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

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Message-ID: <20170725165957.alykngbnrrwn3onw@hirez.programming.kicks-ass.net>
Date:   Tue, 25 Jul 2017 18:59:57 +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 09:49:00AM -0700, Paul E. McKenney wrote:
> On Tue, Jul 25, 2017 at 06:33:18PM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 24, 2017 at 02:58:16PM -0700, Paul E. McKenney wrote:
> > > The sys_membarrier() system call has proven too slow for some use
> > > cases, which has prompted users to instead rely on TLB shootdown.
> > > Although TLB shootdown is much faster, it has the slight disadvantage
> > > of not working at all on arm and arm64.  This commit therefore adds
> > > an expedited option to the sys_membarrier() system call.
> > 
> > > @@ -64,6 +65,10 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> > >  		if (num_online_cpus() > 1)
> > >  			synchronize_sched();
> > >  		return 0;
> > > +	case MEMBARRIER_CMD_SHARED_EXPEDITED:
> > > +		if (num_online_cpus() > 1)
> > > +			synchronize_sched_expedited();
> > > +		return 0;
> > 
> > So you now give unprivileged userspace the means to IPI the entire
> > machine?
> > 
> > So what do we do when someone goes and does:
> > 
> > 	for (;;)
> > 		sys_membarrier(MEMBARRIER_CMD_SHARED_EXPEDITED, 0);
> > 
> > on us?
> 
> The same thing that happens when they call munmap().

munmap() TLB invalidate is limited to those CPUs that actually ran
threads of their process, while this is machine wide.