linux-kernel - Re: [PATCH v4 3.0-rc2-tip 4/22] 4: Uprobes: register/unregister probes.

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Date:	Thu, 16 Jun 2011 10:56:39 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jonathan Corbet <corbet@....net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3.0-rc2-tip 4/22]  4: Uprobes: register/unregister
 probes.

* Peter Zijlstra <peterz@...radead.org> [2011-06-10 01:03:26]:

> On Tue, 2011-06-07 at 18:29 +0530, Srikar Dronamraju wrote:
> > +/*
> > + * There could be threads that have hit the breakpoint and are entering the
> > + * notifier code and trying to acquire the uprobes_treelock. The thread
> > + * calling delete_uprobe() that is removing the uprobe from the rb_tree can
> > + * race with these threads and might acquire the uprobes_treelock compared
> > + * to some of the breakpoint hit threads. In such a case, the breakpoint hit
> > + * threads will not find the uprobe. Finding if a "trap" instruction was
> > + * present at the interrupting address is racy. Hence provide some extra
> > + * time (by way of synchronize_sched() for breakpoint hit threads to acquire
> > + * the uprobes_treelock before the uprobe is removed from the rbtree.
> > + */
> 
> 'some' extra time doesn't really sound convincing to me. Either it is
> sufficient to avoid the race or it is not. It reads to me like: we add a
> delay so that the race mostly doesn't occur. Not good ;-)

The extra time provided is sufficient to avoid the race. So will modify
it to mean "sufficient" instead of "some".

> 
> > +static void delete_uprobe(struct uprobe *uprobe)
> > +{
> > +       unsigned long flags;
> > +
> > +       synchronize_sched();
> > +       spin_lock_irqsave(&uprobes_treelock, flags);
> > +       rb_erase(&uprobe->rb_node, &uprobes_tree);
> > +       spin_unlock_irqrestore(&uprobes_treelock, flags);
> > +       iput(uprobe->inode);
> > +} 
> 
> Also what are the uprobe lifetime rules here? Does it still exist after
> this returns?
> 
> The comment in del_consumer() that says: 'drop creation ref' worries me
> and makes me thing that is the last reference around and the uprobe will
> be freed right there, which clearly cannot happen since its not yet
> removed from the RB-tree.
> 

When del_consumer() is called in unregister_uprobe() it has atleast two
(or more if the uprobe is hit) references. One at the creation time and
the other thro find_uprobe() called in unregister_uprobe before
del_consumer. So the reference lost in del_consumer is never the last
reference.  I added a commented this as creation reference so that the
find_uprobe and the put_uprobe() before return would match.

If the comment is confusing I can delete it or reword it as suggested by
Steven Rostedt which is  /* Have caller drop the creation ref */

I would prefer to delete the comment.

-- 
Thanks and Regards
Srikar
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