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Date: Mon, 21 Nov 2016 16:25:42 +0100
From: Volker Lendecke <Volker.Lendecke@SerNet.DE>
To: Konstantin Belousov <kostikbel@gmail.com>
Cc: freebsd-hackers@freebsd.org
Subject: Re: process shared mutexes?
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On Mon, Nov 21, 2016 at 05:10:40PM +0200, Konstantin Belousov wrote:
> Please see the libthr(3) man page, in particular, read the RUN-TIME
> SETTINGS section, the description of the kern.ipc.umtx_vnode_persistent
> sysctl.
> 
> Does setting the sysctl to 1 allow your program to run ?

Yes, that does make it work. The description says that the umtx vnode
is dropped on the last munmap. If I #if 0 the middle mmap, it works,
although there is no mmap around anymore. So the description is not
100% accurate I'd say.

When does the recycling happen exactly? Can it break running
applications?

And -- how can I make sure for Samba that this is set properly at
runtime? We already have a runtime mutex test for some ancient Linux
kernels that were broken. We could add this as a subtest too. But --
what happens if the admin resets this while Samba is running? Does
the kernel make sure that existing files still get the correct
behaviour when the sysctl changes?

Thanks!

Volker