Re: [PATCH 1/4 v2] hvc_console: remove tty->low_latency

[Hendrik Brueckner <brueckner@linux.vnet.ibm.com>]

On Wed, Jan 14, 2009 at 08:04:04AM +1100, Benjamin Herrenschmidt wrote:
> > So the simplest is to never set it.
> 
> I already applied your previous patch. Please send an incremental fix.
Here is the incremental patch for your powerpc tree along with a summary
as patch description.

Regards,
Hendrik

--
This patch removes the tty->low_latency setting.

For irq based hvc_console backends the tty->low_latency must be set to 0,
because the tty_flip_buffer_push() function must not be called from IRQ context
(see drivers/char/tty_buffer.c).

For polled backends, the low_latency setting causes the bug trace below, because
tty_flip_buffer_push() is called within an atomic context and subsequent calls
might sleep due to mutex_lock.

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

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
---
 drivers/char/hvc_console.c |    2 --
 1 file changed, 2 deletions(-)

--- a/drivers/char/hvc_console.c
+++ b/drivers/char/hvc_console.c
@@ -318,8 +318,6 @@ static int hvc_open(struct tty_struct *t
 	} /* else count == 0 */
 
 	tty->driver_data = hp;
-	if (!hp->irq_requested)
-		tty->low_latency = 1; /* Makes flushes to ldisc synchronous. */
 
 	hp->tty = tty;