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

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Date:	Tue, 16 Feb 2010 09:57:46 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	kosaki.motohiro@...fujitsu.com,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Nicholas Miell <nmiell@...cast.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>, mingo@...e.hu,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, Valdis.Kletnieks@...edu, dhowells@...hat.com
Subject: Re: [RFC patch] introduce sys_membarrier(): process-wide memory barrier (v9)

> 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@...ux.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.



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