linux-kernel - Re: [PATCH] sysfs: Cache the last sysfs_dirent to improve readdir scalability v2

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Date:	Fri, 1 Jan 2010 15:10:49 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Borislav Petkov <petkovbb@...glemail.com>,
	David Airlie <airlied@...ux.ie>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg KH <greg@...ah.com>, Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH] sysfs: Cache the last sysfs_dirent to improve readdir
 scalability v2

On Fri, 1 Jan 2010, Eric W. Biederman wrote:
>  		mutex_unlock(&sysfs_mutex);
> +		ret = filldir(dirent, name, len, filp->f_pos, ino, type);
> +		mutex_lock(&sysfs_mutex);
> +		if (ret < 0)
> +			break;
> +	}
> +	mutex_unlock(&sysfs_mutex);
> +	if ((filp->f_pos > 1) && !pos) { /* EOF */
> +		filp->f_pos = INT_MAX;
> +		filp->private_data = NULL;
>  	}
>  	return 0;

That

	mutex_lock(&sysfs_mutex);
	if (ret < 0)
		break;

looks just silly. We know 'pos' is non-NULL, so the break will effectively 
just be a "mutex_unlock + return 0", and we just did the mutex_lock, so 
why not instead do

	if (ret < 0)
		return 0;
	mutex_lock(&sysfs_mutex);

there?

Not that it really _matters_, but it seems way clearer, no?

But other than that mindless nit, I can't see anything wrong with your 
logic, and it looks ok to me from just reading the patch itself.

So I guess that's an "Ack", although I'd prefer it to get some more 
testing and perhaps go through Greg's tree as sysfs patches usually go.

And by "testing" I mean both the "yes, this second version also breaks the 
lockdep chain and avoids the warning", but also some kind of actual 
testing of /sysfs itself. If there is any.

		Linus
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