On Wed, May 11, 2016 at 11:26:02AM -0700, Jason Low wrote:
> On Wed, 2016-05-11 at 13:49 +0200, Peter Zijlstra wrote:
> > > static inline bool rwsem_try_write_lock(long count, struct rw_semaphore
> > > *sem)
> > > {
> > > /*
> > > + * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
> > > */
> > > + if (count != RWSEM_WAITING_BIAS)
> > > + return false;
> > > +
> > > + /*
> > > + * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
> > > + * are other tasks on the wait list, we need to add on WAITING_BIAS.
> > > + */
> > > + count = list_is_singular(&sem->wait_list) ?
> > > + RWSEM_ACTIVE_WRITE_BIAS :
> > > + RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;
> > > +
> > > + if (cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count) ==
> > > RWSEM_WAITING_BIAS) {
> > > rwsem_set_owner(sem);
> > > return true;
> > > }
> >
> > Right; so that whole thing works because we're holding sem->wait_lock.
> > Should we clarify that someplace?
>
> Yup, we can mention that the rwsem_try_write_lock() function must be
> called with the wait_lock held.
Also try to explain _why_ it must be held.
Thanks!
