Re: [PATCH 4/5] locking/rwsem: Avoid deceiving lock spinners

[Jason Low]

On Tue, 2015-02-03 at 11:43 -0800, Tim Chen wrote:
> On Tue, 2015-02-03 at 09:54 -0800, Jason Low wrote:
> > On Tue, 2015-02-03 at 09:16 -0800, Tim Chen wrote:
> > > > > >  
> > > > > > +	if (READ_ONCE(sem->owner))
> > > > > > +		return true; /* new owner, continue spinning */
> > > > > > +
> > > > > 
> > > > > Do you have some comparison data of whether it is more advantageous
> > > > > to continue spinning when owner changes?  After the above change, 
> > > > > rwsem will behave more like a spin lock for write lock and 
> > > > > will keep spinning when the lock changes ownership.
> > > > 
> > > > But recall we still abort when need_resched, so the spinning isn't
> > > > infinite. Never has been.
> > > > 
> > > > >  Now during heavy
> > > > > lock contention, if we don't continue spinning and sleep, we may use the
> > > > > clock cycles for actually running other threads. 
> > > > 
> > > > Under heavy contention, time spinning will force us to ultimately block
> > > > anyway.
> > > 
> > > The question is under heavy contention, if we are going to block anyway,
> > > won't it be more advantageous not to continue spinning so we can use
> > > the cycles for useful task?
> > 
> > Hi Tim,
> > 
> > Now that we have the OSQ logic, under heavy contention, there will still
> > only be 1 thread that is spinning on owner at a time. 
> 
> That's true.  We cannot have the lock grabbed by a new write 
> contender as any new writer contender of the lock will be 
> queued by the OSQ logic. Only the
> thread doing the optimistic spin is attempting write lock.  
> In other word, switching of write owner of the rwsem to a new
> owner cannot happen.

Another thread can still obtain the write lock in the fast path though
right? We try to obtain the write lock once before calling
rwsem_down_write_failed().


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