linux-kernel - Re: [PATCH v2] locking/lockdep: Fix meaningless usages output of lock classes

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Date:   Mon, 28 Jun 2021 17:17:33 -0400
From:   Waiman Long <llong@...hat.com>
To:     Xiongwei Song <sxwjean@...com>, peterz@...radead.org,
        mingo@...hat.com, will@...nel.org, boqun.feng@...il.com
Cc:     linux-kernel@...r.kernel.org, Xiongwei Song <sxwjean@...il.com>
Subject: Re: [PATCH v2] locking/lockdep: Fix meaningless usages output of lock
 classes

On 6/28/21 11:17 AM, Xiongwei Song wrote:
> From: Xiongwei Song <sxwjean@...il.com>
>
> When enabling CONFIG_LOCK_STAT, then CONFIG_LOCKDEP is forcedly enabled.
> We can get output from /proc/lockdep, which currently includes usages of
> lock classes. But the usages are meaningless, see the output below:
>
> / # cat /proc/lockdep
> all lock classes:
> ffffffff9af63350 ....: cgroup_mutex
>
> ffffffff9af54eb8 ....: (console_sem).lock
>
> ffffffff9af54e60 ....: console_lock
>
> ffffffff9ae74c38 ....: console_owner_lock
>
> ffffffff9ae74c80 ....: console_owner
>
> ffffffff9ae66e60 ....: cpu_hotplug_lock
>
> Only one usage context for each lock, this is because each usage is only
> changed in mark_lock() that is in CONFIG_PROVE_LOCKING defined section,
> however in the test situation, it's not.
>
> The fix is to move the usages reading and seq_print from
> CONFIG_PROVE_LOCKING undefined setcion to its defined section. Also,
> locks_after list of lock_class is empty when CONFIG_PROVE_LOCKING
> undefined, so do the same thing as what have done for usages of lock
> classes.
With this patch, CONFIG_LOCKDEP without CONFIG_PROVE_LOCKING will make 
/proc/lockdep displays just the list of lock classes with their 
associated lock keys. I think it is worth explicitly saying that in the 
commit log.
> Signed-off-by: Xiongwei Song <sxwjean@...il.com>
> ---
>   kernel/locking/lockdep_proc.c | 24 +++++++++++++-----------
>   1 file changed, 13 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/locking/lockdep_proc.c b/kernel/locking/lockdep_proc.c
> index 806978314496..a1ec2652d492 100644
> --- a/kernel/locking/lockdep_proc.c
> +++ b/kernel/locking/lockdep_proc.c
> @@ -70,23 +70,25 @@ static int l_show(struct seq_file *m, void *v)
>   #ifdef CONFIG_DEBUG_LOCKDEP
>   	seq_printf(m, " OPS:%8ld", debug_class_ops_read(class));
>   #endif
> -#ifdef CONFIG_PROVE_LOCKING
> -	seq_printf(m, " FD:%5ld", lockdep_count_forward_deps(class));
> -	seq_printf(m, " BD:%5ld", lockdep_count_backward_deps(class));
> -#endif
> +	if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
> +		seq_printf(m, " FD:%5ld", lockdep_count_forward_deps(class));
> +		seq_printf(m, " BD:%5ld", lockdep_count_backward_deps(class));
>   
> -	get_usage_chars(class, usage);
> -	seq_printf(m, " %s", usage);
> +		get_usage_chars(class, usage);
> +		seq_printf(m, " %s", usage);
> +	}
>   
>   	seq_printf(m, ": ");
>   	print_name(m, class);
>   	seq_puts(m, "\n");
>   
> -	list_for_each_entry(entry, &class->locks_after, entry) {
> -		if (entry->distance == 1) {
> -			seq_printf(m, " -> [%p] ", entry->class->key);
> -			print_name(m, entry->class);
> -			seq_puts(m, "\n");
> +	if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
> +		list_for_each_entry(entry, &class->locks_after, entry) {
> +			if (entry->distance == 1) {
> +				seq_printf(m, " -> [%p] ", entry->class->key);
> +				print_name(m, entry->class);
> +				seq_puts(m, "\n");
> +			}
>   		}
>   	}
>   	seq_puts(m, "\n");

Maybe you can remove the blank lines in this case by moving the last 
seq_puts() inside the if loop. The blank lines are not really needed 
without the associated locks_after information.

Cheers,
Longman