Christian Brauner
2019-04-24 9:17 ` Enrico Weigelt, metux IT consult
2019-04-24 9:11 ` Enrico Weigelt, metux IT consult
2019-04-24 8:56 ` Enrico Weigelt, metux IT consult
2019-04-24 8:20 ` Enrico Weigelt, metux IT consult
2019-04-19 15:43 ` Oleg Nesterov
2019-04-19 18:12 ` Joel Fernandes
2019-04-18 18:44 ` Jonathan Kowalski
2019-04-18 18:57 ` Daniel Colascione
2019-04-18 19:14 ` Linus Torvalds
2019-04-19 19:05 ` Joel Fernandes
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