linux-kernel - Re: [PATCH 1/4] hvc_console: do not set low

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Message-Id: <1998eda55e236b70f63c7fb4b956b487@bga.com>
Date:	Tue, 13 Jan 2009 10:03:54 -0600
From:	Milton Miller <miltonm@....com>
To:	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Benjiman Herrenschmidt <benh@...nel.crashing.org>,
	linuxppc-dev list <linuxppc-dev@...abs.org>,
	Joe Peterson <joe@...rush.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] hvc_console: do not set low_latency


On Jan 13, 2009, at 5:35 AM, Hendrik Brueckner wrote:

> Here is yet another bug trace related to the low_latency issue, that is
> experienced using the hvc_iucv backend.  The hvc_iucv backend now also 
> uses irq
> notifiers.
> The bug trace below appears if hvc_kick() is called.
>
> On Tue, Jan 13, 2009 at 10:04:22AM +0100, Christian Borntraeger wrote:
>> Am Donnerstag 08 Januar 2009 schrieb Milton Miller:
>>> hvc_console is setting low_latency unconditionally, but some clients 
>>> are
>>> interrupt driven and will call hvc_poll from irq context.  This will 
>>> cause
>>> tty_flip_buffer_push to be called from irq context, and it very 
>>> clearly
>>> states it must not be called from IRQ when low_latency is specified.
> It seems that if low_latency is set, tty_flip_buffer_push() should 
> also not be
> called within an atomic context, because echo_char_raw() and other 
> echo* calls
> might sleep.
>
> Christian's patch solves this problem for irq driven backends.
> However, there might be still a problem with polled backends since the 
> khvcd()
> thread calls hvc_poll() while hvc_structs_lock is held.
>
> I think the hvc_udbg backend is based on polling.
> David, could you check if you experience any problems with your 
> hvc_udbg
> backend on latest git.
>


So the simplest is to never set it.

milton

> BUG: sleeping function called from invalid context at 
> /root/cvs/linux-2.6.git/kernel/mutex.c:207
> in_atomic(): 1, irqs_disabled(): 0, pid: 748, name: khvcd
> 1 lock held by khvcd/748:
>  #0:  (hvc_structs_lock){--..}, at: [<00000000002ceb50>] 
> khvcd+0x58/0x12c
> CPU: 0 Not tainted 2.6.29-rc1git #29
> Process khvcd (pid: 748, task: 000000002fb9a480, ksp: 000000002f66bd78)
> 070000000000000a 000000002f66ba00 0000000000000002 (null)
>        000000002f66baa0 000000002f66ba18 000000002f66ba18 
> 0000000000104f08
>        ffffffffffffc000 000000002f66bd78 (null) (null)
>        000000002f66ba00 000000000000000c 000000002f66ba00 
> 000000002f66ba70
>        0000000000466af8 0000000000104f08 000000002f66ba00 
> 000000002f66ba50
> Call Trace:
> ([<0000000000104e7c>] show_trace+0x138/0x158)
>  [<0000000000104f62>] show_stack+0xc6/0xf8
>  [<0000000000105740>] dump_stack+0xb0/0xc0
>  [<000000000013144a>] __might_sleep+0x14e/0x17c
>  [<000000000045e226>] mutex_lock_nested+0x42/0x3b4
>  [<00000000002c443e>] echo_char_raw+0x3a/0x9c
>  [<00000000002c688c>] n_tty_receive_buf+0x1154/0x1208
>  [<00000000002ca0a2>] flush_to_ldisc+0x152/0x220
>  [<00000000002ca1da>] tty_flip_buffer_push+0x6a/0x90
>  [<00000000002cea74>] hvc_poll+0x244/0x2c8
>  [<00000000002ceb68>] khvcd+0x70/0x12c
>  [<000000000015bbd0>] kthread+0x68/0xa0
>  [<0000000000109d5a>] kernel_thread_starter+0x6/0xc
>  [<0000000000109d54>] kernel_thread_starter+0x0/0xc
> 1 lock held by khvcd/748:
>  #0:  (hvc_structs_lock){--..}, at: [<00000000002ceb50>] 
> khvcd+0x58/0x12c
>
>> This wont work, since the call to notifier_add is done later:
>> What about:
>> ---
>>  drivers/char/hvc_console.c |    5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> Index: linux-2.6/drivers/char/hvc_console.c
>> ===================================================================
>> --- linux-2.6.orig/drivers/char/hvc_console.c
>> +++ linux-2.6/drivers/char/hvc_console.c
>> @@ -318,8 +318,6 @@ static int hvc_open(struct tty_struct *t
>>  	} /* else count == 0 */
>>
>>  	tty->driver_data = hp;
>> -	tty->low_latency = 1; /* Makes flushes to ldisc synchronous. */
>> -
>>  	hp->tty = tty;
>>
>>  	spin_unlock_irqrestore(&hp->lock, flags);
>> @@ -327,6 +325,9 @@ static int hvc_open(struct tty_struct *t
>>  	if (hp->ops->notifier_add)
>>  		rc = hp->ops->notifier_add(hp, hp->data);
>>
>> +	if (!hp->irq_requested)
>> +		tty->low_latency = 1; /* Makes flushes to ldisc synchronous. */
>> +
>>  	/*
>>  	 * If the notifier fails we return an error.  The tty layer
>>  	 * will call hvc_close() after a failed open but we don't want to 
>> clean
>
> Acked-by: Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>
>

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