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Re: [PATCH] xfs: prevent NMI timeouts in cmn_err

To: xfs@xxxxxxxxxxx
Subject: Re: [PATCH] xfs: prevent NMI timeouts in cmn_err
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 3 Dec 2010 15:38:46 +1100
In-reply-to: <1291341315-31338-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1291341315-31338-1-git-send-email-david@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Fri, Dec 03, 2010 at 12:55:15PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 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:

        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);

        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?


Dave Chinner

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