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Date: Tue, 17 Mar 2015 19:08:24 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: josh@joshtriplett.org
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        linux-kernel@vger.kernel.org,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
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        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: <20150317180824.GK23123@twins.programming.kicks-ass.net>
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On Tue, Mar 17, 2015 at 10:57:50AM -0700, josh@joshtriplett.org wrote:
> 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.

Sure; but why bother with pointless fluff like that? We can always
introduce the private flag if and when it starts to make sense having
it.