linux-kernel - Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu

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Date:	Mon, 23 Apr 2012 09:40:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Anton Arapov <anton@...hat.com>
Subject: Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu_id

On Mon, 2012-04-23 at 12:54 +0530, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@...radead.org> [2012-04-23 09:14:00]:
> 
> > On Fri, 2012-04-20 at 20:37 +0200, Oleg Nesterov wrote:
> > > Say, a user wants to probe /sbin/init only. What if init forks?
> > > We should remove breakpoints from child->mm somehow. 
> > 
> > How is that hard? dup_mmap() only copies the VMAs, this doesn't actually
> > copy the breakpoint. So the child doesn't have a breakpoint to be
> > removed.
> > 
> 
> Because the pages are COWED, the breakpoint gets copied over to the
> child. If we dont want the breakpoints to be not visible to the child,
> then we would have to remove them explicitly based on the filter (i.e if
> and if we had inserted breakpoints conditionally based on filter). 

I thought we didn't COW shared maps since the fault handler will fill in
the pages right and only anon stuff gets copied.

> Once we add the conditional breakpoint insertion (which is tricky),

How so?

>  we have
> to support conditional breakpoint removal in the dup_mmap() thro the
> uprobe_mmap hook (which I think is not that hard).  Conditional removal
> of breakpoints in fork path would just be an extension of the
> conditional breakpoint insertion.

Right, I don't think that removal is particularly hard if needed.
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