linux-kernel - Re: [PATCH 0/6] CLONE_FD: Task exit notification via file descriptor

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Date:	Fri, 13 Mar 2015 14:33:44 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	David Drysdale <drysdale@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>, Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thiago Macieira <thiago.macieira@...el.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	X86 ML <x86@...nel.org>
Subject: Re: [PATCH 0/6] CLONE_FD: Task exit notification via file descriptor

On Fri, Mar 13, 2015 at 12:42 PM, Josh Triplett <josh@...htriplett.org> wrote:
> On Fri, Mar 13, 2015 at 04:05:29PM +0000, David Drysdale wrote:
>> On Fri, Mar 13, 2015 at 1:40 AM, Josh Triplett <josh@...htriplett.org> wrote:
>> > This patch series introduces a new clone flag, CLONE_FD, which lets the caller
>> > handle child process exit notification via a file descriptor rather than
>> > SIGCHLD.  CLONE_FD makes it possible for libraries to safely launch and manage
>> > child processes on behalf of their caller, *without* taking over process-wide
>> > SIGCHLD handling (either via signal handler or signalfd).
>>
>> Hi Josh,
>>
>> From the overall description (i.e. I haven't looked at the code yet)
>> this looks very interesting.  However, it seems to cover a lot of the
>> same ground as the process descriptor feature that was added to FreeBSD
>> in 9.x/10.x:
>>   https://www.freebsd.org/cgi/man.cgi?query=pdfork&sektion=2
>
> Interesting.
>
>> I think it would ideally be nice for a userspace library developer to be
>> able to do subprocess management (without SIGCHLD) in a similar way
>> across both platforms, without lots of complicated autoconf shenanigans.
>>
>> So could we look at the overlap and seeing if we can come up with
>> something that covers your requirements and also allows for something
>> that looks like FreeBSD's process descriptors?
>
> Agreed; however, I think it's reasonable to provide appropriate Linux
> system calls, and then let glibc or libbsd or similar provide the
> BSD-compatible calls on top of those.  I don't think the kernel
> interface needs to exactly match FreeBSD's, as long as it's a superset
> of the functionality.

We need to be careful with things like read(2), though.  It's hard to
write a glibc function that makes read(2) do something other than what
the kernel thinks.  Similarly, poll(2) is defined by the kernel.  It
would be really nice to be consistent here.

--Andy
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