linux-kernel - Re: [patch 01/27] fs: cleanup files

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Date:	Sat, 25 Apr 2009 04:20:49 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	npiggin@...e.de
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [patch 01/27] fs: cleanup files_lock

[Alan Cc'ed due to tty part of it]

On Sat, Apr 25, 2009 at 11:20:21AM +1000, npiggin@...e.de wrote:

>  	set_bit(TTY_PTY_LOCK, &tty->flags); /* LOCK THE SLAVE */
>  	filp->private_data = tty;
> -	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 there any problem with just shifting mutex_unlock down from several lines
above?


(in do_tty_hangup)
> +	mutex_lock(&tty_mutex);
> +
>  	/* inuse_filps is protected by the single kernel lock */
>  	lock_kernel();

isn't it too early?

> @@ -553,8 +566,7 @@ static void do_tty_hangup(struct work_st
>  	}
>  	spin_unlock(&redirect_lock);
>  
> -	check_tty_count(tty, "do_tty_hangup");
> -	file_list_lock();

i.e. why not here?

> +	__check_tty_count(tty, "do_tty_hangup");
>  	/* This breaks for file handles being sent over AF_UNIX sockets ? */
>  	list_for_each_entry(filp, &tty->tty_files, f_u.fu_list) {
>  		if (filp->f_op->write == redirected_tty_write)

> @@ -1467,9 +1479,9 @@ static void release_one_tty(struct kref
>  	tty_driver_kref_put(driver);
>  	module_put(driver->owner);
>  
> -	file_list_lock();
> +	mutex_lock(&tty_mutex);
>  	list_del_init(&tty->tty_files);
> -	file_list_unlock();
> +	mutex_unlock(&tty_mutex);

Umm... why is it safe from the deadlock POV?

> @@ -1836,8 +1849,12 @@ got_driver:
>  		return PTR_ERR(tty);
>  
>  	filp->private_data = tty;
> -	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);

a) why not simply shift mutex_unlock from several lines above?
b) that code really looks b0rken - what happens if you block on that
mutex_lock and somebody else comes and sees (at least) inconsistent
tty->count?

====

Could you split that into direct move (one patch) + changes?

> +/**
> + *	mark_files_ro - mark all files read-only
> + *	@sb: superblock in question
> + *
> + *	All files are marked read-only.  We don't care about pending
> + *	delete files so this should be used in 'force' mode only.
> + */
> +void mark_files_ro(struct super_block *sb)

BTW, I'd rather merge mnt_write_count one first, so reordering of those
would be appreciated; mnt_write_count + move that function + this patch
is the order I'd prefer.
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