linux-kernel - Re: [PATCH v2 06/14] locks: don't walk inode->i_flock list in locks

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Date:	Thu, 13 Jun 2013 15:45:46 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	viro@...iv.linux.org.uk, matthew@....cx, dhowells@...hat.com,
	sage@...tank.com, smfrench@...il.com, swhiteho@...hat.com,
	Trond.Myklebust@...app.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-afs@...ts.infradead.org,
	ceph-devel@...r.kernel.org, linux-cifs@...r.kernel.org,
	samba-technical@...ts.samba.org, cluster-devel@...hat.com,
	linux-nfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	piastryyy@...il.com
Subject: Re: [PATCH v2 06/14] locks: don't walk inode->i_flock list in
 locks_show

On Tue, Jun 11, 2013 at 07:09:00AM -0400, Jeff Layton wrote:
> When we convert over to using the i_lock to protect the i_flock list,
> that will introduce a potential lock inversion problem in locks_show.
> When we want to walk the i_flock list, we'll need to take the i_lock.
> 
> Rather than do that, just walk the global blocked_locks list and print
> out any that are blocked on the given lock.

I'm OK with this as obviously /proc/locks shouldn't be the common case,
but it still bugs me a bit that we're suddenly making it something like

	O(number of held locks * number of waiters)

where it used to be

	O(number of held lock + number of waiters)

I wonder if there's any solution that's just as easy and avoids scanning
the blocked list each time.

--b.

> 
> Signed-off-by: Jeff Layton <jlayton@...hat.com>
> ---
>  fs/locks.c |    6 ++++--
>  1 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index e451d18..3fd27f0 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -2249,8 +2249,10 @@ static int locks_show(struct seq_file *f, void *v)
>  
>  	lock_get_status(f, fl, *((loff_t *)f->private), "");
>  
> -	list_for_each_entry(bfl, &fl->fl_block, fl_block)
> -		lock_get_status(f, bfl, *((loff_t *)f->private), " ->");
> +	list_for_each_entry(bfl, &blocked_list, fl_link) {
> +		if (bfl->fl_next == fl)
> +			lock_get_status(f, bfl, *((loff_t *)f->private), " ->");
> +	}
>  
>  	return 0;
>  }
> -- 
> 1.7.1
> 
--
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