[PATCH] xfs: prevent NMI timeouts in cmn_err

[Dave Chinner]

On Fri, Dec 03, 2010 at 12:55:15PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner at redhat.com>
> 
> We currently have a global error message buffer in cmn_err that is
> protected by a spin lock that disables interrupts.  Recently there
> have been reports of NMI timeouts occurring when the console is
> being flooded by SCSI error reports due to cmn_err() getting stuck
> trying to print to the console while holding this lock (i.e. with
> interrupts disabled). The NMI watchdog is seeing this CPU as
> non-responding and so is triggering a panic.  While the trigger for
> the reported case is SCSI errors, pretty much anything that spams
> the kernel log could cause this to occur.
> 
> Realistically the only reason that we have the intemediate message
> buffer is to prepend the correct kernel log level prefix to the log
> message. The only reason we have the lock is to protect the global
> message buffer and the only reason the message buffer is global is
> to keep it off the stack. Hence if we can avoid needing a global
> message buffer we avoid needing the lock, and we can do this with a
> small amount of cleanup and some preprocessor tricks:
> 
> 	1. clean up xfs_cmn_err() panic mask functionality to avoid
> 	   needing debug code in xfs_cmn_err()
> 	2. remove the couple of "!" message prefixes that still exist that
> 	   the existing cmn_err() code steps over.
> 	3. redefine CE_* levels directly to KERN_*
> 	4. redefine cmn_err() and friends to use printk() directly
> 	   via variable argument length macros.
> 
> By doing this, we can completely remove the cmn_err() code and the
> lock that is causing the problems, and rely solely on printk()
> serialisation to ensure that we don't get garbled messages.
> 
> A series of followup patches is really needed to clean up all the
> cmn_err() calls and related messages properly, but that results in a
> series that is not easily back portable to enterprise kernels. Hence
> this initial fix is only to address the direct problem in the lowest
> impact way possible.

FWIW, while these macros are the best way to make a simple backport
is possible, I just discovered that mainline has a %pV format
operator that allows an implementation like:

void
xfs_fs_cmn_err(
	const char              *lvl,
	struct xfs_mount        *mp,
	const char              *fmt,
	...)
{
	struct va_format        vaf;
	va_list                 args;

	va_start(args, fmt);
	vaf.fmt = fmt;
	vaf.va = &args;

	printk("%sFilesystem %s: %pV", lvl, mp->m_fsname, &vaf);
	va_end(args);

	BUG_ON(strncmp(lvl, KERN_EMERG, strlen(KERN_EMERG)) == 0);
}

Would this be a preferable method for replacing the existing
implementations, or are the macros good enough as the first step of
a mainline cleanup?

Cheers,,,

Dave.
-- 
Dave Chinner
david at fromorbit.com