Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Sep 19, 2024 at 02:39:13PM +0800, Lai Jiangshan wrote:
> On Tue, Sep 17, 2024 at 10:34 PM Boqun Feng <boqun.feng@gmail.com> wrote:
> 
> > +static void hazptr_context_snap_readers_locked(struct hazptr_reader_tree *tree,
> > +                                              struct hazptr_context *hzcp)
> > +{
> > +       lockdep_assert_held(hzcp->lock);
> > +
> > +       for (int i = 0; i < HAZPTR_SLOT_PER_CTX; i++) {
> > +               /*
> > +                * Pairs with smp_store_release() in hazptr_{clear,free}().
> > +                *
> > +                * Ensure
> > +                *
> > +                * <reader>             <updater>
> > +                *
> > +                * [access protected pointers]
> > +                * hazptr_clear();
> > +                *   smp_store_release()
> > +                *                      // in reader scan.
> > +                *                      smp_load_acquire(); // is null or unused.
> > +                *                      [run callbacks] // all accesses from
> > +                *                                      // reader must be
> > +                *                                      // observed.
> > +                */
> > +               hazptr_t val = smp_load_acquire(&hzcp->slots[i]);
> > +
> > +               if (!is_null_or_unused(val)) {
> > +                       struct hazptr_slot_snap *snap = &hzcp->snaps[i];
> > +
> > +                       // Already in the tree, need to remove first.
> > +                       if (!is_null_or_unused(snap->slot)) {
> > +                               reader_del(tree, snap);
> > +                       }
> > +                       snap->slot = val;
> > +                       reader_add(tree, snap);
> > +               }
> > +       }
> > +}
> 
> Hello
> 
> I'm curious about whether there are any possible memory leaks here.
> 
> It seems that call_hazptr() never frees the memory until the slot is
> set to another valid value.
> 
> In the code here, the snap is not deleted when hzcp->snaps[i] is null/unused
> and snap->slot is not which I think it should be.
> 
> And it can cause unneeded deletion and addition of the snap if the slot
> value is unchanged.
> 

I think you're right. (Although the node will be eventually deleted at
cleanup_hazptr_context(), however there could be a long-live
hazptr_context). It should be:

		hazptr_t val = smp_load_acquire(&hzcp->slots[i]);
		struct hazptr_slot_snap *snap = &hzcp->snaps[i];

		if (val != snap->slot) { // val changed, need to update the tree node.
			// Already in the tree, need to remove first.
			if (!is_null_or_unused(snap->slot)) {
				reader_del(tree, snap);
			}

			// use the latest snapshot.
			snap->slot = val;

			// Add it into tree if there is a reader
			if (!is_null_or_unused(val))
				reader_add(tree, snap);
		}

Regards,
Boqun

> I'm not so sure...
> 
> Thanks
> Lai