Date: Mon, 6 Sep 2004 09:46:22 -0700
From: "Randy.Dunlap" <rddunlap@osdl.org>
To: Russell King <rmk+lkml@arm.linux.org.uk>
Cc: James@superbug.demon.co.uk, dth@ncc1701.cistron.net,
       linux-kernel@vger.kernel.org
Subject: Re: Linux serial console patch
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On Mon, 6 Sep 2004 16:52:30 +0100 Russell King wrote:

| On Mon, Sep 06, 2004 at 04:45:22PM +0100, James Courtier-Dutton wrote:
| > Russell King wrote:
| > > On Mon, Sep 06, 2004 at 10:32:27AM +0000, Danny ter Haar wrote:
| > > 
| > >>James Courtier-Dutton  <James@superbug.demon.co.uk> wrote:
| > >>
| > >>>>I have read your posts to lkml containing your serial console flow control
| > >>>>patches firstly for 2.4.x and then for 2.6.x kernels.
| > >>>
| > >>>Does this fix junk being output from the serial console?
| > >>>If one is using Pentium 4 HT, it seems that both CPU cores try to send 
| > >>>characters to the serial port at the same time, resulting in lost 
| > >>>characters as one CPU over writes the output from the other.
| > >>
| > >>We have multiple P4-HT enabled servers with debian installed & serial
| > >>console enabled (RPB++ ;-) and _i_ have never seen this behaviour.
| > > 
| > > 
| > > I don't think this is a serial problem as such, but a problem with the
| > > kernel console subsystem (printk) itself.  Maybe James can provide an
| > > example output to confirm exactly what he's seeing.
| > > 
| > 
| > http://www.superbug.demon.co.uk/latency/
| > 
| > There are 2 oops traces there. At about line 176, the corruption starts.
| 
| They both look like a two printk's overlapping each other, which isn't
| unreasonable since one is an oops.  We try real hard to get oopses
| out, which means "busting" the printk spinlocks.  The side effect of
| busting those spinlocks is of course no console locking.
| 
| It may be annoying, but unless some SMP person wants to fix the spinlock
| busting to be a little more inteligent, you can expect this situation
| to continue.

I've seen it enough times that I would like to see it fixed,
and David Howells (RH) has posted a patch for it several times.

I'm woondering if this:
http://linux.bkbits.net:8080/linux-2.5/diffs/kernel/printk.c@1.38?nav=index.html|src/|src/kernel|hist/kernel/printk.c
is supposed to be a patch for the problem in the 'latency' log above.
If so, it's not as good a solution as David Howells's patch is.
His latest (AFAIK) is:
http://marc.theaimsgroup.com/?l=linux-kernel&m=105730993512692&w=2

--
~Randy
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