Messages in this thread | | | Date | Tue, 4 Jun 2019 19:02:18 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v8 17/19] locking/rwsem: Merge owner into count on x86-64 |
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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.
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