From: Ed Tomlinson <tomlins@cam.org>
Organization: me
To: Andrew Morton <akpm@osdl.org>
Subject: Re: tickle nmi watchdog whilst doing serial writes.
Date: Sat, 14 May 2005 06:31:57 -0400
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Cc: Dave Jones <davej@redhat.com>, linux-kernel@vger.kernel.org
References: <20050513184806.GA24166@redhat.com> <20050514065753.GA28213@redhat.com> <20050514000723.73bd6e5a.akpm@osdl.org>
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Hi,

Suspect this can be triggered using alt+sysreq+T on a busy system with a slow 
serial console.  Might be a easy way to see if this patch fixes the issue?

Ed Tomlinson

On Saturday 14 May 2005 03:07, Andrew Morton wrote:
> Dave Jones <davej@redhat.com> wrote:
> >
> > On Fri, May 13, 2005 at 11:43:31PM -0700, Andrew Morton wrote:
> >  > Dave Jones <davej@redhat.com> wrote:
> >  > >
> >  > > This was fun. I inserted a music CD with some obnoxious copy-protection
> >  > >  on it into the drive, and lots of SCSI errors went zipping over to
> >  > >  the serial console. Unfortunatly, the box was also compiling a kernel,
> >  > >  playing oggs, and doing a number of other things at the same time,
> >  > >  so this happened..
> >  > > 
> >  > >  NMI Watchdog detected LOCKUP on CPU2CPU 2
> >  > 
> >  > OK..  But calling touch_nmi_watchdog() at 1MHz seems a bit excessive, and
> >  > might perturb the finely-tuned timing in there.
> >  > 
> >  > How's about this?
> > 
> > Umm..  Despite it being past my bedtime, I'm pretty sure I'm
> > missing something here...
> > 
> >  > +		while (!(serial_in(up, UART_MSR) & UART_MSR_CTS) && --tmout)
> >  >  			udelay(1);
> > 
> > I don't see how this is any better than the current code.
> > We're doing 1000000 udelays. Whilst we're doing that,
> > the nmi watchdog goes bonkers.
> >  
> >  > +		if (tmout < 1000000)
> >  > +			touch_nmi_watchdog();
> > 
> > So by the time we do this, its already triggered.
> 
> But the NMI watchdog won't expire after one second - normally it's set to
> fixe seconds.
> 
> > How about..
> > 
> > --- linux-2.6.11/drivers/serial/8250.c~	2005-05-14 02:49:02.000000000 -0400
> > +++ linux-2.6.11/drivers/serial/8250.c	2005-05-14 02:54:30.000000000 -0400
> > @@ -40,6 +40,7 @@
> >  #include <linux/serial_core.h>
> >  #include <linux/serial.h>
> >  #include <linux/serial_8250.h>
> > +#include <linux/nmi.h>
> >  
> >  #include <asm/io.h>
> >  #include <asm/irq.h>
> > @@ -2099,8 +2100,15 @@ static inline void wait_for_xmitr(struct
> >  	if (up->port.flags & UPF_CONS_FLOW) {
> >  		tmout = 1000000;
> >  		while (--tmout &&
> > -		       ((serial_in(up, UART_MSR) & UART_MSR_CTS) == 0))
> > +		       ((serial_in(up, UART_MSR) & UART_MSR_CTS) == 0)) {
> > +			int cnt=0;
> >  			udelay(1);
> > +			cnt++;
> > +			if (cnt==100) {
> > +				touch_nmi_watchdog();
> > +				cnt=0;
> > +			}
> > +		}
> 
> <obwhitespacewhine> spose so.
> 
> --- 25/drivers/serial/8250.c~tickle-nmi-watchdog-whilst-doing-serial-writes	2005-05-14 00:03:09.000000000 -0700
> +++ 25-akpm/drivers/serial/8250.c	2005-05-14 00:06:53.000000000 -0700
> @@ -40,6 +40,7 @@
>  #include <linux/serial_core.h>
>  #include <linux/serial.h>
>  #include <linux/serial_8250.h>
> +#include <linux/nmi.h>
>  
>  #include <asm/io.h>
>  #include <asm/irq.h>
> @@ -2098,9 +2099,11 @@ static inline void wait_for_xmitr(struct
>  	/* Wait up to 1s for flow control if necessary */
>  	if (up->port.flags & UPF_CONS_FLOW) {
>  		tmout = 1000000;
> -		while (--tmout &&
> -		       ((serial_in(up, UART_MSR) & UART_MSR_CTS) == 0))
> +		while (!(serial_in(up, UART_MSR) & UART_MSR_CTS) && --tmout) {
>  			udelay(1);
> +			if ((tmout % 1000) == 0)
> +				touch_nmi_watchdog();
> +		}
>  	}
>  }
>  
> _
> 
> -
-
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