linux-kernel - Re: [PATCH v5 3.1.0-rc4-tip 1/26] uprobes: Auxillary routines to insert, find, delete uprobes

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Message-ID: <20110926120229.GC4072@linux.vnet.ibm.com>
Date:	Mon, 26 Sep 2011 17:32:29 +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>,
	Andi Kleen <andi@...stfloor.org>,
	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>,
	LKML <linux-kernel@...r.kernel.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v5 3.1.0-rc4-tip 1/26]   uprobes: Auxillary routines to
 insert, find, delete uprobes

* Peter Zijlstra <peterz@...radead.org> [2011-09-26 13:18:40]:

> On Tue, 2011-09-20 at 17:29 +0530, Srikar Dronamraju wrote:
> > +static struct uprobe *__insert_uprobe(struct uprobe *uprobe)
> > +{
> > +       struct rb_node **p = &uprobes_tree.rb_node;
> > +       struct rb_node *parent = NULL;
> > +       struct uprobe *u;
> > +       int match;
> > +
> > +       while (*p) {
> > +               parent = *p;
> > +               u = rb_entry(parent, struct uprobe, rb_node);
> > +               match = match_uprobe(uprobe, u);
> > +               if (!match) {
> > +                       atomic_inc(&u->ref);
> > +                       return u;
> > +               }
> > +
> > +               if (match < 0)
> > +                       p = &parent->rb_left;
> > +               else
> > +                       p = &parent->rb_right;
> > +
> > +       }
> > +       u = NULL;
> > +       rb_link_node(&uprobe->rb_node, parent, p);
> > +       rb_insert_color(&uprobe->rb_node, &uprobes_tree);
> > +       /* get access + drop ref */
> > +       atomic_set(&uprobe->ref, 2);
> > +       return u;
> > +} 
> 
> If you ever want to make a 'lockless' lookup work you need to set the
> refcount of the new object before its fully visible, instead of after.
> 

Agree, 

> Now much of a problem now since its fully serialized by that
> uprobes_treelock thing.
> 

Will stick with this for now; If and when we do a lockless lookup we
could fix this.

-- 
Thanks and Regards
Srikar
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