Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

["Paul E. McKenney" <paulmck@us.ibm.com>]

On Sat, Nov 18, 2006 at 10:02:17PM +0300, Oleg Nesterov wrote:
> On 11/17, Paul E. McKenney wrote:
> >
> >  int srcu_read_lock(struct srcu_struct *sp)
> >  {
> >  	int idx;
> > +	struct srcu_struct_array *sap;
> >
> >  	preempt_disable();
> >  	idx = sp->completed & 0x1;
> > -	barrier();  /* ensure compiler looks -once- at sp->completed. */
> > -	per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
> > -	srcu_barrier();  /* ensure compiler won't misorder critical section. */
> > +	sap = rcu_dereference(sp->per_cpu_ref);
> > +	if (likely(sap != NULL)) {
> > +		barrier();  /* ensure compiler looks -once- at sp->completed. */
> > +		per_cpu_ptr(rcu_dereference(sap),
> > +			    smp_processor_id())->c[idx]++;
> > +		smp_mb();
> > +		preempt_enable();
> > +		return idx;
> > +	}
> > +	if (mutex_trylock(&sp->mutex)) {
> > +		preempt_enable();
> > +		if (sp->per_cpu_ref == NULL)
> > +			sp->per_cpu_ref = alloc_srcu_struct_percpu();
> > +		if (sp->per_cpu_ref == NULL) {
> > +			atomic_inc(&sp->hardluckref);
> > +			mutex_unlock(&sp->mutex);
> > +			return -1;
> > +		}
> > +		mutex_unlock(&sp->mutex);
> > +		return srcu_read_lock(sp);
> > +	}
> >  	preempt_enable();
> > -	return idx;
> > +	atomic_inc(&sp->hardluckref);
> > +	return -1;
> >  }
> 
> This is a real nitpick, but in theory we have a possibility for the livelock.
> 
> Suppose that synchronize_srcu() takes sp->mutex and fails to allocate
> sp->per_cpu_ref. If we have a flow of srcu_read_lock/srcu_read_unlock,
> this loop in synchronize_srcu()
> 
>   	while (srcu_readers_active_idx(sp, idx))
>   		schedule_timeout_interruptible(1);
> 
> may spin unpredictably long, because we use the same sp->hardluckref for
> accounting.

Excellent point -- hardluckref also needs to be a two-element array.

						Thanx, Paul