linux-kernel - Re: [PATCH 06/13] tracing: remove dump_ran check in __ftrace

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Date:	Mon, 11 Mar 2013 11:58:17 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jovi Zhang <bookjovi@...il.com>
Cc:	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 06/13] tracing: remove dump_ran check in __ftrace_dump

On Mon, 2013-03-11 at 23:35 +0800, Jovi Zhang wrote:
> On Mon, Mar 11, 2013 at 10:08 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Mon, 2013-03-11 at 15:13 +0800, zhangwei(Jovi) wrote:
> >> It's reasonable to call __ftrace_dump function not only once,
> >> so remove the dump_ran variable checking.
> >
> > This needs a little more work. On an oops, I only want it dumped once,
> > because a crash can cause another crash while its dumping, and without
> > that check in will corrupt the buffer.
> For reclusive dumping, it's already under the protection of
> ftrace_dump_lock spinlock,
> I missed something? would you explain more on this case?

Actually, it matters if it was called from NMI context or not. If an NMI
triggered and did a dump while a reader was reading the buffer, the NMI
can corrupt the buffer. It will print fine for the NMI, but if the
reader continues, there's a chance it can get messed up and corrupt the
buffer. The "dump once" is a paranoid method to say we only dump it once
on oops and stop (hence the ftrace_kill() there too).

Now I think there's a possible deadlock here as well. If the dump was
caused by something other than an NMI lock up, and while it is dumping
the NMI goes off and triggers a bug, it too can enter this and that
arch_spin_lock() will cause a deadlock. This will be something I need to
clean up as well.

Thanks for calling my attention to this.

-- Steve

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