Robust mutexes implementation

[Jilles Tjoelker]

On Fri, May 13, 2016 at 11:19:16PM +0300, Konstantin Belousov wrote:
> On Fri, May 13, 2016 at 05:37:16PM +0200, Jilles Tjoelker wrote:
> > On Mon, May 09, 2016 at 05:51:07AM +0300, Konstantin Belousov wrote:
> > The mmap() page in POSIX.1-2008tc1 XSH 3 has:

> > ] The state of synchronization objects such as mutexes, semaphores,
> > ] barriers, and conditional variables placed in shared memory mapped
> > ] with MAP_SHARED becomes undefined when the last region in any process
> > ] containing the synchronization object is unmapped.

> > This is new in issue 7 (SUSv4):

> > ] Austin Group Interpretations 1003.1-2001 #078 and #079 are applied,
> > ] clarifying page alignment requirements and adding a note about the
> > ] state of synchronization objects becoming undefined when a shared
> > ] region is unmapped.

> Very interesting, thanks.

> 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.

> > > Current updates to the patch
> > > https://kib.kiev.ua/kib/pshared/robust.4.patch

> Do you have any further notes, or do you want to give the patch more time ?
> If not, are you fine with 'Reviewed by' attribution ?

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?

'Reviewed by: jilles' is fine otherwise.

-- 
Jilles Tjoelker