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

Mathieu Desnoyers
2020-07-16T18:58:45Z

----- On Jul 16, 2020, at 12:03 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

> ----- On Jul 16, 2020, at 11:46 AM, Mathieu Desnoyers
> mathieu.desnoyers@efficios.com wrote:
>
>> ----- On Jul 16, 2020, at 12:42 AM, Nicholas Piggin npiggin@gmail.com 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.
>
> Actually, sorry, the above is not quite right. It's been a while
> since I looked into the details of membarrier.
>
> The smp_mb() at the beginning of membarrier() needs to be paired with a
> smp_mb() _after_ rq->curr is switched back to the user thread, so the
> memory barrier is between store to rq->curr and following user-space
> accesses.
>
> The smp_mb() at the end of membarrier() needs to be paired with the
> smp_mb__after_spinlock() at the beginning of schedule, which is
> between accesses to userspace memory and switching rq->curr to kthread.
>
> As to *why* this ordering is needed, I'd have to dig through additional
> scenarios from https://lwn.net/Articles/573436/. Or maybe Paul remembers ?

Thinking further about this, I'm beginning to consider that maybe we have been
overly cautious by requiring memory barriers before and after store to rq->curr.

If CPU0 observes a CPU1's rq->curr->mm which differs from its own process (current)
while running the membarrier system call, it necessarily means that CPU1 had
to issue smp_mb__after_spinlock when entering the scheduler, between any user-space
loads/stores and update of rq->curr.

Requiring a memory barrier between update of rq->curr (back to current process's
thread) and following user-space memory accesses does not seem to guarantee
anything more than what the initial barrier at the beginning of __schedule already
provides, because the guarantees are only about accesses to user-space memory.

Therefore, with the memory barrier at the beginning of __schedule, just observing that
CPU1's rq->curr differs from current should guarantee that a memory barrier was issued
between any sequentially consistent instructions belonging to the current process on
CPU1.

Or am I missing/misremembering an important point here ?

Thanks,

Mathieu

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

--
Mathieu Desnoyers
EfficiOS Inc.