linux-kernel - Re: [PATCH] qspinlock: no need to check return value of debugfs

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Date:   Wed, 23 Jan 2019 09:14:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] qspinlock: no need to check return value of
 debugfs_create functions

On Tue, Jan 22, 2019 at 04:21:43PM +0100, Greg Kroah-Hartman wrote:
> When calling debugfs functions, there is no need to ever check the
> return value.  The function can work or not, but the code logic should
> never do something different based on this.

So I've seen you do a fair number of these patches; but I don't
fully understand.

The existing code rolls back the created files such that we either have
all files or none at all. Why is this wrong?

It for some daft reason one of the debugfs calls fails (imagine this was
a module and we did modprobe while under memory pressure), why should we
present a partial interface to the 'user' ?

> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Will Deacon <will.deacon@....com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  kernel/locking/qspinlock_stat.h | 18 ++++--------------
>  1 file changed, 4 insertions(+), 14 deletions(-)
> 
> diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
> index 42d3d8dc8f49..766fa0c8c115 100644
> --- a/kernel/locking/qspinlock_stat.h
> +++ b/kernel/locking/qspinlock_stat.h
> @@ -213,9 +213,6 @@ static int __init init_qspinlock_stat(void)
>  	struct dentry *d_qstat = debugfs_create_dir("qlockstat", NULL);
>  	int i;
>  
> -	if (!d_qstat)
> -		goto out;
> -
>  	/*
>  	 * Create the debugfs files
>  	 *
> @@ -224,20 +221,13 @@ static int __init init_qspinlock_stat(void)
>  	 * performance.
>  	 */
>  	for (i = 0; i < qstat_num; i++)
> -		if (!debugfs_create_file(qstat_names[i], 0400, d_qstat,
> -					 (void *)(long)i, &fops_qstat))
> -			goto fail_undo;
> +		debugfs_create_file(qstat_names[i], 0400, d_qstat,
> +				    (void *)(long)i, &fops_qstat);
>  
> -	if (!debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
> -				 (void *)(long)qstat_reset_cnts, &fops_qstat))
> -		goto fail_undo;
> +	debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
> +			    (void *)(long)qstat_reset_cnts, &fops_qstat);
>  
>  	return 0;
> -fail_undo:
> -	debugfs_remove_recursive(d_qstat);
> -out:
> -	pr_warn("Could not create 'qlockstat' debugfs entries\n");
> -	return -ENOMEM;
>  }
>  fs_initcall(init_qspinlock_stat);
>  
> -- 
> 2.20.1
>