On Wed, 2014-04-30 at 11:08 -0700, Tim Chen wrote:
> On Wed, 2014-04-30 at 20:04 +0200, Peter Zijlstra wrote:
> > 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 ?
>
> Yeah, we should just write it as
> old = cmpxchg(&sem->count, count,
> RWSEM_ACTIVE_WRITE_BIAS);
Oops, I mean
old = cmpxchg(&sem->count, count, count +
RWSEM_ACTIVE_WRITE_BIAS);
for count == 0, we need sem->count to be RWSEM_ACTIVE_WRITE_BIAS,
for count == RWSEM_WAITING_BIAS, we need sem->count to be RWSEM_WAITING_BIAS +
RWSEM_ACTIVE_WRITE_BIAS
Tim
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