Re: [PATCH RFC 3/9] RCU: Preemptible RCU

Peter Zijlstra
2007-09-21T15:47:19Z

On Fri, 21 Sep 2007 10:40:03 -0400 Steven Rostedt <rostedt@goodmis.org>
wrote:

> On Mon, Sep 10, 2007 at 11:34:12AM -0700, Paul E. McKenney wrote:

> Can you have a pointer somewhere that explains these states. And not a
> "it's in this paper or directory". Either have a short discription here,
> or specify where exactly to find the information (perhaps a
> Documentation/RCU/preemptible_states.txt?).
>
> Trying to understand these states has caused me the most agony in
> reviewing these patches.
>
> > + */
> > +
> > +enum rcu_try_flip_states {
> > + rcu_try_flip_idle_state, /* "I" */
> > + rcu_try_flip_waitack_state, /* "A" */
> > + rcu_try_flip_waitzero_state, /* "Z" */
> > + rcu_try_flip_waitmb_state /* "M" */
> > +};

I thought the 4 flip states corresponded to the 4 GP stages, but now
you confused me. It seems to indeed progress one stage for every 4 flip
states.

Hmm, now I have to puzzle how these 4 stages are required by the lock
and unlock magic.

> > +/*
> > + * Return the number of RCU batches processed thus far. Useful for debug
> > + * and statistics. The _bh variant is identical to straight RCU.
> > + */
>
> If they are identical, then why the separation?

I guess a smaller RCU domain makes for quicker grace periods.

> > +void __rcu_read_lock(void)
> > +{
> > + int idx;
> > + struct task_struct *me = current;
>
> Nitpick, but other places in the kernel usually use "t" or "p" as a
> variable to assign current to. It's just that "me" thows me off a
> little while reviewing this. But this is just a nitpick, so do as you
> will.

struct task_struct *curr = current;

is also not uncommon.

> > + int nesting;
> > +
> > + nesting = ORDERED_WRT_IRQ(me->rcu_read_lock_nesting);
> > + if (nesting != 0) {
> > +
> > + /* An earlier rcu_read_lock() covers us, just count it. */
> > +
> > + me->rcu_read_lock_nesting = nesting + 1;
> > +
> > + } else {
> > + unsigned long oldirq;
>
> > +
> > + /*
> > + * Disable local interrupts to prevent the grace-period
> > + * detection state machine from seeing us half-done.
> > + * NMIs can still occur, of course, and might themselves
> > + * contain rcu_read_lock().
> > + */
> > +
> > + local_irq_save(oldirq);
>
> Isn't the GP detection done via a tasklet/softirq. So wouldn't a
> local_bh_disable be sufficient here? You already cover NMIs, which would
> also handle normal interrupts.

This is also my understanding, but I think this disable is an
'optimization' in that it avoids the regular IRQs from jumping through
these hoops outlined below.

> > +
> > + /*
> > + * Outermost nesting of rcu_read_lock(), so increment
> > + * the current counter for the current CPU. Use volatile
> > + * casts to prevent the compiler from reordering.
> > + */
> > +
> > + idx = ORDERED_WRT_IRQ(rcu_ctrlblk.completed) & 0x1;
> > + smp_read_barrier_depends(); /* @@@@ might be unneeded */
> > + ORDERED_WRT_IRQ(__get_cpu_var(rcu_flipctr)[idx])++;
> > +
> > + /*
> > + * Now that the per-CPU counter has been incremented, we
> > + * are protected from races with rcu_read_lock() invoked
> > + * from NMI handlers on this CPU. We can therefore safely
> > + * increment the nesting counter, relieving further NMIs
> > + * of the need to increment the per-CPU counter.
> > + */
> > +
> > + ORDERED_WRT_IRQ(me->rcu_read_lock_nesting) = nesting + 1;
> > +
> > + /*
> > + * Now that we have preventing any NMIs from storing
> > + * to the ->rcu_flipctr_idx, we can safely use it to
> > + * remember which counter to decrement in the matching
> > + * rcu_read_unlock().
> > + */
> > +
> > + ORDERED_WRT_IRQ(me->rcu_flipctr_idx) = idx;
> > + local_irq_restore(oldirq);
> > + }
> > +}

> > +/*
> > + * Attempt a single flip of the counters. Remember, a single flip does
> > + * -not- constitute a grace period. Instead, the interval between
> > + * at least three consecutive flips is a grace period.
> > + *
> > + * If anyone is nuts enough to run this CONFIG_PREEMPT_RCU implementation
>
> Oh, come now! It's not "nuts" to use this ;-)
>
> > + * on a large SMP, they might want to use a hierarchical organization of
> > + * the per-CPU-counter pairs.
> > + */

Its the large SMP case that's nuts, and on that I have to agree with
Paul, its not really large SMP friendly.