linux-kernel - Re: [patch 16/20] XEN-paravirt: Add the Xen virtual console driver.

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Date:	Sun, 14 Jan 2007 00:37:52 +0000
From:	Alan <alan@...rguk.ukuu.org.uk>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org, xen-devel@...ts.xensource.com,
	Ian Pratt <ian.pratt@...source.com>,
	Christian Limpach <Christian.Limpach@...cam.ac.uk>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [patch 16/20] XEN-paravirt: Add the Xen virtual console driver.

> +#endif
> +		tty_insert_flip_char(xencons_tty, buf[i], 0);

Please use the defines like TTY_NORMAL not just 0. 

> +		if ((xencons_tty->flags & (1 << TTY_DO_WRITE_WAKEUP)) &&
> +		    (xencons_tty->ldisc.write_wakeup != NULL))
> +			(xencons_tty->ldisc.write_wakeup)(xencons_tty);
> +	}

You are't allowed to derefence and call ldisc methods without
holding the lock. You can replace that chunk with a call to the helper
function tty_wakeup(tty). Small but real race condition otherwise as you
xencons_tty->ldisc may be changing as you call it.

> +static inline int __xencons_put_char(int ch)
> +{
> +	char _ch = (char)ch;
> +	if ((wp - wc) == wbuf_size)
> +		return 0;
> +	wbuf[WBUF_MASK(wp++)] = _ch;
> +	return 1;
> +}

A lot of very confusing sign stuff here - you turn an int into a char and
put it into a uchar array

> +	for (i = 0; i < count; i++)
> +		if (!__xencons_put_char(buf[i]))
> +			break;

The int coming from a uchar array

Don't think its wrong - just acutely weird and perhaps could be tidier

> +static void xencons_close(struct tty_struct *tty, struct file *filp)
> +{
> +	unsigned long flags;
> +
> +	mutex_lock(&tty_mutex);

It would be good in future if you could avoid using tty_mutex and use a
private lock. At the moment vt "borrows" it and there are a couple of
incestuous spots but the plan is to eventually fix them and make it
private to tty_io.

Andrew: No objection to this tty stuff being merged provided the bugs
noted above (not worried about the sign stuff) are fixed before it goes
on to Linus.

Alan
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