From: Joel Fernandes <joel@joelfernandes.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Christian Brauner <christian@brauner.io>,
linux-kernel@vger.kernel.org, luto@amacapital.net,
rostedt@goodmis.org, dancol@google.com, sspatil@google.com,
jannh@google.com, surenb@google.com, timmurray@google.com,
Jonathan Kowalski <bl0pbl33p@gmail.com>,
torvalds@linux-foundation.org, kernel-team@android.com,
Andrew Morton <akpm@linux-foundation.org>,
Arnd Bergmann <arnd@arndb.de>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Ingo Molnar <mingo@kernel.org>, Jann Horn <jann@thejh.net>,
linux-kselftest@vger.kernel.org, Michal Hocko <mhocko@suse.com>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>,
Serge Hallyn <serge@hallyn.com>, Shuah Khan <shuah@kernel.org>,
Stephen Rothwell <sfr@canb.auug.org.au>,
Thomas Gleixner <tglx@linutronix.de>,
Tycho Andersen <tycho@tycho.ws>,
viro@zeniv.linux.org.uk, linux-api@vger.kernel.org
Subject: Re: [PATCH v1 1/2] Add polling support to pidfd
Date: Mon, 29 Apr 2019 10:02:45 -0400 [thread overview]
Message-ID: <20190429140245.GB233442@google.com> (raw)
In-Reply-To: <20190428162405.GA6757@redhat.com>
On Sun, Apr 28, 2019 at 06:24:06PM +0200, Oleg Nesterov wrote:
> Thanks for cc'ing me...
>
> On 04/26, Christian Brauner wrote:
> >
> > On Thu, Apr 25, 2019 at 03:00:09PM -0400, Joel Fernandes (Google) wrote:
> > > +static unsigned int pidfd_poll(struct file *file, struct poll_table_struct *pts)
> > > +{
> > > + struct task_struct *task;
> > > + struct pid *pid;
> > > + int poll_flags = 0;
> > > +
> > > + /*
> > > + * 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);
> > > + pid = file->private_data;
> > > + task = pid_task(pid, PIDTYPE_PID);
> > > + WARN_ON_ONCE(task && !thread_group_leader(task));
> > > +
> > > + if (!task || (task->exit_state && thread_group_empty(task)))
> > > + poll_flags = POLLIN | POLLRDNORM;
>
> Joel, I still can't understand why do we need tasklist... and I don't really
> understand the comment. The code looks as if you are trying to avoid poll_wait(),
> but this would be strange.
>
> OK, why can't pidfd_poll() do
>
> poll_wait(file, &pid->wait_pidfd, pts);
>
> rcu_read_lock();
> task = pid_task(pid, PIDTYPE_PID);
> if (!task || task->exit_state && thread_group_empty(task))
> poll_flags = POLLIN | ...;
> rcu_read_unlock();
>
> return poll_flags;
>
> ?
Oh that's much better Oleg, and would avoid the race I had in mind: Basically
I was acquiring the tasklist_lock to avoid a case where a polling task is not
woken up because it was added to the waitqueue too late. The reading of the
exit_state and the conditional adding to the wait queue, needed to be atomic.
Otherwise something like the following may be possible:
Task A (poller) Task B (exiting task being polled)
------------ ----------------
poll() called
exit_state is set to non-zero
read exit_state
wake_up_all()
add_wait_queue()
----------------------------------------------
However, in your code above, it is avoided because we get:
Task A (poller) Task B (exiting task being polled)
------------ ----------------
poll() called
add_wait_queue()
exit_state is set to non-zero
read exit_state
remove_wait_queue()
wake_up_all()
I don't see any other issues with your code above so I can try it out and
update the patches. Thanks.
> > > +static void do_notify_pidfd(struct task_struct *task)
> >
> > Maybe a short command that this helper can only be called when we know
> > that task is a thread-group leader wouldn't hurt so there's no confusion
> > later.
>
> Not really. If the task is traced, do_notify_parent() (and thus do_notify_pidfd())
> can be called to notify the debugger even if the task is not a leader and/or if
> it is not the last thread. The latter means a spurious wakeup for pidfd_poll().
Seems like you are replying to Christian's point. I agree with you.
> > > +{
> > > + struct pid *pid;
> > > +
> > > + lockdep_assert_held(&tasklist_lock);
> > > +
> > > + pid = get_task_pid(task, PIDTYPE_PID);
> > > + wake_up_all(&pid->wait_pidfd);
> > > + put_pid(pid);
>
> Why get/put?
Yes, pid_task() should do it. Will update it. Thanks!
- Joel
next prev parent reply other threads:[~2019-04-29 14:02 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-25 19:00 [PATCH v1 1/2] Add polling support to pidfd Joel Fernandes (Google)
2019-04-25 19:00 ` [PATCH v1 2/2] Add selftests for pidfd polling Joel Fernandes (Google)
2019-04-25 20:00 ` Tycho Andersen
2019-04-26 13:47 ` Joel Fernandes
2019-04-25 21:29 ` Christian Brauner
2019-04-25 22:07 ` Daniel Colascione
2019-04-26 17:26 ` Joel Fernandes
2019-04-26 19:35 ` Daniel Colascione
2019-04-26 20:31 ` Joel Fernandes
2019-04-26 13:42 ` Joel Fernandes
2019-04-25 22:24 ` [PATCH v1 1/2] Add polling support to pidfd Christian Brauner
2019-04-26 14:23 ` Joel Fernandes
2019-04-26 15:21 ` Christian Brauner
2019-04-26 15:31 ` Christian Brauner
2019-04-28 16:24 ` Oleg Nesterov
2019-04-29 14:02 ` Joel Fernandes [this message]
2019-04-29 14:07 ` Joel Fernandes
2019-04-29 14:25 ` Oleg Nesterov
2019-04-29 14:20 ` Oleg Nesterov
2019-04-29 16:32 ` Joel Fernandes
2019-04-30 11:53 ` Oleg Nesterov
2019-04-30 12:07 ` Oleg Nesterov
2019-04-30 15:49 ` Joel Fernandes
2019-04-26 14:58 ` Christian Brauner
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190429140245.GB233442@google.com \
--to=joel@joelfernandes.org \
--cc=akpm@linux-foundation.org \
--cc=arnd@arndb.de \
--cc=bl0pbl33p@gmail.com \
--cc=christian@brauner.io \
--cc=dancol@google.com \
--cc=ebiederm@xmission.com \
--cc=gregkh@linuxfoundation.org \
--cc=jann@thejh.net \
--cc=jannh@google.com \
--cc=kernel-team@android.com \
--cc=linux-api@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-kselftest@vger.kernel.org \
--cc=luto@amacapital.net \
--cc=mhocko@suse.com \
--cc=mingo@kernel.org \
--cc=oleg@redhat.com \
--cc=peterz@infradead.org \
--cc=rostedt@goodmis.org \
--cc=serge@hallyn.com \
--cc=sfr@canb.auug.org.au \
--cc=shuah@kernel.org \
--cc=sspatil@google.com \
--cc=surenb@google.com \
--cc=tglx@linutronix.de \
--cc=timmurray@google.com \
--cc=torvalds@linux-foundation.org \
--cc=tycho@tycho.ws \
--cc=viro@zeniv.linux.org.uk \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).