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Date:   Thu, 17 Jun 2021 23:12:46 +0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Xiongwei Song <sxwjean@me.com>
Cc:     peterz@infradead.org, mingo@redhat.com, will@kernel.org,
        longman@redhat.com, linux-kernel@vger.kernel.org,
        Xiongwei Song <sxwjean@gmail.com>
Subject: Re: [PATCH 3/3] locking/lockdep: print possible warning after
 counting deps
Message-ID: <YMtmbjw5EGdXRdD8@boqun-archlinux>
References: <20210617142828.346111-1-sxwjean@me.com>
 <20210617142828.346111-4-sxwjean@me.com>
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In-Reply-To: <20210617142828.346111-4-sxwjean@me.com>
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Hi,

On Thu, Jun 17, 2021 at 10:28:28PM +0800, Xiongwei Song wrote:
> From: Xiongwei Song <sxwjean@gmail.com>
> 
> The graph walk might hit error when counting dependencies. Once the
> return value is negative, print a warning to reminder users.
> 

Thanks for the improvement, but please see below:

> Suggested-by: Waiman Long <longman@redhat.com>
> Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
> ---
>  kernel/locking/lockdep.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index cb94097014d8..cfe0f4374594 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -2028,8 +2028,12 @@ static unsigned long __lockdep_count_forward_deps(struct lock_list *this)
>  {
>  	unsigned long  count = 0;
>  	struct lock_list *target_entry;
> +	enum bfs_result ret;
> +
> +	ret = __bfs_forwards(this, (void *)&count, noop_count, NULL, &target_entry);
>  
> -	__bfs_forwards(this, (void *)&count, noop_count, NULL, &target_entry);
> +	if (bfs_error(ret))
> +		print_bfs_bug(ret);

Here print_bfs_bug() will eventually call debug_locks_off_graph_unlock()
to release the graph lock, and the caller (lockdep_count_fowards_deps())
will also call graph_unlock() afterwards, and that means we unlock
*twice* if a BFS error happens... although in that case, lockdep should
stop working so messing up with the graph lock may not hurt anything,
but still, I don't think we want to do that.

So probably you can open-code __lockdep_count_forward_deps() into
lockdep_count_forwards_deps(), and call print_bfs_bug() or
graph_unlock() accordingly. The body of __lockdep_count_forward_deps()
is really small, so I think it's OK to open-code it into its caller.

Regards,
Boqun

>  
>  	return count;
>  }
> @@ -2053,8 +2057,12 @@ static unsigned long __lockdep_count_backward_deps(struct lock_list *this)
>  {
>  	unsigned long  count = 0;
>  	struct lock_list *target_entry;
> +	enum bfs_result ret;
> +
> +	ret = __bfs_backwards(this, (void *)&count, noop_count, NULL, &target_entry);
>  
> -	__bfs_backwards(this, (void *)&count, noop_count, NULL, &target_entry);
> +	if (bfs_error(ret))
> +		print_bfs_bug(ret);
>  
>  	return count;
>  }
> -- 
> 2.30.2
>