----- On Jul 21, 2020, at 11:06 AM, Peter Zijlstra peterz@infradead.org wrote:

On Tue, Jul 21, 2020 at 08:04:27PM +1000, Nicholas Piggin wrote:

quoted

That being said, the x86 sync core gap that I imagined could be fixed
by changing to rq->curr == rq->idle test does not actually exist because
the global membarrier does not have a sync core option. So fixing the
exit_lazy_tlb points that this series does *should* fix that. So
PF_KTHREAD may be less problematic than I thought from implementation
point of view, only semantics.

So I've been trying to figure out where that PF_KTHREAD comes from,
commit 227a4aadc75b ("sched/membarrier: Fix p->mm->membarrier_state racy
load") changed 'p->mm' to '!(p->flags & PF_KTHREAD)'.

So the first version:

 https://lkml.kernel.org/r/20190906031300.1647-5-mathieu.desnoyers@efficios.com

appears to unconditionally send the IPI and checks p->mm in the IPI
context, but then v2:

 https://lkml.kernel.org/r/20190908134909.12389-1-mathieu.desnoyers@efficios.com

has the current code. But I've been unable to find the reason the
'p->mm' test changed into '!(p->flags & PF_KTHREAD)'.

Looking back at my inbox, it seems like you are the one who proposed to
skip all kthreads: 

https://lkml.kernel.org/r/20190904124333.GQ2332@hirez.programming.kicks-ass.net

The comment doesn't really help either; sure we have the whole lazy mm
thing, but that's ->active_mm, not ->mm.

Possibly it is because {,un}use_mm() do not have sufficient barriers to
make the remote p->mm test work? Or were we over-eager with the !p->mm
doesn't imply kthread 'cleanups' at the time?

The nice thing about adding back kthreads to the threads considered for membarrier
IPI is that it has no observable effect on the user-space ABI. No pre-existing kthread
rely on this, and we just provide an additional guarantee for future kthread
implementations.

Also, I just realized, I still have a fix for use_mm() now
kthread_use_mm() that seems to have been lost.

I suspect we need to at least document the memory barriers in kthread_use_mm and
kthread_unuse_mm to state that they are required by membarrier if we want to
ipi kthreads as well.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com