* Peter Zijlstra <peterz@xxxxxxxxxxxxx> [2011-09-27 13:41:21]:
> On Mon, 2011-09-26 at 21:14 +0530, Srikar Dronamraju wrote:
> > > Why not something like:
> > >
> > >
> > > +static struct uprobe *__find_uprobe(struct inode * inode, loff_t offset,
> > > bool inode_only)
> > > +{
> > > struct uprobe u = { .inode = inode, .offset = inode_only ? 0 : offset };
> > > + struct rb_node *n = uprobes_tree.rb_node;
> > > + struct uprobe *uprobe;
> > > struct uprobe *ret = NULL;
> > > + int match;
> > > +
> > > + while (n) {
> > > + uprobe = rb_entry(n, struct uprobe, rb_node);
> > > + match = match_uprobe(&u, uprobe);
> > > + if (!match) {
> > > if (!inode_only)
> > > atomic_inc(&uprobe->ref);
> > > + return uprobe;
> > > + }
> > > if (inode_only && uprobe->inode == inode)
> > > ret = uprobe;
> > > + if (match < 0)
> > > + n = n->rb_left;
> > > + else
> > > + n = n->rb_right;
> > > +
> > > + }
> > > return ret;
> > > +}
> > >
> >
> > I am not comfortable with this change.
> > find_uprobe() was suppose to return back a uprobe if and only if
> > the inode and offset match,
>
> And it will, because find_uprobe() will never expose that third
> argument.
>
> > However with your approach, we end up
> > returning a uprobe that isnt matching and one that isnt refcounted.
> > Moreover if even if we have a matching uprobe, we end up sending a
> > unrefcounted uprobe back.
>
> Because the matching isn't the important part, you want to return the
> leftmost node matching the specified inode. Also, in that case you
> explicitly don't want the ref, since the first thing you do on the
> call-site is drop the ref if there was a match. You don't care about
> inode:0 in particular, you want a place to start iterating all of
> inode:*.
>
The case of we taking a ref and dropping it would arise if and only if
there is a matching uprobe i.e inode: and 0 offset. I dont think that
would be the common case.
If you arent comfortable passing the rb_node as the third argument, then
we could pass the reference to uprobe itself. But that would mean we do
a redundant dereference everytime.
--
Thanks and Regards
Srikar
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