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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 31.08.17 at 09:15, <chao.gao@xxxxxxxxx> wrote:
> 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.

For non-recursive locks spin_is_locked() only tells you if _any_
CPU in the system currently holds the lock. For recursive ones it
checks whether it's the local CPU that owns the lock.

>>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.

Good.

>>>>> +    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?

If an atomic_t variable fits your needs, I think it should always
be preferred. add_sized() was introduced for a case where an
atomic_t variable would not have been usable. Please also
consult older commits for understanding the background.

>>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.

With it being left unclear whether the counter is meant to
also be protected by the lock: In the patch here you claim it
is, yet by later introducing lock-less readers you weaken
that model. Hence the request to bring things into a
consistent state right away, and ideally also into the final
state.

Jan


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