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Date: Mon, 26 Sep 2011 17:32:29 +0530
From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>,
        Linux-mm <linux-mm@kvack.org>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andi Kleen <andi@firstfloor.org>, Hugh Dickins <hughd@google.com>,
        Christoph Hellwig <hch@infradead.org>,
        Jonathan Corbet <corbet@lwn.net>, Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Oleg Nesterov <oleg@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
        Jim Keniston <jkenisto@linux.vnet.ibm.com>,
        Roland McGrath <roland@hack.frob.com>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v5 3.1.0-rc4-tip 1/26]   uprobes: Auxillary routines to
 insert, find, delete uprobes
Message-ID: <20110926120229.GC4072@linux.vnet.ibm.com>
Reply-To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
References: <20110920115938.25326.93059.sendpatchset@srdronam.in.ibm.com>
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 <1317035920.9084.84.camel@twins>
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* Peter Zijlstra <peterz@infradead.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