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
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