linux-kernel - Re: [PATCH 3/3] ftrace: Revert 8ab2b7efd ftrace: Remove unnecessary disabling of

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Date:	Tue, 7 Jun 2011 18:20:30 +0100
From:	"Richard W.M. Jones" <rjones@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 3/3] ftrace: Revert 8ab2b7efd ftrace: Remove unnecessary
 disabling of

On Tue, Jun 07, 2011 at 01:07:43PM -0400, Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@...hat.com>
> 
> Revert the commit that removed the disabling of interrupts around
> the initial modifying of mcount callers to nops, and update the comment.
> 
> The original comment was outdated and stated that the interrupts were
> being disabled to prevent kstop machine, which was required with the
> old ftrace daemon, but was no longer the case.
> 
> What the comment failed to mention was that interrupts needed to be
> disabled to keep interrupts from preempting the modifying of the code
> and then executing the code that was partially modified.
> 
> Revert the commint and update the comment.
> 
> Reported-by: Richard W.M. Jones <rjones@...hat.com>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> ---
>  kernel/trace/ftrace.c |    7 +++++++
>  1 files changed, 7 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 204b3eb..908038f 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -3330,6 +3330,7 @@ static int ftrace_process_locs(struct module *mod,
>  {
>  	unsigned long *p;
>  	unsigned long addr;
> +	unsigned long flags;
>  
>  	mutex_lock(&ftrace_lock);
>  	p = start;
> @@ -3346,7 +3347,13 @@ static int ftrace_process_locs(struct module *mod,
>  		ftrace_record_ip(addr);
>  	}
>  
> +	/*
> +	 * Disable interrupts to prevent interrupts from executing
> +	 * code that is being modified.
> +	 */
> +	local_irq_save(flags);
>  	ftrace_update_code(mod);
> +	local_irq_restore(flags);
>  	mutex_unlock(&ftrace_lock);
>  
>  	return 0;
> -- 
> 1.7.4.4

ACK.

I have extensively tested this patch because it was required in order
to fix a boot problem with libguestfs on Fedora Rawhide:

https://bugzilla.redhat.com/show_bug.cgi?id=710921

Rich.

-- 
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