[Python-Dev] pthreads, fork, import, and execvp (original) (raw)

Thomas Wouters thomas at python.org
Wed Sep 16 22:34:36 CEST 2009

On Wed, Sep 9, 2009 at 23:56, Nick Coghlan <ncoghlan at gmail.com> wrote:

Thomas Wouters wrote: > Your idea of making this an API called a 'fork lock' or something > sounds good (though I think it needs a better name. PyBeforeFork & > PyAfterFork?). The subprocess module, for example, disables garbage > collection before forking and restores it afterwards to avoid > http://bugs.python.org/issue1336. That type of thing could also be > done in such a function. > > > Unfortunately it's rather hard to make those functions work correctly > with the current API -- we can't provide functions you can just use as > arguments to pthreadatfork because the global interpreter lock is not > re-entrant and we have no way of testing whether the current thread > holds the GIL.

I thought "make sure I have the GIL, either by already having it or waiting for it if I don't yet have it" was the entire point of the PyGILStateEnsure() API? [1]

Hm, yeah. For some reason I was certain it was inappropriate, back when I was trying to create a pthread_atfork-friendly set of functions. At the time I was also hip-deep in the awfulness of Python/thread*.c and its unsafe punning and unwarranted assumptions, so I may have overreacted. I added this as a feature-request issue ( http://bugs.python.org/issue6923 ) and will look at it some more.

In the mean time, I fixed the biggest source of issues by checking in the change to make at least calls to fork() made by Python be safe, also backported to 2.6.

-- Thomas Wouters <thomas at python.org>

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