Re: [RFC patch] introduce sys_membarrier(): process-wide memory barrier (v9)

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> On Fri, Feb 12, 2010 at 05:46:06PM -0500, Mathieu Desnoyers wrote:
> > Here is an implementation of a new system call, sys_membarrier(), which
> > executes a memory barrier on all threads of the current process. It can be used
> > to distribute the cost of user-space memory barriers asymmetrically by
> > transforming pairs of memory barriers into pairs consisting of sys_membarrier()
> > and a compiler barrier. For synchronization primitives that distinguish between
> > read-side and write-side (e.g. userspace RCU, rwlocks), the read-side can be
> > accelerated significantly by moving the bulk of the memory barrier overhead to
> > the write-side.
> > 
> > The first user of this system call is the "liburcu" Userspace RCU implementation
> > found at http://lttng.org/urcu. It aims at greatly simplifying and enhancing the
> > current implementation, which uses a scheme similar to the sys_membarrier(), but
> > based on signals sent to each reader thread.
> > 
> > Editorial question: 
> > 
> > This synchronization only takes care of threads using the current process memory
> > map. It should not be used to synchronize accesses performed on memory maps
> > shared between different processes. Is that a limitation we can live with ?
> 
> Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Yes.

Personally, I think this patch's concept is clear and it can construct the base
of userland lockless programming.
If a userland programmer want to use lockless, hazard pointer is one of common
technique and this syscall help it.