linux-kernel - Re: [RFC] [PATCH 2.6.37-rc5-tip 8/20] 8: uprobes: mmap and fork hooks.

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Date:	Wed, 26 Jan 2011 11:20:39 +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 <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>,
	LKML <linux-kernel@...r.kernel.org>,
	SystemTap <systemtap@...rces.redhat.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC] [PATCH 2.6.37-rc5-tip 8/20]  8: uprobes: mmap and fork
 hooks.

On Wed, 2011-01-26 at 14:33 +0530, Srikar Dronamraju wrote:
> 
> 
> I actually dont like to release the write_lock and then reacquire it.
> write_opcode, which is called thro install_uprobe, i.e to insert the
> actual breakpoint instruction takes a read lock on the mmap_sem.
> Hence uprobe_mmap gets called in context with write lock on mmap_sem
> held, I had to release it before calling install_uprobe. 

Ah, right, so that's going to give you a head-ache ;-)

The moment you release this mmap_sem, the map you're going to install
the probe point in can go away.

The only way to make this work seems to start by holding the mmap_sem
for writing and make a breakpoint install function that assumes its
taken and doesn't try to acquire it again.


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