Re: [PATCH 1/2] kernel, timekeeping, add trylock option to ktime_get_with_offset()

From: Jiri Bohac <jbohac@suse.cz>
To: Prarit Bhargava <prarit@redhat.com>
Cc: linux-kernel@vger.kernel.org,
	John Stultz <john.stultz@linaro.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>,
	Xunlei Pang <pang.xunlei@linaro.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Baolin Wang <baolin.wang@linaro.org>,
	Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH 1/2] kernel, timekeeping, add trylock option to ktime_get_with_offset()
Date: Wed, 6 Jan 2016 17:04:43 +0100	[thread overview]
Message-ID: <20160106160443.GA16110@midget.suse.cz> (raw)
In-Reply-To: <1452085234-10667-2-git-send-email-prarit@redhat.com>

On Wed, Jan 06, 2016 at 08:00:33AM -0500, Prarit Bhargava wrote:
> -ktime_t ktime_get_with_offset(enum tk_offsets offs)
> +ktime_t ktime_get_with_offset(enum tk_offsets offs, int trylock)
>  {
>  	struct timekeeper *tk = &tk_core.timekeeper;
>  	unsigned int seq;
>  	ktime_t base, *offset = offsets[offs];
>  	s64 nsecs;
> +	unsigned long flags = 0;
> +
> +	if (unlikely(!timekeeping_initialized))
> +		return ktime_set(0, 0);
>  
>  	WARN_ON(timekeeping_suspended);
>  
> +	if (trylock && !raw_spin_trylock_irqsave(&timekeeper_lock, flags))
> +		return ktime_set(KTIME_MAX, 0);
> +

Are you trying to avoid a deadlock caused by calling printk() with
timekeeper_lock locked?

I believe this is already unsafe, as explained in the commit log
of 6d9bcb62 (timekeeping: use printk_deferred when holding
timekeeping seqlock).

So directly calling ktime_get() from printk would just turn a
rare deadlock into a certain one - perhaps a good thing?

-- 
Jiri Bohac <jbohac@suse.cz>
SUSE Labs, SUSE CZ