Robust mutexes implementation

[Konstantin Belousov]

On Sat, May 14, 2016 at 12:34:34AM +0200, Jilles Tjoelker wrote:
> On Fri, May 13, 2016 at 11:19:16PM +0300, Konstantin Belousov wrote:
> > BTW, is there a chance of Austin Group get notified of, and possibly
> > adopting, MAP_EXCL flag ?
> 
> You could send a message to the mailing list. MAP_EXCL is not used in
> the FreeBSD base but Debian code search shows Chromium and Hurd ftpfs
> mentioning it in comments.
It is not clear if they reference FreeBSD flag, or just come to this
semi-obvious idea independently.  Anway, it is not used because feature
is not present anywhere (except us).

I do not want to present this on ag list as a stranger with bad language.

> I don't trust the if (umtx_shm_vnobj_persistent) in
> sys/vm/vnode_pager.c:
> 
> +	if (umtx_shm_vnobj_persistent)
> +		umtx_shm_object_terminated(obj);
> 
> If umtx_shm_vnobj_persistent is turned off via sysctl between the check
> in sys/vm/vm_object.c and this one, don't we have a leak?
Removed the test in the vnode_destroy_vobject(), doing the
umtx termination unconditionally on reclaim.

> 
> 'Reviewed by: jilles' is fine otherwise.

Updated patch is at
https://www.kib.kiev.ua/kib/pshared/robust.5.patch

P.S.  I will document UMUTEX_ROBUST/UMUTEX_RB_* and UMTX_OP_ROBUST_LISTS
after the feature is committed.