linux-kernel - Re: [PATCH 6/6] kexec jump: fix for ftrace

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Date:	Fri, 08 Aug 2008 10:59:31 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Machek <pavel@....cz>, nigel@...el.suspend2.net,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>, mingo@...e.hu,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Kexec Mailing List <kexec@...ts.infradead.org>
Subject: Re: [PATCH 6/6] kexec jump: fix for ftrace

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


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