Re: [Xen-devel] [PATCH v5 1/4] VT-d PI: track the number of vcpus on pi blocking list

[Chao Gao <chao.gao@xxxxxxxxx>]

On Thu, Aug 31, 2017 at 01:42:53AM -0600, Jan Beulich wrote:
>>>> On 31.08.17 at 00:57, <chao.gao@xxxxxxxxx> wrote:
>> On Wed, Aug 30, 2017 at 10:00:49AM -0600, Jan Beulich wrote:
>>>>>> On 16.08.17 at 07:14, <chao.gao@xxxxxxxxx> wrote:
>>>> @@ -100,6 +101,24 @@ void vmx_pi_per_cpu_init(unsigned int cpu)
>>>>      spin_lock_init(&per_cpu(vmx_pi_blocking, cpu).lock);
>>>>  }
>>>>  
>>>> +static void vmx_pi_add_vcpu(struct pi_blocking_vcpu *pbv,
>>>> +                            struct vmx_pi_blocking_vcpu *vpbv)
>>>> +{
>>>> +    ASSERT(spin_is_locked(&vpbv->lock));
>>>
>>>You realize this is only a very weak check for a non-recursive lock?
>> 
>> I just thought the lock should be held when adding one entry to the
>> blocking list. Do you think we should remove this check or make it
>> stricter?
>
>Well, the primary purpose of my comment was to make you aware
>of the fact. If the weak check is good enough for you, then fine.

To be honest, I don't know the difference between weak check and tight
check.

>Removing the check would be a bad idea imo (but see also below);
>tightening might be worthwhile, but might also go too far (depending
>mainly on how clearly provable it is that all callers actually hold the
>lock).

IMO, the lock was introduced (not by me) to protect the blocking list.
list_add() and list_del() should be performed with the lock held. So I
think it is clear that all callers should hold the lock.

>
>>>> +    add_sized(&vpbv->counter, 1);
>>>> +    ASSERT(read_atomic(&vpbv->counter));
>>>
>>>Why add_sized() and read_atomic() when you hold the lock?
>>>
>> 
>> In patch 3, frequent reading the counter is used to find a suitable
>> vcpu and we can use add_sized() and read_atomic() to avoid acquiring the
>> lock. In one word, the lock doesn't protect the counter.
>
>In that case it would be more natural to switch to the atomic
>accesses there. Plus you still wouldn't need read_atomic()
>here, with the lock held. Furthermore I would then wonder
>whether it wasn't better to use atomic_t for the counter at

Is there some basic guide on when it is better to use read_atomic()
and add_sized() and when it is better to define a atomic variable
directly?

>that point. Also with a lock-less readers the requirement to
>hold a lock here (rather than using suitable LOCKed accesses)
>becomes questionable too.

As I said above, I think the lock is used to protect the list.

I think this patch has two parts:
1. Move all list operations to two inline functions. (with this, adding
a counter is easier and don't need add code in several places.)

2. Add a counter.

Thanks
Chao

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