linux-kernel - Re: [RFC][PATCH 5/5 v2] x86/nmi: Perform a safe NMI stack trace on all CPUs

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Date:	Fri, 27 Jun 2014 10:40:59 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Petr Mládek <pmladek@...e.cz>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Michal Hocko <mhocko@...e.cz>,
	Jan Kara <jack@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Dave Anderson <anderson@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Konstantin Khlebnikov <koct9i@...il.com>
Subject: Re: [RFC][PATCH 5/5 v2] x86/nmi: Perform a safe NMI stack trace on
 all CPUs

On Fri, 27 Jun 2014 16:32:46 +0200
Petr Mládek <pmladek@...e.cz> wrote:

 
> > +/*
> > + * It is not safe to call printk() directly from NMI handlers.
> > + * It may be fine if the NMI detected a lock up and we have no choice
> > + * but to do so, but doing a NMI on all other CPUs to get a back trace
> > + * can be done with a sysrq-l. We don't want that to lock up, which
> > + * can happen if the NMI interrupts a printk in progress.
> > + *
> > + * Instead, we redirect the vprintk() to this nmi_vprintk() that writes
> > + * the content into a per cpu seq_buf buffer. Then when the NMIs are
> > + * all done, we can safely dump the contents of the seq_buf to a printk()
> > + * from a non NMI context.
> > + */
> 
> I would move this comment above #define NMI_BUF_SIZE
> It should be on top because it helps to understand many other tricks that are
> used above.
> 

I have no problem with moving it. This started out as a simple comment
and then I just got carried away ;-)

-- Steve
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