Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd
To:     Daniel Colascione <dancol@google.com>,
        Christian Brauner <christian@brauner.io>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Steven Rostedt <rostedt@goodmis.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Linus Torvalds <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 <linux-fsdevel@vger.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <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 <kernel-team@android.com>,
        Tycho Andersen <tycho@tycho.ws>
References: <20190411175043.31207-1-joel@joelfernandes.org>
 <20190416120430.GA15437@redhat.com> <20190416192051.GA184889@google.com>
 <20190417130940.GC32622@redhat.com>
 <CAG48ez2Q0t2-rZz8wmGeih=NigYL-CT3+E6NgiPT0AkOQAUPMQ@mail.gmail.com>
 <CEB62F51-E8BF-4EFB-B438-8AF76633FF00@brauner.io>
 <20190419190247.GB251571@google.com>
 <20190419191858.iwcvqm6fihbkaata@brauner.io>
 <20190419194902.GE251571@google.com>
 <CAKOZueu3PzD_QgROXFRs8FvD=HTKJLsHDK9GrUdbFoMzx_7HEA@mail.gmail.com>
 <CAHrFyr4REuKFuARAXnhBC67TGzyDGmMONc7XjfUGvf+Qfm7EuQ@mail.gmail.com>
 <CAKOZueuBTwyrLtpVQaoTAgjGSgeHY7392sKv5uAYdy4XKNHKHQ@mail.gmail.com>
From:   "Enrico Weigelt, metux IT consult" <lkml@metux.net>
Organization: metux IT consult
Message-ID: <f631a11b-bdaa-1b07-dda5-028bf9efbd58@metux.net>
Date:   Wed, 24 Apr 2019 10:20:34 +0200
User-Agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.2.1
MIME-Version: 1.0
In-Reply-To: <CAKOZueuBTwyrLtpVQaoTAgjGSgeHY7392sKv5uAYdy4XKNHKHQ@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 19.04.19 23:21, Daniel Colascione wrote:

>> EPOLLIN on a pidfd could very well mean that data can be read via
>> a read() on the pidfd *other* than the exit status. The read could e.g.
>> give you a lean struct that indicates the type of state transition: NOTIFY_EXIT,
>> NOTIFY_EXEC, etc.. This way we are not bound to a specific poll event indicating
>> a specific state.
>> Though there's a case to be made that EPOLLHUP could indicate process exit
>> and EPOLLIN a state change + read().
> 
> And do you imagine making read() destructive? Does that read() then
> reset the POLLIN state? You're essentially proposing that a pidfd
> provide an "event stream" interface, delivering notifications packets
> that indicate state changes like "process exited" or "process stopped"
> or "process execed". While this sort of interface is powerful and has
> some nice properties that tools like debuggers and daemon monitors
> might want to use, I think it's too complicated and error prone for
> the overwhelmingly common case of wanting to monitor process lifetime.

I don't think it's so complicated. read() + comparing a few bits
(eg. strncmp(), if the packets are strings) really isn't a big deal.

Actually, I proposed quite the same (before I read this mail).

> I like Linus' idea of just making waitid(2) (not waitpid(2), as I
> mistakenly mentioned earlier) on a pidfd act *exactly* like a
> waitid(2) on the corresponding process and making POLLIN just mean
> "waitid will succeed". It's a nice simple model that's easy to reason
> about and that makes it easy to port existing code to pidfds.

Okay, that's probably a good starting point. We could add more
sophisticated monitoring later.


--mtx


-- 
Enrico Weigelt, metux IT consult
Free software and Linux embedded engineering
info@metux.net -- +49-151-27565287