Re: [PATCH v8 17/19] locking/rwsem: Merge owner into count on x86-64

From: Waiman Long
Date: Tue Jun 04 2019 - 11:51:25 EST

On 6/4/19 5:45 AM, Peter Zijlstra wrote:
> On Mon, May 20, 2019 at 04:59:16PM -0400, Waiman Long wrote:
>> With separate count and owner, there are timing windows where the two
>> values are inconsistent. That can cause problem when trying to figure
>> out the exact state of the rwsem. For instance, a RT task will stop
>> optimistic spinning if the lock is acquired by a writer but the owner
>> field isn't set yet. That can be solved by combining the count and
>> owner together in a single atomic value.
> I just realized we can use cmpxchg_double() here (where available of
> course).

Does the 2 doubles need to be 128-bit aligned to use cmpxchg_double()? I
don't think we can guarantee that unless we explicitly set this alignment.