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

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Message-ID: <20170727101426.gvbympkitoqyvfxj@hirez.programming.kicks-ass.net>
Date:   Thu, 27 Jul 2017 12:14:26 +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 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().

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.