Re: [PATCH 05/10] Introduce ridr_get_new_above()

[Nadia Derbey]

Tim Pepper wrote:
> On Tue 29 Apr at 16:33:09 +0200 Nadia.Derbey@xxxxxxxx said:
>
> > [PATCH 05/10]
> >
> > This patch introduces the ridr_get_new_above() routine, and some common code
> > between the idr an ridr API's.
>
>
> The ridr_get_new_above() is the first place we see something really
> different compared to idr (an RCU addition).  This is a lot of patching
> so far for what would be a small incremental change otherwise.
>
>
> > Index: linux-2.6.25-mm1/include/linux/idr.h
> > ===================================================================
> > --- linux-2.6.25-mm1.orig/include/linux/idr.h	2008-04-29 13:08:00.000000000 +0200
> > +++ linux-2.6.25-mm1/include/linux/idr.h	2008-04-29 14:08:47.000000000 +0200
> > @@ -71,6 +71,27 @@ struct idr {
> > }
> > #define DEFINE_IDR(name)	struct idr name = IDR_INIT(name)
> >
> > +/* Actions to be taken after a call to _idr_sub_alloc */
> > +#define IDR_DONE         -1
> > +#define IDR_NEED_TO_GROW -2
> > +#define IDR_NOMORE_SPACE -3
> > +#define IDR_GO_TOP       -4
> > +#define IDR_GO_UP        -5
>
>
> This stuff is useful on its own in as much as it improves code
> readability.  A lot of this patch (the "some common code between the
> idr and ridr API's" part) could be a standalone patch distinct from the
> ridr series and then be at the head of your stack.
>
>
> > +			return action;
> > +		case IDR_DONE:
> > +			goto end_loop;
> > +		case IDR_GO_UP:
> > +			continue;
> > +		case IDR_GO_TOP:
> > +			goto restart;
> > +		default:
> > +			m = action;
> > 			break;
> > +		}
> > +		BUG_ON(m < 0);
>
>
> Why the added BUG_ON()?  These couple hunks are a bit muddled, so maybe I
> missed something.  But I don't see anything different in how m or bm are
> being manipulated such that m<0 is anymore likely after this patch.
>
>
> > Index: linux-2.6.25-mm1/lib/ridr.c
> > ===================================================================
> > --- linux-2.6.25-mm1.orig/lib/ridr.c	2008-04-29 13:23:17.000000000 +0200
> > +++ linux-2.6.25-mm1/lib/ridr.c	2008-04-29 14:03:35.000000000 +0200
> > @@ -11,6 +11,23 @@
> > static struct kmem_cache *ridr_layer_cache;
> >
> >
> > +static struct ridr_layer *get_from_free_list(struct ridr *idp)
> > +{
> > +	struct ridr_layer *q;
> > +	struct idr_layer *p;
> > +	unsigned long flags;
> > +
> > +	spin_lock_irqsave(&idp->lock, flags);
> > +	if ((q = idp->id_free)) {
> > +		p = ridr_to_idr(q);
> > +		idp->id_free = p->ary[0];
> > +		idp->id_free_cnt--;
> > +		p->ary[0] = NULL;
> > +	}
> > +	spin_unlock_irqrestore(&idp->lock, flags);
> > +	return(q);
> > +}
> > +
>
>
> idr's alloc_layer() in disguise?
>
>
> > +static int sub_alloc(struct ridr *idp, int *starting_id,
> > +			struct ridr_layer **rpa, struct idr_layer **pa)
>
>
> ...
> More or less duplication
> ...
>
>
> > +		 * Create the layer below if it is missing.
> > +		 */
> > +		if (!p->ary[m]) {
> > +			new = get_from_free_list(idp);
> > +			if (!new)
> > +				return -1;
> > +			rcu_assign_pointer(p->ary[m], new);
> > +			p->count++;
> > +		}
> > +		pa[l] = p;
> > +		rpa[l--] = idr_to_ridr(p);
> > +		p = p->ary[m];
> > +	}
> > +
> > +end_loop:
> > +	pa[l] = p;
> > +	rpa[l] = idr_to_ridr(p);
> > +	return id;
> > +}
>
>
> Oh but wait..there's some RCU-ness tucked in there.
>
>
> > +
> > +static int ridr_get_empty_slot(struct ridr *idp, int starting_id,
> > +			      struct ridr_layer **rpa, struct idr_layer **pa)
> > +{
> > +	struct ridr_layer *p, *rnew;
> > +	int layers, v, id;
> > +	unsigned long flags;
> > +
> > +	id = starting_id;
> > +build_up:
> > +	p = idp->top;
> > +	layers = idp->layers;
> > +	if (unlikely(!p)) {
> > +		p = get_from_free_list(idp);
> > +		if (!p)
> > +			return -1;
> > +		layers = 1;
> > +	}
> > +	/*
> > +	 * Add a new layer to the top of the tree if the requested
> > +	 * id is larger than the currently allocated space.
> > +	 */
> > +	while (layers < MAX_LEVEL - 1 && id >= (1 << (layers * IDR_BITS))) {
>
>                ^^                   ^^
> Dropped some parens.  Otherwise more duplication...
>
>
> > +				rnew->idr.ary[0] = NULL;
> > +				rnew->idr.bitmap = rnew->idr.count = 0;
> > +				__move_to_free_list(idp, rnew);
> > +			}
> > +			spin_unlock_irqrestore(&idp->lock, flags);
> > +			return -1;
> > +		}
> > +		_idr_set_new_slot(ridr_to_idr(rnew), ridr_to_idr(p));
> > +		p = rnew;
> > +	}
> > +	rcu_assign_pointer(idp->top, p);
> > +	idp->layers = layers;
> > +	v = sub_alloc(idp, &id, rpa, pa);
> > +	if (v == IDR_NEED_TO_GROW)
> > +		goto build_up;
> > +	return(v);
> > +}
>
>
> Some more RCU.
>
>
> > +static int ridr_get_new_above_int(struct ridr *idp, void *ptr, int starting_id)
> > +{
> > +	struct ridr_layer *rpa[MAX_LEVEL];
> > +	struct idr_layer *pa[MAX_LEVEL];
> > +	int id;
> > +
> > +	id = ridr_get_empty_slot(idp, starting_id, rpa, pa);
> > +	if (id >= 0) {
> > +		/*
> > +		 * Successfully found an empty slot.  Install the user
> > +		 * pointer and mark the slot full.
> > +		 */
> > +		rcu_assign_pointer(pa[0]->ary[id & IDR_MASK],
> > +				(struct ridr_layer *)ptr);
> > +		pa[0]->count++;
> > +		_idr_mark_full(pa, id);
> > +	}
> > +
> > +	return id;
> > +}
>
>
> And other line of RCU.
>
> OK.  So at this point in patch 5/10 we've got 3 lines of new code and
> hundreds of lines of duplicated code?
>
> A while more looking through the rest of the patches for the rest of the
> context and I might be able to actually think about the implications of
> these three lines being where they are.
>
> Locking changes are complicated enough without all this obfuscation!
> I understand the desire to not break IDR, but...

OK, you convinced me. I'll re-send a new patchset that is incremental on 
top of idr. Sorry (and thanks a lot) for the painful review!

Regards,
Nadia
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