[PATCH] nptl: Add backoff mechanism to spinlock loop

[Guo, Wangyang]

Noah Goldstein wrote:
> > >> +                 atomic_spin_nop ();
> > >> +             while (--spin_count > 0);
> > >> +             /* Binary exponential backoff, prepare for next loop.  */
> > >> +             exp_backoff <<= 1;
> > >>             }
> > >>           while (LLL_MUTEX_READ_LOCK (mutex) != 0
> > >Does this load not already prevent against the 'CAS storm'?
> > This just prevent CAS in a long held lock.
> > But if multiple threads waiting for lock at the same time, suppose 
> > many of them are going to read lock state, once the lock owner release the lock at this point, those waiter threads will be convinced lock is unlocked.
> > The next step, they will all do try lock at the same time. Backoff is introduced to solve this problem.
> 
> The loop isn't spinning on CAS failure which is typically where you see the poor performance.
> I get that there can still be some contention on the CAS, but shouldn't the read check limit the evict ping-ponging?

Yes, read check can help. 
But in a very high contention case, it becomes more easier to meet the above problem that needs backoff.
> >
> > >>                  || LLL_MUTEX_TRYLOCK (mutex) != 0);
> >