Subject: Re: [PATCH 1/4] hvc_console: do not set low_latency

From: Christian Borntraeger <[email-masked]>
Date: Tue, 13 Jan 2009 10:04:22 +0100

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.
> 
> Looking back through history:
> v2.6.16-rc1 via 33f0f88f1c51ae5c2d593d26960c760ea154c2e2
>     [PATCH] TTY layer buffering revamp
> 
> added this new api.
> 
> v2.6.16-rc3 via 8977d929e49021d9a6e031310aab01fa72f849c2
>     [PATCH] tty buffering stall fix
> 
> claims to fix a stall discovered with hvc_console
> 
> v2.6.16-rc5 via fb5c594c2acc441f0d2d8f457484a0e0e9285db3
>    [PATCH] Fix race condition in hvc console.
> 
> said set this flag to avoid a stall problem, and was merged through
> the powerpc arch tree.
> 
> Without searching for email discussions, it would appear to be an
> overlapping "fix", but one that did not consider all users.
> 
> ---
> This version continues to set low_latency if irqs are not flagged to
> be in use, as requested by paulus.   Do all hvc drivers identify the
> interrupt this way?  (A quick look at hvc_iucv says they lock to bh
> and are not irq driven, the rest would have used the irq before that
> patch).
> 
> Having the flag set for purely polled drivers will save delaying
> the work when receiving input for 1 jiffie.
> 
> 
> Index: work.git/drivers/char/hvc_console.c
> ===================================================================
> --- work.git.orig/drivers/char/hvc_console.c	2009-01-08 
03:01:24.000000000 -0600
> +++ work.git/drivers/char/hvc_console.c	2009-01-08 03:01:51.000000000 -0600
> @@ -318,7 +318,8 @@ static int hvc_open(struct tty_struct *t
>  	} /* else count == 0 */
> 
>  	tty->driver_data = hp;
> -	tty->low_latency = 1; /* Makes flushes to ldisc synchronous. */
> +	if (!hp->irq_requested)
> +		tty->low_latency = 1; /* Makes flushes to ldisc synchronous. */
> 
>  	hp->tty = tty;
> 
> 

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