Re: [PATCH v8 15/19] locking/rwsem: Adaptive disabling of reader optimistic spinning

[Peter Zijlstra]

On Mon, May 20, 2019 at 04:59:14PM -0400, Waiman Long wrote:
> +static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
> +                                           long last_rowner)
> +{
> +     long owner = atomic_long_read(&sem->owner);
> +
> +     if (!(owner & RWSEM_READER_OWNED))
> +             return false;
> +
> +     owner       &= ~RWSEM_OWNER_FLAGS_MASK;
> +     last_rowner &= ~RWSEM_OWNER_FLAGS_MASK;
> +     if ((owner != last_rowner) && rwsem_try_read_lock_unqueued(sem)) {

just because I'm struggling with sleep deprivation and the big picture
isn't making sense,.. you can write that like:

        ((owner ^ last_rowner) & ~RWSEM_OWNER_FLAGS_MASK)

> +             lockevent_inc(rwsem_opt_rlock2);
> +             lockevent_add(rwsem_opt_fail, -1);
> +             return true;
> +     }
> +     return false;
> +}