Date: Thu, 08 Jan 2009 23:09:16 -0800 From: Julian Elischer <julian@elischer.org> To: Brian Fundakowski Feldman <green@freebsd.org> Cc: hackers@freebsd.org, jasone@freebsd.org Subject: Re: threaded, forked, rethreaded processes will deadlock Message-ID: <4966F81C.3070406@elischer.org> In-Reply-To: <20090109053117.GB2825@green.homeunix.org> References: <20090109031942.GA2825@green.homeunix.org> <Pine.GSO.4.64.0901082237001.28531@sea.ntplx.net> <20090109053117.GB2825@green.homeunix.org>
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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)
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