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

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Date:	Mon, 16 Mar 2015 18:24:45 +0100
From:	Peter Zijlstra <peterz@...radead.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>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Howells <dhowells@...hat.com>,
	Nick Piggin <npiggin@...nel.dk>
Subject: Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier
 (x86) (v12)

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.
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