linux-kernel - Re: [REGRESSION] 2.6.39-rc1: f23eb2b breaks console output

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Date:	Mon, 4 Apr 2011 12:59:09 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Sitsofe Wheeler <sitsofe@...oo.com>
Cc:	Alexander Beregalov <a.beregalov@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg KH <gregkh@...e.de>, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [REGRESSION] 2.6.39-rc1: f23eb2b breaks console output

On Mon, Apr 4, 2011 at 12:42 PM, Sitsofe Wheeler <sitsofe@...oo.com> wrote:
> On Mon, Apr 04, 2011 at 11:15:10PM +0400, Alexander Beregalov wrote:
>>
>> commit f23eb2b2b28547fc70df82dd5049eb39bec5ba12 tty: stop using
>> "delayed_work" in the tty layer
>> breaks console output for me.
>
> Same here - doing a dmesg in an xterm makes the machine hang for about
> 10 seconds and latencytop whinges that a raw SCSI command took 4000ms to
> complete (uniprocessor x86 EeePC 900).
>
> Additionally, you won't see this problem if you have CONFIG_PREEMPT set
> but as my config has CONFIG_PREEMPT_VOLUNTARY=y I do see this problem.
> Reverting f23eb2b2b28547fc70df82dd5049eb39bec5ba12 fixed the problem for
> me too.

Does this patch make a difference?

The tty ldisc flushing code had this crazy "reflush if we're full"
logic, which was apparently hiding a problem in N_TTY handling where
it wouldn't wake things up when the receive-room was opened up again.

When I made it not do timers, the "reflush" logic basically made for
an infinite loop (before, it used to instead basically poll for the
event on each timer tick instead).

The attached patch removes the reflush logic, and makes N_TTY instead
wake up the work when opening up the receive buffer again.

               Linus

View attachment "patch.diff" of type "text/x-patch" (1520 bytes)