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Subject: Re: [RFC] [PATCH 2.6.37-rc5-tip 5/20]  5: Uprobes:
 register/unregister probes.
From: Peter Zijlstra <peterz@infradead.org>
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Date: Wed, 26 Jan 2011 11:10:56 +0100
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To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>, linux-mm@kvack.org, Arnaldo Carvalho de Melo <acme@infradead.org>, Linus Torvalds <torvalds@linux-foundation.org>, Ananth N Mavinakayanahalli <ananth@in.ibm.com>, Christoph Hellwig <hch@infradead.org>, Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>, Oleg Nesterov <oleg@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, SystemTap <systemtap@sources.redhat.com>, Jim Keniston <jkenisto@linux.vnet.ibm.com>, Frederic Weisbecker <fweisbec@gmail.com>, Andi Kleen <andi@firstfloor.org>, LKML <linux-kernel@vger.kernel.org>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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On Wed, 2011-01-26 at 13:17 +0530, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@infradead.org> [2011-01-25 13:15:45]:
>=20
> > > +
> > > +       if (atomic_read(&uprobe->ref) =3D=3D 1) {
> > > +               synchronize_sched();
> > > +               rb_erase(&uprobe->rb_node, &uprobes_tree);
> >=20
> > How is that safe without holding the treelock?
>=20
> Right,=20
> Something like this should be good enuf right?
>=20
> if (atomic_read(&uprobe->ref) =3D=3D 1) {
> 	synchronize_sched();
> 	spin_lock_irqsave(&treelock, flags);
> 	rb_erase(&uprobe->rb_node, &uprobes_tree);
> 	spin_lock_irqrestore(&treelock, flags);
> 	iput(uprobe->inode);
> }
> =09

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

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