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Date: Mon, 16 Mar 2015 18:24:45 +0100
From: Peter Zijlstra <peterz@infradead.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>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        David Howells <dhowells@redhat.com>, Nick Piggin <npiggin@kernel.dk>
Subject: Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier
 (x86) (v12)
Message-ID: <20150316172445.GI21418@twins.programming.kicks-ass.net>
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On Mon, Mar 16, 2015 at 03:43:56PM +0000, Mathieu Desnoyers wrote:
> > > +enum {
> > > +	/*
> > > +	 * Private flag set: only synchronize across a single process. If this
> > > +	 * flag is not set, it means "shared": synchronize across multiple
> > > +	 * processes.  The shared mode is useful for shared memory mappings
> > > +	 * across processes.
> > > +	 */
> > > +	MEMBARRIER_PRIVATE_FLAG = (1 << 0),
> > > +
> > > +	/*
> > > +	 * Expedited flag set: adds some overhead, fast execution (few
> > > +	 * microseconds).  If this flag is not set, it means "delayed": low
> > > +	 * overhead, but slow execution (few milliseconds).
> > > +	 */
> > > +	MEMBARRIER_EXPEDITED_FLAG = (1 << 1),
> > 
> > 
> > I suppose this is an unprivileged syscall; so what do we do about:
> > 
> > 	for (;;)
> > 		sys_membar(EXPEDITED);
> > 
> > Which would spray the entire system with IPIs at break neck speed.
> 
> Currently, combining EXPEDITED with non-PRIVATE returns -EINVAL.

Ah, tl;dr that bit. This patch really had _too_ much verbiage.

> Therefore, if someone cares about issuing barriers on the entire
> system, the only option is to use non-EXPEDITED, which rely on
> synchronize_rcu().
> 
> The only way to invoke expedited barriers in a loop is:
> 
> for (;;)
>         sys_membarrier(MEMBARRIER_EXPEDITED | MEMBARRIER_PRIVATE);
> 
> Which will only send IPIs to the CPU running threads from the same
> process.

That is indeed less of a problem.