linux-kernel - Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option

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Message-ID: <20170727133729.syymgkfslzam732w@hirez.programming.kicks-ass.net>
Date:   Thu, 27 Jul 2017 15:37:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com
Subject: Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option

On Thu, Jul 27, 2017 at 05:56:59AM -0700, Paul E. McKenney wrote:
> On Thu, Jul 27, 2017 at 12:14:26PM +0200, Peter Zijlstra wrote:
> > On Tue, Jul 25, 2017 at 12:36:12PM -0700, Paul E. McKenney wrote:
> > > This horse is already out, so trying to shut the gate won't be effective.
> > 
> > So I'm not convinced it is. The mprotect() hack isn't portable as we've
> > established and on x86 where it does work, it doesn't (much) perturb
> > tasks not related to our process because we keep a tight mm_cpumask().
> 
> Wrong.  People are using it today, portable or not.  If we want them
> to stop using it, we need to give them an alternative.  Period.

What's wrong? The mprotect() hack isn't portable, nor does it perturb
other users much.

I would much rather they use this than your
synchronize_sched_expedited() thing. Its much better behaved. And
they'll run into pain the moment they start using ARM,PPC,S390,etc..