Re: [External] Re: [PATCH v4 2/3] io_uring: avoid ring quiesce while registering/unregistering eventfd

[Usama Arif <usama.arif@bytedance.com>]

On 03/02/2022 19:12, Jens Axboe wrote:
> On 2/3/22 12:05 PM, Usama Arif wrote:
>>
>>
>> On 03/02/2022 18:49, Jens Axboe wrote:
>>> On 2/3/22 11:24 AM, Usama Arif wrote:
>>>> -static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
>>>> +static void io_eventfd_signal(struct io_ring_ctx *ctx)
>>>>    {
>>>> -	if (likely(!ctx->cq_ev_fd))
>>>> -		return false;
>>>> +	struct io_ev_fd *ev_fd;
>>>> +
>>>> +	rcu_read_lock();
>>>> +	/* rcu_dereference ctx->io_ev_fd once and use it for both for checking and eventfd_signal */
>>>> +	ev_fd = rcu_dereference(ctx->io_ev_fd);
>>>> +
>>>> +	if (likely(!ev_fd))
>>>> +		goto out;
>>>>    	if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
>>>> -		return false;
>>>> -	return !ctx->eventfd_async || io_wq_current_is_worker();
>>>> +		goto out;
>>>> +
>>>> +	if (!ctx->eventfd_async || io_wq_current_is_worker())
>>>> +		eventfd_signal(ev_fd->cq_ev_fd, 1);
>>>> +
>>>> +out:
>>>> +	rcu_read_unlock();
>>>>    }
>>>
>>> This still needs what we discussed in v3, something ala:
>>>
>>> /*
>>>    * This will potential race with eventfd registration, but that's
>>>    * always going to be the case if there is IO inflight while an eventfd
>>>    * descriptor is being registered.
>>>    */
>>> if (!rcu_dereference_raw(ctx->io_ev_fd))
>>> 	return;
>>>
>>> rcu_read_lock();
>>
>> Hmm, so i am not so worried about the registeration, but actually
>> worried about unregisteration.
>> If after the check and before the rcu_read_lock, the eventfd is
>> unregistered won't we get a NULL pointer exception at
>> eventfd_signal(ev_fd->cq_ev_fd, 1)?
> 
> You need to check it twice, that's a hard requirement. The first racy
> check is safe because we don't care if we miss a notification, once
> inside rcu_read_lock() it needs to be done properly of course. Like you
> do below, that's how it should be done.
> 
>>> I wonder if we can get away with assigning ctx->io_ev_fd to NULL when we
>>> do the call_rcu(). The struct itself will remain valid as long as we're
>>> under rcu_read_lock() protection, so I think we'd be fine? If we do
>>> that, then we don't need any rcu_barrier() or synchronize_rcu() calls,
>>> as we can register a new one while the previous one is still being
>>> killed.
>>>
>>> Hmm?
>>>
>>
>> We would have to remove the check that ctx->io_ev_fd != NULL. That we
>> would also result in 2 successive calls to io_eventfd_register without
>> any unregister in between being successful? Which i dont think is the
>> right behaviour?
>>
>> I think the likelihood of hitting the rcu_barrier itself is quite low,
>> so probably the cost is low as well.
> 
> Yeah it might very well be. To make what I suggested work, we'd need a
> way to mark the io_ev_fd as going away. Which would be feasible, as we
> know the memory will remain valid for us to check. So it could
> definitely work, you'd just need a check for that.
> 
>> Thanks, will do that this in the next patchset with the above
>> io_eventfd_signal changes if those look ok as well?
> 
> The code you pasted looked good. Consider the "is unregistration in
> progress" suggestion as well, as it would be nice to avoid any kind of
> rcu synchronization if at all possible.
> 


Thanks for the review comments! I think all of them should have been 
addressed now in v5. I also removed ring quiesce from io_uring_register 
as the remaining 2 opcodes don't need them (Thanks Pavel for confirming 
that!)

Regards,
Usama