linux-kernel - Re: [PATCH] workaround minor lockdep bug triggered by mm_take_all

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Message-Id: <20080804144114.e323a47f.akpm@linux-foundation.org>
Date:	Mon, 4 Aug 2008 14:41:14 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andrea Arcangeli <andrea@...ranet.com>
Cc:	penberg@...helsinki.fi, a.p.zijlstra@...llo.nl, davej@...hat.com,
	rdreier@...co.com, torvalds@...ux-foundation.org,
	davem@...emloft.net, jeremy@...p.org, hugh@...itas.com,
	mingo@...e.hu, linux-kernel@...r.kernel.org, arjan@...radead.org
Subject: Re: [PATCH] workaround minor lockdep bug triggered by
 mm_take_all_locks

On Mon, 4 Aug 2008 23:30:18 +0200
Andrea Arcangeli <andrea@...ranet.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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