Re: [PATCH 08/11] membarrier: Provide core serializing command (v2)

[Peter Zijlstra]

On Mon, Jan 29, 2018 at 06:36:05PM +0000, Mathieu Desnoyers wrote:
> ----- On Jan 29, 2018, at 1:15 PM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:

> > Aaah, its the case where we do not pass through switch_mm(), the partial
> > comment got to me. I only realized after reading the next patch.
> 
> Indeed, if we read the entire comment, it's made clear that this case is for
> when switch_mm is not invoked, where the current mm is changed without going
> through switch_mm(), when scheduling between uthread->kthread->uthread for
> instance.
> 
>         /*
>          * When transitioning from a kernel thread to a userspace
>          * thread, mmdrop()'s implicit full barrier is required by the
>          * membarrier system call, because the current active_mm can
>          * become the current mm without going through switch_mm().
>          * membarrier also requires a core serializing instruction
>          * before going back to user-space after storing to rq->curr.
>          */
> 
> Is there something I should improve in the wording of this added
> sentence to make it clearer ?

Can be improved I think, its got two unqualified "membarrier"s in and
its a bit mixed up. I'm having a major case of the mondays (brain just
won't start today), but maybe something like:

	When we switched through a kernel thread, the loop in
	membarrier_{private,global}_expedited() can have observed that
	kernel thread and not issued an IPI. We will also not pass
	through switch_mm(). Membarrier requires a barrier after writing
	rq->curr and returning to userspace, so provide them here:

	  - a full memory barrier for {PRIVATE,GLOBAL}_EXPEDITED
	  - a sync_core for SYNC_CORE



Also I think changing the changlog to state where we need core-sync
would be good. Currently the x86 patch does that, but not this one,
while this introduces the feature.