linux-kernel - Re: [PATCH 2/2] lockdep: Don't only check recursive read locks once in a sequence

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Date:	Wed, 18 Nov 2009 17:43:03 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ming Lei <tom.leiming@...il.com>
Subject: Re: [PATCH 2/2] lockdep: Don't only check recursive read locks once
 in a sequence

Frederic Weisbecker wrote:
> Say we have the following locks:
> A (rwlock, Aw: writelock, Ar: recursive read lock)
> B (normal lock)
> 
> and the following sequences:
> Ar -> B -> Ar
> Aw -> B
> 
> This won't be detected as a lock inversion

"""
read-preference <==> read-recursive ability  (rwlock)
otherwise ==> read-recursive disability      (rwsem)
"""

If "B -> Ar" is always after "Ar", it's NOT a really
lock inversion because rwlock is read-preference, we
can ignore all "Ar" which are after "B".

If sometimes "B -> Ar" is not after "Ar",
then we have these sequences:
B -> Ar
Aw -> B

Lockdep can detects it now(without this patch applied).

Maybe I have misunderstood your patch.

> because in the sequence
> of locks held by the current task, if we have a same class acquired
> as read-recursive several times, only the first one will be checked
> in the tree (although all of them are checked for deadlocks in the
> current held sequence).
> 
> Fix it by always adding recursive read locks in the dependency tree.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ming Lei <tom.leiming@...il.com>
> ---
>  kernel/lockdep.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> index a6f7440..13d1d54 100644
> --- a/kernel/lockdep.c
> +++ b/kernel/lockdep.c
> @@ -1949,9 +1949,9 @@ static int validate_chain(struct task_struct *curr, struct lockdep_map *lock,
>  			hlock->read = 2;
>  		/*
>  		 * Add dependency only if this lock is not the head
> -		 * of the chain, and if it's not a secondary read-lock:
> +		 * of the chain.
>  		 */
> -		if (!chain_head && ret != 2)
> +		if (!chain_head)
>  			if (!check_prevs_add(curr, hlock))
>  				return 0;
>  		graph_unlock();

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