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

[Waiman Long <waiman.long@hpe.com>]

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.

Cheers,
Longman