On Wed, 2014-04-30 at 10:27 +0200, Peter Zijlstra wrote:
> On Mon, Apr 28, 2014 at 03:09:01PM -0700, Davidlohr Bueso wrote:
> > +/*
> > + * Try to acquire write lock before the writer has been put on wait queue.
> > + */
> > +static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> > +{
> > + long count = ACCESS_ONCE(sem->count);
> > +retry:
> > + if (count == RWSEM_WAITING_BIAS) {
> > + count = cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
> > + RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS);
> > + /* allow write lock stealing, try acquiring the write lock. */
> > + if (count == RWSEM_WAITING_BIAS)
> > + goto acquired;
> > + else if (count == 0)
> > + goto retry;
> > + } else if (count == 0) {
> > + count = cmpxchg(&sem->count, 0, RWSEM_ACTIVE_WRITE_BIAS);
> > + if (count == 0)
> > + goto acquired;
> > + else if (count == RWSEM_WAITING_BIAS)
> > + goto retry;
> > + }
> > + return false;
> > +
> > +acquired:
> > + return true;
> > +}
>
> Could we have written that like:
>
> static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> {
> long old, count = ACCESS_ONCE(sem->count);
>
> for (;;) {
> if (!(count == 0 || count == RWSEM_WAITING_BIAS))
> return false;
>
> old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_BIAS);
> if (old == count)
> return true;
>
> count = old;
> }
> }
>
> ?
Yeah that's a lot nicer.
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