linux-kernel - Re: [RFC] [PATCH 2.6.37-rc5-tip 5/20] 5: Uprobes: register/unregister probes.

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Date:	Wed, 26 Jan 2011 11:10:56 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	linux-mm@...ck.org, Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	SystemTap <systemtap@...rces.redhat.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC] [PATCH 2.6.37-rc5-tip 5/20]  5: Uprobes:
 register/unregister probes.

On Wed, 2011-01-26 at 13:17 +0530, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@...radead.org> [2011-01-25 13:15:45]:
> 
> > > +
> > > +       if (atomic_read(&uprobe->ref) == 1) {
> > > +               synchronize_sched();
> > > +               rb_erase(&uprobe->rb_node, &uprobes_tree);
> > 
> > How is that safe without holding the treelock?
> 
> Right, 
> Something like this should be good enuf right?
> 
> if (atomic_read(&uprobe->ref) == 1) {
> 	synchronize_sched();
> 	spin_lock_irqsave(&treelock, flags);
> 	rb_erase(&uprobe->rb_node, &uprobes_tree);
> 	spin_lock_irqrestore(&treelock, flags);
> 	iput(uprobe->inode);
> }
> 	

How is the atomic_read() not racy with a future increment, and what is
that synchronize_sched() thing for?
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