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Subject: Re: [PATCH 6/6] kexec jump: fix for ftrace
From: Huang Ying <ying.huang@intel.com>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>, Pavel Machek <pavel@ucw.cz>,
       nigel@nigel.suspend2.net, "Rafael J. Wysocki" <rjw@sisk.pl>,
       Andrew Morton <akpm@linux-foundation.org>, mingo@elte.hu,
       Linus Torvalds <torvalds@linux-foundation.org>,
       linux-kernel@vger.kernel.org,
       Kexec Mailing List <kexec@lists.infradead.org>
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	 <20080807133805.GD27783@redhat.com>
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Date: Fri, 08 Aug 2008 10:59:31 +0800
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On Thu, 2008-08-07 at 09:38 -0400, Vivek Goyal wrote:
[...]
> What kind of problem we run into if we don't disable the ftracer?
> 
> I think there are too many #ifdefs now and probably we can at least
> get rid if #ifdef CONFIG_FTRACE thing.
> 
> I think ftracer needs to export the function to enable the tracer
> back (tracer_enable()) so that we don't directly play with ftrace_enabled
> variable. tracer_enable() can be do {} while{0} in case of CONFIG_FTRACE=n
> so that we can get rid of #ifdefs here.

The ftracer issue for kexec is reported by Dhaval Giani and fixed by
Ingo as in following thread:

http://lkml.org/lkml/2008/2/19/175

After some testing, I found that if we enable ftrace before
restore_processor_state(), system will hang. I think maybe ftracer
depends on some processor state that we destroyed during kexec and
restored by restore_processor_state(). So I move save_processor_state()
and restore_processor_state() into machine_kexec() and enable ftrace
after restore_processor_state().

The #ifdef CONFIG_FTRACE should be removed. I think an interface like
irq_save/restore is good for this.

saved_ftrace_enabled = ftrace_save_enabled()
<...>
ftrace_restore_enabled(saved_ftrace_enabled)

Best Regards,
Huang Ying