Date: Tue, 15 Mar 2011 20:22:50 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
cc: Peter Zijlstra <peterz@infradead.org>, 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>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Christoph Hellwig <hch@infradead.org>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Andi Kleen <andi@firstfloor.org>, Oleg Nesterov <oleg@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jim Keniston <jkenisto@linux.vnet.ibm.com>,
        Roland McGrath <roland@hack.frob.com>,
        SystemTap <systemtap@sources.redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [PATCH v2 2.6.38-rc8-tip 4/20] 4: uprobes: Adding and remove a
 uprobe in a rb tree.
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On Tue, 15 Mar 2011, Srikar Dronamraju wrote:
> * Thomas Gleixner <tglx@linutronix.de> [2011-03-15 14:38:33]:
> > > +/*
> > > + * Find a uprobe corresponding to a given inode:offset
> > > + * Acquires treelock
> > > + */
> > > +static struct uprobe *find_uprobe(struct inode * inode, loff_t offset)
> > > +{
> > > +	struct uprobe *uprobe;
> > > +	unsigned long flags;
> > > +
> > > +	spin_lock_irqsave(&treelock, flags);
> > > +	uprobe = __find_uprobe(inode, offset, NULL);
> > > +	spin_unlock_irqrestore(&treelock, flags);
> > 
> > What's the calling context ? Do we really need a spinlock here for
> > walking the rb tree ?
> > 
> 
> find_uprobe() gets called from unregister_uprobe and on probe hit from
> uprobe_notify_resume. I am not sure if its a good idea to walk the tree
> as and when the tree is changing either because of a insertion or
> deletion of a probe.

I know that you cannot walk the tree lockless except you would use
some rcu based container for your probes.

Though my question is more whether this needs to be a spinlock or if
that could be replaced by a mutex. At least there is no reason to
disable interrupts. You cannot trap into a probe from the thread in
which you are installing/removing it.

Thanks,

	tglx
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