Re: [PATCH v8 14/19] locking/rwsem: Enable time-based spinning on reader-owned rwsem

[Peter Zijlstra]

On Mon, May 20, 2019 at 04:59:13PM -0400, Waiman Long wrote:
> +static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
> +{
> +     long owner = atomic_long_read(&sem->owner);
> +
> +     while (owner & RWSEM_READER_OWNED) {
> +             if (owner & RWSEM_NONSPINNABLE)
> +                     break;
> +             owner = atomic_long_cmpxchg(&sem->owner, owner,
> +                                         owner | RWSEM_NONSPINNABLE);
> +     }
> +}

--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -206,12 +206,13 @@ static inline void rwsem_set_nonspinnabl
 {
        long owner = atomic_long_read(&sem->owner);
 
-       while (owner & RWSEM_READER_OWNED) {
+       do {
+               if (!(owner & RWSEM_READER_OWNED))
+                       break;
                if (owner & RWSEM_NONSPINNABLE)
                        break;
-               owner = atomic_long_cmpxchg(&sem->owner, owner,
-                                           owner | RWSEM_NONSPINNABLE);
-       }
+       } while (!atomic_long_try_cmpxchg(&sem->owner, &owner,
+                                         owner | RWSEM_NONSPINNABLE));
 }
 
 /*