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