[Devel] [PATCH vz9 v1 06/63] dm-ploop: convert enospc handling to use lockless lists

[Pavel Tikhomirov]

On 2/3/25 15:57, Alexander Atanasov wrote:
> On 3.02.25 9:49, Pavel Tikhomirov wrote:
>>
>>
>> On 2/3/25 15:42, Alexander Atanasov wrote:
>>> On 3.02.25 9:27, Pavel Tikhomirov wrote:
>>>>
>>>>
>>>> On 2/3/25 14:45, Alexander Atanasov wrote:
>>>>> On 3.02.25 8:01, Pavel Tikhomirov wrote:
>>>>>>
>>>>>>> @@ -166,7 +171,6 @@ static bool ploop_try_delay_enospc(struct 
>>>>>>> ploop_rq *prq, struct pio *pio)
>>>>>>>       bool delayed = true;
>>>>>>>       unsigned long flags;
>>>>>>> -    spin_lock_irqsave(&ploop->deferred_lock, flags);
>>>>>>>       if (unlikely(ploop->wants_suspend)) {
>>>>>>>           delayed = false;
>>>>>>>           goto unlock;
>>>>>>> @@ -176,10 +180,11 @@ static bool ploop_try_delay_enospc(struct 
>>>>>>> ploop_rq *prq, struct pio *pio)
>>>>>>>       pr_err_once(PL_FMT("underlying disk is almost full"),
>>>>>>>           ploop_device_name(ploop));
>>>>>>> +    spin_lock_irqsave(&ploop->deferred_lock, flags);
>>>>>>>       ploop->event_enospc = true;
>>>>>>> -    list_add_tail(&pio->list, &ploop->enospc_pios);
>>>>>>> -unlock:
>>>>>>>       spin_unlock_irqrestore(&ploop->deferred_lock, flags);
>>>>>>> +    llist_add((struct llist_node *)(&pio->list), &ploop- 
>>>>>>> >enospc_pios);
>>>>>>> +unlock:
>>>>>>>       if (delayed)
>>>>>>>           mod_timer(&ploop->enospc_timer, jiffies + 
>>>>>>> PLOOP_ENOSPC_TIMEOUT);
>>>>>>
>>>>>> Can you please explain why we need to take defered_lock around 
>>>>>> ploop- >event_enospc setting after your patch? (It looks that this 
>>>>>> lock does not do anything now.)
>>>>>>
>>>>>
>>>>> see static int ploop_get_event(...), without lock event can be missed
>>>>
>>>> That is not an explanation. How exactly can it be missed?
>>>
>>>
>>
>> Other cpu can set "ploop->event_enospc = true" here before lock (i.e. 
>> it was set to true twice), that would lead to emiting only one event 
>> for two sets.
>>
>>>          spin_lock_irq(&ploop->deferred_lock);
>>>          if (ploop->event_enospc) {                    <- while emit 
>>> other cpu can set
>>>                  ret = (DMEMIT("event_ENOSPC\n")) ? 1 : 0;
>>>                  if (ret)
>>>                          ploop->event_enospc = false; <- next cleaars 
>>> here -> second event lost
>>
>> Not lost, you just emit once for two sets, which as you can see from 
>> above comment is possible even with locks everywhere. So your 
>> explanation does not prove that we need this lock AFAICS.
> 
> 
> It is okay to emit once for two events set befere we EMIT.
> It is not okay to miss an event set after we emit.

What is the difference between event set before EMIT and event set just 
after emit before reseting event_enospc to false? Why should they be 
handled differently?

> 
> 1. Event(s) one or more
> 2. Start to EMIT
> 3. Userspace gets first event does what it does to handle it.
> 4. A new event comes - userspace is not notified

You would probably agree if all happens without concurrency exactly as 
you've wrote, a new event will also be notified.

So we talk about the case of concurrency, that means that in one thread 
event_enospc = true in ploop_try_delay_enospc was set after DMEMIT in 
ploop_get_event and before event_enospc = false in ploop_get_event in 
other thread.

In this case userspace is notified about both events in one emit, just 
about one event it is notified slightly earlier when it actually set 
true to variable. The question is - Why is it bad? I don't see why.

> 5. End of EMIT
> 
> at 4. we have a lost event - an event userspace will not receive.
> Cost is minimal as we do not expect to see much of this so the 
> correctness is more important here.
> 
> 
> And this can happen for sure - we have a lots of writes , they get 
> queued. Userspace frees some space , writes are retried but again space 
> is over because they need more space than it is freed. Without the 
> second event we get stuck here, userspace will not free more space.
> And pios will hang an retry.
> 
>>
>>>          }
>>>          spin_unlock_irq(&ploop->deferred_lock);
>>
>>
>>
>>>
>>>
>>>
>>
> 

-- 
Best regards, Tikhomirov Pavel
Senior Software Developer, Virtuozzo.