linux-kernel - Re: [PATCH v5 3.1.0-rc4-tip 4/26] uprobes: Define hooks for mmap/munmap.

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Date:	Tue, 27 Sep 2011 18:29:00 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jonathan Corbet <corbet@....net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3.1.0-rc4-tip 4/26]   uprobes: Define hooks for
 mmap/munmap.

* Peter Zijlstra <peterz@...radead.org> [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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