linux-kernel - Re: 'scheduling while atomic' during ppp connection on 2.6.37.1 and 2.6.38

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Date:	Mon, 21 Mar 2011 10:15:41 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Jack Stone <jwjstone@...tmail.fm>, Mac <kmac@...zta.fm>,
	linux-kernel@...r.kernel.org, linux-ppp@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Jiri Slaby <jirislaby@...il.com>
Subject: Re: 'scheduling while atomic' during ppp connection on 2.6.37.1 and
 2.6.38

On 03/20/2011 10:58 PM, Alan Cox wrote:
>> +        spin_lock_irqsave(&dc->spin_mutex, flags);
>>           if (port->port.count)
>>               room = kfifo_avail(&port->fifo_ul);
>> -        mutex_unlock(&port->tty_sem);
>> +        spin_unlock_irqrestore(&dc->spin_mutex, flags);
> 
> dc->spin_mutex does not protect port->port.count.

Neither port->tty_sem did.

Anyway is the test needed at all? I.e. could
tty->ops->write/chars_in_buffer/ntty_write_room be called with
port->port.count == 0 at all?

And the lock should not as well be needed. Kfifo assures atomicity where
there is only one reader and one writer which should be the case here.
Unless tty->ops->write can be called in parallel. And it should not,
that's what's tty->atomic_write_lock for.

thanks,
-- 
js
suse labs
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