linux-kernel - Re: [PATCH 3/3] livepatch: Cleanup message handling in klp_try_switch

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Date:   Wed, 24 Apr 2019 12:41:00 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Petr Mladek <pmladek@...e.com>
cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] livepatch: Cleanup message handling in
 klp_try_switch_task()

On Wed, 24 Apr 2019, Petr Mladek wrote:

> WARN_ON_ONCE() could not be called safely under rq lock because
> of console deadlock issues. Fortunately, simple printk_deferred()
> is enough because the warning is printed from a well defined
> location and context.
> 
> Also klp_try_switch_task() is called under klp_mutex.
> Therefore, the buffer for debug messages could be static.
> 
> Signed-off-by: Petr Mladek <pmladek@...e.com>
> ---
>  kernel/livepatch/transition.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> index 9c89ae8b337a..e8183d18227f 100644
> --- a/kernel/livepatch/transition.c
> +++ b/kernel/livepatch/transition.c
> @@ -254,6 +254,7 @@ static int klp_check_stack_func(struct klp_func *func,
>  static int klp_check_stack(struct task_struct *task, char *err_buf)
>  {
>  	static unsigned long entries[MAX_STACK_ENTRIES];
> +	static int enosys_warned;
>  	struct stack_trace trace;
>  	struct klp_object *obj;
>  	struct klp_func *func;
> @@ -263,8 +264,16 @@ static int klp_check_stack(struct task_struct *task, char *err_buf)
>  	trace.nr_entries = 0;
>  	trace.max_entries = MAX_STACK_ENTRIES;
>  	trace.entries = entries;
> +
>  	ret = save_stack_trace_tsk_reliable(task, &trace);
> -	WARN_ON_ONCE(ret == -ENOSYS);
> +	if (ret == -ENOSYS) {
> +		if (!enosys_warned) {
> +			printk_deferred(KERN_WARNING "%s: save_stack_trace_tsk_reliable() not supported on this architecture.\n",
> +					__func__);
> +			enosys_warned = 1;

... abusing the fact that you are also printk maintainer :) ... looking at 
the above, wouldn't it make sense to introduce generic 
printk_deferred_once() instead?

-- 
Jiri Kosina
SUSE Labs