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

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Message-Id: <20170727125659.GO3730@linux.vnet.ibm.com>
Date:   Thu, 27 Jul 2017 05:56:59 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 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.

> And if there are other (unpriv.) means of spraying IPIs around, we
> should most certainly look at fixing those, not just shrug and make
> matters worse.

We need to keep an open mind for a bit.  If there was a trivial
solution, we would have implemented it back in 2010.

							Thanx, Paul