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Date: Tue, 17 Mar 2015 10:57:50 -0700
From: josh@joshtriplett.org
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        linux-kernel@vger.kernel.org,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
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        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>,
        David Howells <dhowells@redhat.com>
Subject: Re: [RFC PATCH v13] sys_membarrier(): system/process-wide memory
 barrier (x86)
Message-ID: <20150317175750.GB4141@cloud>
References: <1426612922-3308-1-git-send-email-mathieu.desnoyers@efficios.com>
 <20150317173035.GI23123@twins.programming.kicks-ass.net>
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On Tue, Mar 17, 2015 at 06:30:35PM +0100, Peter Zijlstra wrote:
> On Tue, Mar 17, 2015 at 01:22:02PM -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) issues a memory barrier on all threads
> > running on the system (~MEMBARRIER_PRIVATE). Both are currently
> > implemented by calling synchronize_sched().
> 
> Then why bother with the flag?

Semantically, MEMBARRIER_PRIVATE is allowed to avoid issuing a barrier
on CPUs not running the current process if it can, while
~MEMBARRIER_PRIVATE may not.  (The latter would be useful for
applications such as system-wide tracing.)  That they're currently both
implemented the same way doesn't mean they're semantically equivalent.

- Josh Triplett