linux-kernel - Re: printk() vs tty

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Date:	Wed, 14 Dec 2011 11:56:46 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <greg@...ah.com>, Theodore Ts'o <tytso@....edu>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: printk() vs tty_io

On Wed, 2011-12-14 at 10:43 +0000, Alan Cox wrote:
> On Tue, 13 Dec 2011 15:52:35 -0800
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > On Tue, Dec 13, 2011 at 11:33 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > I've been poking at reducing the constraints on printk(), like make it
> > > work under rq->lock etc..
> > 
> > You aren't supposed register a console that wakes things up. But the
> > only console that honors that afaik is the traditional vt console.
> > *Maybe* the network console, I didn't check.
> 
> Serial console tries to be good as does netconsole. 

Well, this was the 8250 serial port driver doing wakeups. Its a simple
on-board serial port,

[   29.797007]        [<ffffffff8108a650>] __wake_up_common+0x4e/0x84
[   29.797007]        [<ffffffff8108d6ea>] __wake_up+0x39/0x4d
[   29.797007]        [<ffffffff8135d0db>] tty_wakeup+0x5b/0x60
[   29.797007]        [<ffffffff81373fb9>] uart_write_wakeup+0x21/0x23
[   29.797007]        [<ffffffff81376899>] transmit_chars+0xd8/0x12f
[   29.797007]        [<ffffffff81377f3a>] serial8250_handle_port+0x2d9/0x2fd
[   29.797007]        [<ffffffff81377fcb>] serial8250_handle_irq+0x16/0x1d
[   29.797007]        [<ffffffff81377ff5>] serial8250_default_handle_irq+0x23/0x27
[   29.797007]        [<ffffffff81377ade>] serial8250_interrupt+0x4d/0xc6
[   29.797007]        [<ffffffff810c9ae8>] handle_irq_event_percpu+0xab/0x1fc
[   29.797007]        [<ffffffff810c9c7a>] handle_irq_event+0x41/0x61
[   29.797007]        [<ffffffff810cc4cd>] handle_edge_irq+0xd1/0xf6
[   29.797007]        [<ffffffff8103ab85>] handle_irq+0x24/0x2f
[   29.797007]        [<ffffffff8150acdd>] do_IRQ+0x4d/0xb3

which places the wakeup under uport->lock, serial8250_console_write()
takes uport->lock, under console_sem, connecting the locks.


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