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

From: Thiago Macieira
Date: Sat Mar 14 2015 - 18:26:36 EST

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)
Software Architect - Intel Open Source Technology Center

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