synchronize_qrcu_timeout()

[Oleg Nesterov]

On 06/25, Paul E. McKenney wrote:
>
> On Mon, Jun 25, 2007 at 02:43:32PM +0400, Oleg Nesterov wrote:
> > 
> > Sadly, you can't use srcu/qrcu because it doesn't handle timeouts.  
> 
> Interesting...  So the thought is to have a synchronize_srcu_timeout()
> or something similar that waited for a grace period to elapse or for
> a timeout to expire, whichever comes first?  It should not be too hard
> to arrange something, if needed.

Yes. As for qrcu (see http://marc.info/?t=116484476500001), I think it is easy.
First, we add "int interrupted" into struct qrcu_struct, then something like 
this

        long synchronize_qrcu_timeout(struct qrcu_struct *qp, long tout)
        {
                int idx;

                smp_mb();
                mutex_lock(&qp->mutex);

        again:
                idx = qp->completed & 0x1;

                if (unlikely(qp->interrupted)) {
                        idx ^= 0x1;
                        goto restart;
                }


                if (atomic_read(qp->ctr + idx) == 1)
                        goto out;

                atomic_inc(qp->ctr + (idx ^ 0x1));
                qp->completed++;

                atomic_dec(qp->ctr + idx);
        restart:
                __wait_event_timeout(qp->wq, !atomic_read(qp->ctr + idx), tout);
                if (unlikely(!tout)) {
                        qp->interrupted = 1;
                        goto out;
                }

                if (unlikely(qp->interrupted)) {
                        qp->interrupted = 0;
                        goto again;
                }

        out:
                mutex_unlock(&qp->mutex);
                smp_mb();

                return tout;
        }

Of course, this needs some other changes to implement synchronize_qrcu() on top
of synchronize_qrcu_timeout(). I also think that this doesn't break

        [RFC PATCH] QRCU fastpath optimization
        http://marc.info/?l=linux-kernel&m=117125577710947

Looks like we can do something similar for srcu.

Oleg.

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