Re: [PATCH] locking/rwsem: Optimize write lock slowpath

[Peter Zijlstra <peterz@infradead.org>]

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!