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

From: Waiman Long
Date: Tue Jun 04 2019 - 13:10:12 EST

On 6/4/19 1:02 PM, Peter Zijlstra wrote:
> On Tue, Jun 04, 2019 at 11:47:21AM -0400, Waiman Long wrote:
>> 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.
> It does :/ and yes, we'd need to play games with __align(2*sizeof(long))
> and such.

So do you want this as an option now as it will be x86 specific? Or we
can do that as a follow-up if we want to.