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Subject: Re: [RFC] [PATCH 2.6.37-rc5-tip 8/20]  8: uprobes: mmap and fork
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From: Peter Zijlstra <peterz@infradead.org>
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Date: Wed, 26 Jan 2011 11:20:39 +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 <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>, LKML <linux-kernel@vger.kernel.org>, SystemTap <systemtap@sources.redhat.com>, Jim Keniston <jkenisto@linux.vnet.ibm.com>, Frederic Weisbecker <fweisbec@gmail.com>, Andi Kleen <andi@firstfloor.org>, Andrew Morton <akpm@linux-foundation.org>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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On Wed, 2011-01-26 at 14:33 +0530, Srikar Dronamraju wrote:
>=20
>=20
> 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.=20

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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