linux-kernel - Re: [PATCH v2 1/2] livepatch: Remove duplicate warning about missing reliable stacktrace support

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Message-ID: <20190507004032.2fgddlsycyypqdsn@treble>
Date:   Mon, 6 May 2019 19:40:32 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Jiri Kosina <jikos@...nel.org>, 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 v2 1/2] livepatch: Remove duplicate warning about missing
 reliable stacktrace support

On Tue, Apr 30, 2019 at 11:10:48AM +0200, Petr Mladek wrote:
> WARN_ON_ONCE() could not be called safely under rq lock because
> of console deadlock issues. Fortunately, there is another check
> for the reliable stacktrace support in klp_enable_patch().
> 
> Signed-off-by: Petr Mladek <pmladek@...e.com>
> ---
>  kernel/livepatch/transition.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> index 9c89ae8b337a..8e0274075e75 100644
> --- a/kernel/livepatch/transition.c
> +++ b/kernel/livepatch/transition.c
> @@ -263,8 +263,15 @@ 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);
> +	/*
> +	 * pr_warn() under task rq lock might cause a deadlock.
> +	 * Fortunately, missing reliable stacktrace support has
> +	 * already been handled when the livepatch was enabled.
> +	 */
> +	if (ret == -ENOSYS)
> +		return ret;

I find the comment to be a bit wordy and confusing (and vague).

Also this check is effectively the same as the klp_have_reliable_stack()
check which is done in kernel/livepatch/core.c.  So I think it would be
clearer and more consistent if the same check is done here:

	if (!klp_have_reliable_stack())
		return -ENOSYS;

	ret = save_stack_trace_tsk_reliable(task, &trace);

	[ no need to check ret for ENOSYS here ]

Then, IMO, no comment is needed.

-- 
Josh