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    SubjectRe: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer
    On Sat, Apr 13, 2019 at 01:22:54PM -0400, Waiman Long wrote:
    > +/*
    > + * Try to acquire read lock before the reader is put on wait queue.
    > + * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
    > + * is ongoing.
    > + */
    > +static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
    > +{
    > + long count = atomic_long_read(&sem->count);
    > +
    > + if (RWSEM_COUNT_WLOCKED_OR_HANDOFF(count))
    > + return false;
    > +
    > + count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
    > + if (!RWSEM_COUNT_WLOCKED_OR_HANDOFF(count)) {
    > + rwsem_set_reader_owned(sem);
    > + lockevent_inc(rwsem_opt_rlock);
    > + return true;
    > + }
    > +
    > + /* Back out the change */
    > + atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
    > + return false;
    > +}

    Doesn't a cmpxchg 'loop' make more sense here?

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