On Mon, May 09, 2016 at 10:24:16PM -0400, Waiman Long wrote:
> >>+static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
> >>+{
> >>+ /*
> >>+ * We check the owner value first to make sure that we will only
> >>+ * do a write to the rwsem cacheline when it is really necessary
> >>+ * to minimize cacheline contention.
> >>+ */
> >>+ if (sem->owner != (struct task_struct *)RWSEM_READER_OWNED)
> >>+ sem->owner = (struct task_struct *)RWSEM_READER_OWNED;
> >How much if anything did this optimization matter?
>
> I hadn't run any performance test to verify the effective of this change.
> For a reader-heavy rwsem, this change should be able to save quite a lot of
> needless write to the rwsem cacheline.
Right; I was just wondering.
> >>+static inline bool rwsem_is_writer_owned(struct task_struct *owner)
> >>+{
> >>+ return (unsigned long)owner> RWSEM_READER_OWNED;
> >>+}
> >Tad too clever that; what does GCC generate if you write the obvious:
> >
> > return owner&& owner != RWSEM_READER_OWNER;
>
> You are right. GCC is intelligent enough to make the necessary optimization.
> I will revert it to this form which is more obvious.
Yay! thanks for checking. Sometimes GCC throws a wobbly and does
something unexpected, but it tends to get these 'simple' things right.
