Re: [patch 01/27] fs: cleanup files_lock

[Nick Piggin <npiggin@suse.de>]

On Sat, Apr 25, 2009 at 10:42:34AM +0100, Alan Cox wrote:
> On Sat, 25 Apr 2009 11:20:21 +1000
> npiggin@suse.de wrote:
> 
> > Lock tty_files with tty_mutex, provide helpers to manipulate the per-sb
> > files list, and unexport the files_lock spinlock.
> 
> This looks half like a backward step to me: It swaps clean method calls
> for open coded stuff and it adds more random undocumented uses to
> tty_mutex, which has far too much already.
> 
> I don't think
> 
> -	file_move(filp, &tty->tty_files);
> +
> +	mutex_lock(&tty_mutex);
> +	file_list_del(filp);
> +	list_add(&filp->f_u.fu_list, &tty->tty_files);
> +	mutex_unlock(&tty_mutex);
> 
> is exactly an improvement, nor is 
> 
> -	file_move(filp, &tty->tty_files);
> -	check_tty_count(tty, "tty_open");
> +	mutex_lock(&tty_mutex);
> +	BUG_ON(list_empty(&filp->f_u.fu_list));
> +	file_list_del(filp); /* __dentry_open has put it on the sb list
>   */
> +	list_add(&filp->f_u.fu_list, &tty->tty_files);
> +	__check_tty_count(tty, "tty_open");
> +	mutex_unlock(&tty_mutex);
> 
> The basic idea looks totally sound but it can use its own lock and there
> should be helpers so this stuff doesn't have to get open coded.

Yes, I agree it was silly to try reusing tty_mutex for this, as you
and Al point out. I've just added a new spinlock for the tty layer
for the moment, which makes it much more like a mechanical search/
replace.