linux-kernel - Re: WARNING at tty_buffer.c:428 process_one

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Message-Id: <20130305.155300.1299712636968046625.davem@davemloft.net>
Date:	Tue, 05 Mar 2013 15:53:00 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	peter@...leysoftware.com
Cc:	jslaby@...e.cz, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: WARNING at tty_buffer.c:428 process_one_work()

From: Peter Hurley <peter@...leysoftware.com>
Date: Tue, 05 Mar 2013 15:47:04 -0500

> On Tue, 2013-03-05 at 15:03 -0500, David Miller wrote:
>> From: Jiri Slaby <jslaby@...e.cz>
>> Date: Tue, 05 Mar 2013 20:44:49 +0100
>> 
>> > Hi, I must admit I don't understand. I now checked both of them and they
>> > call uart_handle_sysrq_char unconditionally, or?
>> 
>> Nope, in the sunsab.c receive function, we used to handle the SYSRQ
>> stuff before break checking when TTY is NULL, now we don't.
> 
> Hi David,
> 
> SysRq is signalled first by a BRK condition, then followed by the input
> character indicating which SysRq function to perform.
> 
> sunsab.c: receive_char() is behaving as you would expect.
> 
> First, a BRK status is indicated so uart_handle_break() records a
> timestamp. If the next input is received within 5 sec. of that
> timestamp, the character received is interpreted as a SysRq function --
> handled by uart_handle_sysrq_char().
> 
> Are you observing that SysRq processing is not occurring with this
> driver when only a console exists, or are you hypothesizing that this is
> possible?

Before we go down this road we need to first address the fact that
non-trivial semantic changes we made to these functions without any
of that being documented.

And it was done to a large swath of serial and TTY drivers.

The author of these changes doesn't even grok that receive interrupts
for these devices can be enabled even if TTY is NULL.
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