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

[Peter Zijlstra]

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?