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 17:22:34 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
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, 2011-12-21 at 16:03 +0000, Alan Cox wrote:

> > +	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

Fair enough, if I can push it into serial_core.c I can probably fix that
one weird case as well, I couldn't inside the 8250 driver because the
port lock was taken by the serial core code.

> 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.

Could you explain that more, its not actually connecting with any
neurons..

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

OK, will try and see if I can poke at it a level upwards.. 
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