On Wed, Apr 30, 2014 at 10:50:09AM -0700, Tim Chen wrote:
> 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);
count = RWSEM_WAITING_BIAS
new = RWSEM_WAITING_BIAS + RWSEM_ACTIVE_WRITE_BIAS
new = count + RWSEM_ACTIVE_WRITE_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);
count = 0
new = RWSEM_ACTIVE_WRITE_BIAS
new = count + 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);
>
> Above line won't be correct for the case when count == 0. We are trying
> to acquire write lock, so the sem->count should become
> RWSEM_ACTIVE_WRITE_BIAS, or RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS.
> So we should change the logic to
>
> if (count == RWSEM_WAITING_BIAS)
> old = cmpxchg(&sem->count, count, count +
> RWSEM_ACTIVE_BIAS);
> else
> old = cmpxchg(&sem->count, count, count +
> RWSEM_ACTIVE_WRITE_BIAS);
I think I simply mis-typed it; shouldn't both cases be
RWSEM_ACTIVE_WRITE_BIAS ?
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