On Wed, Jul 14, 2010 at 11:28:17AM -0700, johan...@opensolaris.org wrote:
> On Wed, Jul 14, 2010 at 12:32:26AM -0500, Nicolas Williams wrote:
> > One possible improvement that comes to mind would be a private vforkx()
> > flag to request that threads in the parent other than the one calling
> > vforkx() not be stopped. The child would have to be extremely careful
> > to not call malloc() or do anything that might disturb the state of the
> > parent beyond the current stack frame, which is why such a flag would
> > have to be private and undocumented. But this should not be considered
> > until someone first shows that posix_spawn() performance is a problem
> > and that the parent thread stop/resume overhead is the biggest ticket
> > item.
>
> It's significantly more complicated than that. You'd have to ensure
> that there are no race conditions between the atfork handlers, rtld,
> libc, ...
there are no atfork handlers in the vfork case, and you could make the
rtld block without having to stop all threads in the parent. A careful
examination of this approach will surely show a few more such gotchas,
probably none fatal, but all problematic enough that such a vforkx()
extension would have to be private and undocumented.
> ..., and any application specific locking. [...]
Wouldn't apply here since only posix_spawn(3C) would actually use this.
> The vfork manual is already
> pretty clear that all bets are off should you use this in a
> multi-threaded application:
>
> The use of vfork() or vforkx() in multithreaded applica-
> tions, however, is unsafe due to race conditions that can
> cause the child process to become deadlocked and conse-
> quently block both the child and parent process from execu-
> tion indefinitely.
This doesn't apply to posix_spawn(3C) however. That is, it is possible
to use vforkx() in a thread-safe way, and posix_spawn(3C) does just
that:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Interface Stability | Committed |
|_____________________________|_____________________________|
| MT-Level | MT-Safe |
|_____________________________|_____________________________|
It's also possible to hang yourself with vforkx(). Fortunately
posix_spawn(3C) doesn't do that.
Nico
--
_______________________________________________
on-discuss mailing list
on-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/on-discuss
