Re: [RESEND PATCH v4] x86/hpet: Reduce HPET counter read contention

[Andy Lutomirski]

On Aug 12, 2016 9:31 PM, "Waiman Long" <waiman.long@xxxxxxx> wrote:
>
> On 08/12/2016 01:16 PM, Dave Hansen wrote:
>>
>> On 08/12/2016 10:01 AM, Waiman Long wrote:
>>>
>>> The reason for using a special lock is that I want both sequence number
>>> update and locking to be done together atomically. They can be made
>>> separate as is in the seqlock. However, that will make the code more
>>> complex to make sure that all the threads see a consistent set of lock
>>> state and sequence number.
>>
>> Why do we need a sequence number?  The "cached" HPET itself could be used.
>>
>> I'm thinking something like below could use a spinlock instead of the
>> doing a custom cmpxchg sequence.  The spin_is_locked() should allow the
>> contended "readers" to avoid using atomics.
>>
>> spinlock_t hpet_lock;
>> u32 hpet_value;
>> ...
>> {
>>         u32 old_hpet = READ_ONCE(hpet_value);
>>         u32 new_hpet;
>>
>>         // need to ensure that the spin_is_locked() is ordered after
>>         // the READ_ONCE().
>>         smp_rmb();
>>         // spin_is_locked() doesn't do atomics
>>         if (!spin_is_locked(&hpet_lock)&&  spin_trylock(&hpet_lock)) {
>>
>>                 WRITE_ONCE(hpet_value, real_read_hpet());
>>                 spin_unlock(&hpet_lock);
>>                 return hpet_value;
>>         }
>>         // Contended case.  We spin here waiting for the guy who holds
>>         // the lock to write a new value to 'hpet_value'.
>>         //
>>         // We know that our old_hpet is older than our check for the
>>         // spinlock being locked. So, someone must either have already
>>         // updated it or be updating it.
>>         do {
>>                 cpu_relax();
>>                 // We do not do a rmb() here.  We don't need a guarantee
>>                 // that this read is up-to-date, just that it will
>>                 // _eventually_ see an up-to-date value.
>>                 new_hpet = READ_ONCE(hpet_value);
>>         } while (old_hpet == new_hpet);
>>         return new_hpet;
>> }
>
>
> Yes, I think that work too. I will update my patch accordingly. Thanks for the input.

Why is Dave more convincing than I was a couple months ago when I
asked a similar question? :)

I don't think this is right.  If the HPET ever returns the same value
twice in a row (unlikely because it's generally too slow to read, but
it's plausible that someone will make a fast HPET some day), then this
could deadlock.

Also, does this code need to be NMI-safe?  This implementation is
deadlocky if it's called from an NMI.

The original code was wait-free, right?  That was a nice property, too.

--Andy