linux-kernel - Re: [PATCH] percpu: use notrace variant of preempt_disable/preempt

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Date:   Mon, 7 Nov 2016 12:27:16 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Heiko Carstens <heiko.carstens@...ibm.com>
Cc:     Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
        Sebastian Ott <sebott@...ux.vnet.ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH] percpu: use notrace variant of
 preempt_disable/preempt_enable

On Thu, 3 Nov 2016 14:58:38 +0100
Heiko Carstens <heiko.carstens@...ibm.com> wrote:

> Commit 345ddcc882d8 ("ftrace: Have set_ftrace_pid use the bitmap like
> events do") added a couple of this_cpu_read calls to the ftrace code.
> 
> On x86 this is not a problem, since it has single instructions to read
> percpu data. Other architectures which use the generic variant now
> have additional preempt_disable and preempt_enable calls in the core
> ftrace code. This may lead to recursive calls and in result to a dead
> machine, e.g. if preemption and debugging options are enabled.
> 
> To fix this use the notrace variant of preempt_disable and
> preempt_enable within the generic percpu code.
> 
> Reported-and-bisected-by: Sebastian Ott <sebott@...ux.vnet.ibm.com>
> Tested-by: Sebastian Ott <sebott@...ux.vnet.ibm.com>
> Fixes: 345ddcc882d8 ("ftrace: Have set_ftrace_pid use the bitmap like events do")
> Signed-off-by: Heiko Carstens <heiko.carstens@...ibm.com>

Acked-by: Steven Rostedt <rostedt@...dmis.org>

Feel free to push this in your tree.

-- Steve

> ---
>  include/asm-generic/percpu.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/asm-generic/percpu.h b/include/asm-generic/percpu.h
> index 40e887068da2..0504ef8f3aa3 100644
> --- a/include/asm-generic/percpu.h
> +++ b/include/asm-generic/percpu.h
> @@ -118,9 +118,9 @@ do {									\
>  #define this_cpu_generic_read(pcp)					\
>  ({									\
>  	typeof(pcp) __ret;						\
> -	preempt_disable();						\
> +	preempt_disable_notrace();					\
>  	__ret = raw_cpu_generic_read(pcp);				\
> -	preempt_enable();						\
> +	preempt_enable_notrace();					\
>  	__ret;								\
>  })
>