On Mon, 2014-08-04 at 22:30 -0700, Davidlohr Bueso wrote:
> On Mon, 2014-08-04 at 21:54 -0700, Davidlohr Bueso wrote:
> > > #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
> > > +/*
> > > + * The owner field is set to RWSEM_READ_OWNED if the last owner(s) are
> > > + * readers. It is not reset until a writer takes over and set it to its
> > > + * task structure pointer or NULL when it frees the lock. So a value
> > > + * of RWSEM_READ_OWNED doesn't mean it currently has active readers.
> > > + */
> > > +#define RWSEM_READ_OWNED ((struct task_struct *)-1)
> >
> > Looks rather weird...
>
> Instead of populating owner when taking the reader lock, why not just
> leave it NULL. Then, we can differentiate between the owner being NULL
> either because it is taken by reader(s) or simply because it is not
> taken. So something like this:
>
> static inline bool rwsem_owner_is_reader(struct rw_semaphore *sem)
> {
> return !sem->owner && rwsem_is_locked(sem));
> }
Although we could race between both checks, specially when going into
slowpaths, too bad. Probably a read barrier before calling is_locked
too. Anyway, you see, things start getting funky.
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