linux-kernel - Re: [PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task exit notification via fd

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Date:	Sat, 14 Mar 2015 15:26:05 -0700
From:	Thiago Macieira <thiago.macieira@...el.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	"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>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task exit notification via fd

On Saturday 14 March 2015 15:03:08 Josh Triplett wrote:
> I had to think about this for a while, but I think it makes sense now.
> wait should *not* ever return the PID of an autoreaped process, because
> that would introduce a race condition (the caller cannot safely do
> *anything* with the PID of an autoreaped process, since by the time it
> does, the process may be gone and the PID may be reused).  However, that
> doesn't mean wait cannot block on the process, and then subsequently
> wake up and return -ECHILD (or keep waiting on some other child process
> if there is one).  That's apparently the semantic used with SA_NOCLDWAIT
> or if you have SIGCHLD set to SIG_IGN, and matching that seems
> appropriate.
> 
> Thiago, could your QProcess implementation handle that modified autoreap
> semantic?  The downside there is that if your calling process has a
> process-wide loop that waits for all processes (and explicitly passes
> the Linux-specific __WCLONE or __WALL flag, since your processes
> launched with a 0 signal would count as "clone" children), they'd get
> back the processes you launch, too.  (That would happen with your
> userspace-emulated version too for calls *without* __WCLONE or __WALL.)
> You'd still get the exit status you need via the clonefd, without a
> race, and you wouldn't need to touch process-wide signal handling, so I
> think this should still work and avoid any races.

I don't see why QProcess would have a problem. We don't have such a process-
wide wait loop with __WCLONE or __WALL and I can't think of any reason why 
someone would do that and still expect NPTL to work. Or, put another way, if 
they are using clone/clone4 directly and bypassing NPTL, they're probably in a 
very specialised process that has no business running QProcess in the first 
place. I wouldn't be too worried.

Inside glibc itself, __WCLONE is used only in unit tests and __WALL is used in 
a loop in elf/pldd.c, which is an independent application. Bionic has __WCLONE 
in tests only too.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center

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