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

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Message-ID: <20150316032550.GA1110@thin>
Date:	Sun, 15 Mar 2015 20:25:50 -0700
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>
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.

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

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

- Josh Triplett
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