linux-kernel - 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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Date:	Tue, 15 Mar 2011 20:22:50 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
cc:	Peter Zijlstra <peterz@...radead.org>, 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>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	SystemTap <systemtap@...rces.redhat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.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.

On Tue, 15 Mar 2011, Srikar Dronamraju wrote:
> * Thomas Gleixner <tglx@...utronix.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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