Date: Mon, 4 Aug 2008 14:41:14 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Andrea Arcangeli <andrea@qumranet.com>
Cc: penberg@cs.helsinki.fi, a.p.zijlstra@chello.nl, davej@redhat.com,
       rdreier@cisco.com, torvalds@linux-foundation.org, davem@davemloft.net,
       jeremy@goop.org, hugh@veritas.com, mingo@elte.hu,
       linux-kernel@vger.kernel.org, arjan@infradead.org
Subject: Re: [PATCH] workaround minor lockdep bug triggered by
 mm_take_all_locks
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On Mon, 4 Aug 2008 23:30:18 +0200
Andrea Arcangeli <andrea@qumranet.com> wrote:

> Now perhaps I misunderstood lockdep entirely

You did ;)

At runtime lockdep will "learn" the lock ordering rules, building a
graph in memory.  If it ever sees the thus-learnt rules violated, it
will warn.


Simple example:


static DEFINE_MUTEX(a);
static DEFINE_MUTEX(b);

void f1(void)
{
	mutex_lock(a);
	mutex_lock(b);
}

void f2(void)
{
	mutex_lock(b);
	mutex_lock(a);
}

void doit(void)
{
	f1();
	f2();
}


lockdep will warn here about the ranking violation.  As soon as it
occurs.  Even though the system never deadlocked.

It's very very clever and powerful.
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