Messages in this thread | | | Date | Wed, 30 Apr 2014 20:04:41 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] rwsem: Support optimistic spinning |
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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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