Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier (x86) (v12)

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- Original Message -----
> From: "Josh Triplett" <josh@joshtriplett.org>
> To: "Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>
> Cc: linux-kernel@vger.kernel.org, "KOSAKI Motohiro" <kosaki.motohiro@jp.fujitsu.com>, "Steven Rostedt"
> <rostedt@goodmis.org>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, "Nicholas Miell" <nmiell@comcast.net>,
> "Linus Torvalds" <torvalds@linux-foundation.org>, "Ingo Molnar" <mingo@redhat.com>, "Alan Cox"
> <gnomes@lxorguk.ukuu.org.uk>, "Lai Jiangshan" <laijs@cn.fujitsu.com>, "Stephen Hemminger"
> <stephen@networkplumber.org>, "Andrew Morton" <akpm@linux-foundation.org>, "Thomas Gleixner" <tglx@linutronix.de>,
> "Peter Zijlstra" <peterz@infradead.org>, "David Howells" <dhowells@redhat.com>, "Nick Piggin" <npiggin@kernel.dk>
> Sent: Sunday, March 15, 2015 11:25:50 PM
> Subject: Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier (x86) (v12)
> 
> On Sun, Mar 15, 2015 at 03:24:19PM -0400, Mathieu Desnoyers wrote:
> > Here is an implementation of a new system call, sys_membarrier(), which
> > executes a memory barrier on either all running threads of the current
> > process (MEMBARRIER_PRIVATE_FLAG) or calls synchronize_sched() to issue
> > a memory barrier on all threads running on the system. 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.
> 
> From a quick review, this seems quite reasonable (as it did 5 years
> ago).
> 
> One request: Could you please add a config option (default y) in the
> EXPERT menu to disable this?  You actually seem to already have it
> marked as a cond_syscall.

Sure, done.

> 
> Also, a very minor nit: flags in kernel APIs aren't typically named with
> a _FLAG suffix.

OK, good point.

> 
> With the syscall made optional, and with or without that naming nit
> fixed:
> Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Thanks!

Mathieu

> 
> - Josh Triplett
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com