Messages in this thread | | | Date | Tue, 21 Jul 2020 11:15:13 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode |
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----- 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: > >> 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
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