linux-kernel - Re: [RFC][PATCH 4/7] lockdep: Seperate lock ids for read/write acquires

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Date:	Mon, 18 Apr 2011 12:46:17 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC][PATCH 4/7] lockdep: Seperate lock ids for read/write
 acquires

On Sun, 2011-04-17 at 11:45 +0200, Peter Zijlstra wrote:
>  
> +/*
> + * A lock's class id is used to calculate the chain-key. Since we need to
> + * differentiate between the chains which contain the read acquire of
> + * a lock from the chains having write acquire of the same lock,
> + * we offset the class_idx by MAX_LOCKDEP_KEYS if it is a read acquire.

Don't we only care to do this if we have a recursive read? I thought
simple reads still work fine with the current algorithm?

> + *
> + * Thus the the lock's key during a chain-key calculation can be in the range
> + * 1 to 2 * MAX_LOCKDEP_KEYS - 1.
> + *
> + * LOCKDEP_CHAIN_KEY_BITS holds the number of bits required to
> + * represent this range.
> + */
> +#define LOCKDEP_CHAIN_KEY_BITS	(MAX_LOCKDEP_KEYS_BITS + 1)
>  struct held_lock {
>  	/*
>  	 * One-way hash of the dependency chain up to this point. We
> Index: linux-2.6/kernel/lockdep.c
> ===================================================================
> --- linux-2.6.orig/kernel/lockdep.c
> +++ linux-2.6/kernel/lockdep.c
> @@ -303,8 +303,8 @@ static struct list_head chainhash_table[
>   * unique.
>   */
>  #define iterate_chain_key(key1, key2) \
> -	(((key1) << MAX_LOCKDEP_KEYS_BITS) ^ \
> -	((key1) >> (64-MAX_LOCKDEP_KEYS_BITS)) ^ \
> +	(((key1) << LOCKDEP_CHAIN_KEY_BITS) ^ \
> +	((key1) >> (64 - LOCKDEP_CHAIN_KEY_BITS)) ^ \
>  	(key2))
>  
>  void lockdep_off(void)
> @@ -1988,6 +1988,9 @@ static void check_chain_key(struct task_
>  		if (DEBUG_LOCKS_WARN_ON(id >= MAX_LOCKDEP_KEYS))
>  			return;
>  
> +		if (is_read(hlock->rw_state))
> +			id += MAX_LOCKDEP_KEYS;

Again, isn't this about recursive reads? Or am I just confused ;)

-- Steve

> +
>  		if (prev_hlock && (prev_hlock->irq_context !=
>  							hlock->irq_context))
>  			chain_key = 0;
> @@ -2815,6 +2818,18 @@ static int __lock_acquire(struct lockdep
>  	if (DEBUG_LOCKS_WARN_ON(id >= MAX_LOCKDEP_KEYS))
>  		return 0;
>  
> +	/*
> +	 * Factor in the read/write state in the chain key calculation.
> +	 *
> +	 * Two chains containing lock dependencies in the same order can
> +	 * still differ due to their read/write state
> +	 * eg: lock(A)->Rlock(B) is different from lock(A)->Wlock(B)
> +	 *
> +	 * Hence distinguish between such chains.
> +	 */
> +	if (is_read(rw_state))
> +		id += MAX_LOCKDEP_KEYS;
> +
>  	chain_key = curr->curr_chain_key;
>  	if (!depth) {
>  		if (DEBUG_LOCKS_WARN_ON(chain_key != 0))
> 


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