linux-kernel - Re: [RFC][PATCH 7/7] serial, 8250: Mostly avoid wakeups from under port->lock

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Date:	Wed, 21 Dec 2011 16:03:23 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Ted Ts'o" <tytso@....edu>, Greg KH <greg@...ah.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	akpm@...ux-foundation.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 7/7] serial, 8250: Mostly avoid wakeups from under
 port->lock

On Wed, 21 Dec 2011 11:57:46 +0100
Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> While not a strict requirement (anymore), it is nice not to issue
> wakeups (or have them connected by locks) from console output.
> 
> This patch avoids doing most wakeups from under port->lock, which is
> connected to 8250console_write(). Only seriously challenged UARTs will
> still do the wakeup, but since I don't actually have such a creature,
> I'm less inclined to fix it up.

Gak..

> +	BUG_ON(!state);
> +	tty = state->port.tty;
> +	tty_kref_get(tty);
> +	spin_unlock_irqrestore(&up->port.lock, flags);
> +	tty_wakeup(tty);
> +	tty_kref_put(tty);

driver innards shouldn't know this stuff and this makes it worse rather
than cleaning it up

The basic idea looks fine but I really don't want magic lock hackery in
the 8250 driver. We need a way of generalising this so the code is
cleaner and the locking internal knowledge stays out of the driver itself.

Also I think it's probably buggy - sending the x_char is forward progress
so probably needs to cause a wakeup.

So for the moment NAK, but worthy of figuring out how to do it right
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