Re: [PATCH v6] x86/hpet: Reduce HPET counter read contention

[Waiman Long]

On 08/25/2016 02:50 PM, Dave Hansen wrote:
> On 08/12/2016 05:59 PM, Waiman Long wrote:
> > + * The lock and the hpet value are stored together and can be read in a
> > + * single atomic 64-bit read. It is explicitly assumed that arch_spinlock_t
> > + * is 32 bits in size.
> This requirement forces us to give up all of the goodness of lockdep.
> Is this strictly a performance optimization or is there some function
> requirement behind it as well?

Yes, it is mostly performance optimization. Using a full spinlock will 
require additional synchronization code like a memory barrier to prevent 
race between the lock and HPET value with respect to the readers.

It is a simple lock that won't have additional locks nested inside. So I 
wonder if there is any value in having the lockdep functionality for 
this lock.

Cheers,
Longman