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

From: Mathieu Desnoyers <[email-masked]>
Date: Thu, 16 Jul 2020 11:46:22 -0400 (EDT)

----- On Jul 16, 2020, at 12:42 AM, Nicholas Piggin [email-masked] wrote:
> I should be more complete here, especially since I was complaining
> about unclear barrier comment :)
> 
> 
> CPU0                     CPU1
> a. user stuff            1. user stuff
> b. membarrier()          2. enter kernel
> c. smp_mb()              3. smp_mb__after_spinlock(); // in __schedule
> d. read rq->curr         4. rq->curr switched to kthread
> e. is kthread, skip IPI  5. switch_to kthread
> f. return to user        6. rq->curr switched to user thread
> g. user stuff            7. switch_to user thread
>                         8. exit kernel
>                         9. more user stuff
> 
> What you're really ordering is a, g vs 1, 9 right?
> 
> In other words, 9 must see a if it sees g, g must see 1 if it saw 9,
> etc.
> 
> Userspace does not care where the barriers are exactly or what kernel
> memory accesses might be being ordered by them, so long as there is a
> mb somewhere between a and g, and 1 and 9. Right?

This is correct. Note that the accesses to user-space memory can be
done either by user-space code or kernel code, it doesn't matter.
However, in order to be considered as happening before/after
either membarrier or the matching compiler barrier, kernel code
needs to have causality relationship with user-space execution,
e.g. user-space does a system call, or returns from a system call.

In the case of io_uring, submitting a request or returning from waiting
on request completion appear to provide this causality relationship.

Thanks,

Mathieu

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