linux-kernel - Re: [PATCH RFC 1/2] Add polling support to pidfd

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Message-ID: <f631a11b-bdaa-1b07-dda5-028bf9efbd58@metux.net>
Date:   Wed, 24 Apr 2019 10:20:34 +0200
From:   "Enrico Weigelt, metux IT consult" <lkml@...ux.net>
To:     Daniel Colascione <dancol@...gle.com>,
        Christian Brauner <christian@...uner.io>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Jann Horn <jannh@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrei Vagin <avagin@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, Michal Hocko <mhocko@...e.com>,
        Nadav Amit <namit@...are.com>, Serge Hallyn <serge@...lyn.com>,
        Shuah Khan <shuah@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Taehee Yoo <ap420073@...il.com>, Tejun Heo <tj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        kernel-team <kernel-team@...roid.com>,
        Tycho Andersen <tycho@...ho.ws>
Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd

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@...ux.net -- +49-151-27565287