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

[Peter Zijlstra]

On Tue, Jul 25, 2017 at 10:50:13PM +0000, Mathieu Desnoyers wrote:
> This would implement a MEMBARRIER_CMD_PRIVATE_EXPEDITED (or such) flag
> for expedited process-local effect. This differs from the "SHARED" flag,
> since the SHARED flag affects threads accessing memory mappings shared
> across processes as well.
> 
> I wonder if we could create a MEMBARRIER_CMD_SHARED_EXPEDITED behavior
> by iterating on all memory mappings mapped into the current process,
> and build a cpumask based on the union of all mm masks encountered ?
> Then we could send the IPI to all cpus belonging to that cpumask. Or
> am I missing something obvious ?

I would readily object to such a beast. You far too quickly end up
having to IPI everybody because of some stupid shared map or something
(yes I know, normal DSOs are mapped private).

The whole private_expedited is only palatable because we can only hinder
our own threads (much).