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Date: Wed, 14 Dec 2011 09:05:27 -0500
From: "Ted Ts'o" <tytso@mit.edu>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg KH <greg@kroah.com>, linux-kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@elte.hu>
Subject: Re: printk() vs tty_io
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On Wed, Dec 14, 2011 at 11:56:46AM +0100, Peter Zijlstra wrote:
> 
> 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.

printk() goes through serial8250_console_write() which writes to the
UART device using a busy loop, with local IRQ's disabled.  So it's not
serial8250_console_write() which is waking things up; this is coming
form interrupt handler running on some different CPU.

Am I missing something?

						- Ted