[RFC PATCH 4/7] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode

[peterz at infradead.org]

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

> 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)'.

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?

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