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Date: Sun, 5 Feb 2012 20:00:22 +0100
From: Jilles Tjoelker <jilles@stack.nl>
To: Daniel Eischen <deischen@freebsd.org>
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Cc: Garrett Cooper <yanegomi@gmail.com>, freebsd-hackers@freebsd.org,
	Ivan Voras <ivoras@freebsd.org>, Xin LI <delphij@delphij.net>,
	Jan Mikkelsen <janm-freebsd-hackers@transactionware.com>,
	davidxu@freebsd.org
Subject: Re: sem(4) lockup in python?
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On Sun, Feb 05, 2012 at 01:32:42PM -0500, Daniel Eischen wrote:
> On Sun, 5 Feb 2012, Ivan Voras wrote:
> > On 5 February 2012 11:44, Garrett Cooper <yanegomi@gmail.com> wrote:

> >>    'make MAKE_JOBS_NUMBER=1' is the workground used right now..

> > David Xu suggested that it is a bug in Python - it doesn't set
> > process-shared attribute when it calls sem_init(), but i've tried
> > patching it (replacing the port patchfile file the one I've attached)
> > and I still get the hang.

> I don't understand how process shared semaphores can work.  Perhaps
> I'm dumb and ignorant, but a sem_id_t is an allocated struct.   The
> actual kernel sem_id is inside the struct.  Isn't this the same
> reason pthread_mutex_t and pthread_cond_t cannot be process-shared?

That's how the old implementation works. It does not support
process-shared semaphores although they may happen to work in some
specific cases.

However, in 9.0, sem_t works differently and contains the actual lock
word directly, so that process-shared semaphores work. The
implementation is in lib/libc/gen/sem_new.c. The pshared flag to
sem_init() is not a no-op because it tells the kernel to allow for use
from multiple processes.

Note that the old implementation is still present as well, for
compatibility with old binaries.

-- 
Jilles Tjoelker