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

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Message-ID: <1181775493.9326.1426510824610.JavaMail.zimbra@efficios.com>
Date:	Mon, 16 Mar 2015 13:00:24 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nicholas Miell <nmiell@...cast.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Alan Cox <gnomes@...rguk.ukuu.org.uk>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	David Howells <dhowells@...hat.com>,
	Nick Piggin <npiggin@...nel.dk>
Subject: Re: [RFC PATCH] sys_membarrier(): system/process-wide memory
 barrier (x86) (v12)

----- Original Message -----
> From: "Josh Triplett" <josh@...htriplett.org>
> To: "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
> Cc: linux-kernel@...r.kernel.org, "KOSAKI Motohiro" <kosaki.motohiro@...fujitsu.com>, "Steven Rostedt"
> <rostedt@...dmis.org>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, "Nicholas Miell" <nmiell@...cast.net>,
> "Linus Torvalds" <torvalds@...ux-foundation.org>, "Ingo Molnar" <mingo@...hat.com>, "Alan Cox"
> <gnomes@...rguk.ukuu.org.uk>, "Lai Jiangshan" <laijs@...fujitsu.com>, "Stephen Hemminger"
> <stephen@...workplumber.org>, "Andrew Morton" <akpm@...ux-foundation.org>, "Thomas Gleixner" <tglx@...utronix.de>,
> "Peter Zijlstra" <peterz@...radead.org>, "David Howells" <dhowells@...hat.com>, "Nick Piggin" <npiggin@...nel.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@...htriplett.org>

Thanks!

Mathieu

> 
> - Josh Triplett
> 

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