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

Mathieu Desnoyers
2020-07-20T16:46:58Z

----- On Jul 19, 2020, at 11:03 PM, Nicholas Piggin npiggin@gmail.com wrote:

> Excerpts from Mathieu Desnoyers's message of July 17, 2020 11:42 pm:
>> ----- On Jul 16, 2020, at 7:26 PM, Nicholas Piggin npiggin@gmail.com wrote:
>> [...]
>>>
>>> membarrier does replace barrier instructions on remote CPUs, which do
>>> order accesses performed by the kernel on the user address space. So
>>> membarrier should too I guess.
>>>
>>> Normal process context accesses like read(2) will do so because they
>>> don't get filtered out from IPIs, but kernel threads using the mm may
>>> not.
>>
>> But it should not be an issue, because membarrier's ordering is only with
>> respect
>> to submit and completion of io_uring requests, which are performed through
>> system calls from the context of user-space threads, which are called from the
>> right mm.
>
> Is that true? Can io completions be written into an address space via a
> kernel thread? I don't know the io_uring code well but it looks like
> that's asynchonously using the user mm context.

Indeed, the io completion appears to be signaled asynchronously between kernel
and user-space. Therefore, both kernel and userspace code need to have proper
memory barriers in place to signal completion, otherwise user-space could read
garbage after it notices completion of a read.

I did not review the entire io_uring implementation, but the publish side
for completion appears to be:

static void __io_commit_cqring(struct io_ring_ctx *ctx)
{
struct io_rings *rings = ctx->rings;

/* order cqe stores with ring update */
smp_store_release(&rings->cq.tail, ctx->cached_cq_tail);

if (wq_has_sleeper(&ctx->cq_wait)) {
wake_up_interruptible(&ctx->cq_wait);
kill_fasync(&ctx->cq_fasync, SIGIO, POLL_IN);
}
}

The store-release on tail should be paired with a load_acquire on the
reader-side (it's called "read_barrier()" in the code):

tools/io_uring/queue.c:

static int __io_uring_get_cqe(struct io_uring *ring,
struct io_uring_cqe **cqe_ptr, int wait)
{
struct io_uring_cq *cq = &ring->cq;
const unsigned mask = *cq->kring_mask;
unsigned head;
int ret;

*cqe_ptr = NULL;
head = *cq->khead;
do {
/*
* It's necessary to use a read_barrier() before reading
* the CQ tail, since the kernel updates it locklessly. The
* kernel has the matching store barrier for the update. The
* kernel also ensures that previous stores to CQEs are ordered
* with the tail update.
*/
read_barrier();
if (head != *cq->ktail) {
*cqe_ptr = &cq->cqes[head & mask];
break;
}
if (!wait)
break;
ret = io_uring_enter(ring->ring_fd, 0, 1,
IORING_ENTER_GETEVENTS, NULL);
if (ret < 0)
return -errno;
} while (1);

return 0;
}

So as far as membarrier memory ordering dependencies are concerned, it relies
on the store-release/load-acquire dependency chain in the completion queue to
order against anything that was done prior to the completed requests.

What is in-flight while the requests are being serviced provides no memory
ordering guarantee whatsoever.

> How about other memory accesses via kthread_use_mm? Presumably there is
> still ordering requirement there for membarrier,

Please provide an example case with memory accesses via kthread_use_mm where
ordering matters to support your concern.

> so I really think
> it's a fragile interface with no real way for the user to know how
> kernel threads may use its mm for any particular reason, so membarrier
> should synchronize all possible kernel users as well.

I strongly doubt so, but perhaps something should be clarified in the documentation
if you have that feeling.

Thanks,

Mathieu

>
> Thanks,
> Nick

--
Mathieu Desnoyers
EfficiOS Inc.