process shared mutexes?

[Volker Lendecke]

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