Brian Fundakowski Feldman wrote:
On Thu, Jan 08, 2009 at 11:09:16PM -0800, Julian Elischer wrote:
Brian Fundakowski Feldman wrote:
On Thu, Jan 08, 2009 at 10:44:20PM -0500, Daniel Eischen wrote:
On Thu, 8 Jan 2009, Brian Fundakowski Feldman wrote:
It appears that the post-fork hooks for malloc(3) are somewhat broken such that
when a threaded program forks, and then its child attempts to go threaded, it
deadlocks because it already appears to have locks held. I am not familiar
enough with the current libthr/libc/rtld-elf interaction that I've been able
to fix it myself, unfortunately.
There's really nothing to fix - according to POSIX you are only
allowed to call async-signal-safe functions in the child forked
from a threaded process. If you are trying to do anything other
than that, it may or may not work on FreeBSD, but it is not
guaranteed and is not portable.
The rationale is that what is the point of forking and creating
more threads, when you can just as easily create more threads in
the parent without forking? The only reason to fork from a threaded
process is to call one of the exec() functions.
Well, it worked until the last major set of changes to malloc. For me, the
point
was that I was able to have transparent background worker threads in any program
regardless of its architecture, using the standard pthread fork hooks. Could
you
point me to the POSIX section covering fork and threads? If it's really not
supposed to work then that's fine, but there's an awful lot of code there
dedicated
to support going threaded again after a fork.
Practically, you can't go threaded again after a fork
(by which I mean creating new threads or use things
like mutexes etc.) in any posix system I know of.
It would require that:
The forking thread stop until:
Every other thread has released every resource it owns
and reports itself to be in a "safe quiescent state",
or at least report every resource it owns, especially
locks,
and
After the fork:
The child, post fork, to take ownership of all
of them, and free them.
You might be able to do that in a simple
threaded program, but consider then that the libraries may have
threads running in them of which you are unaware, and that
some of the resources may interract with resources owned by the
forking thread.
Add to this that there may be a signal thrown into this mix as well
(signals are the bane of thread developement)
Well, I wouldn't mind showing all of you what I can of what I had been doing
with the background threads -- it works pretty well modulo this particular
malloc lock bug. Due to it being inappropriate to share library resources
between a child and parent for an open socket connection, I always considered
the only "safe" behavior to be going single-threaded for the duration of the
fork
processes in both the parent and child, and the pthread_atfork(3) hooks have
been
sufficient to do so.
ahhhh
well going single threaded for the duration of the fork, changes
everything!
_______________________________________________
freebsd-hackers@freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-hackers
To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
