Re: [RFC][PATCH 3/4] ntp: Use printk_deferred in leapsecond path

From: John Stultz <john.stultz@linaro.org>
To: Jiri Bohac <jbohac@suse.cz>
Cc: lkml <linux-kernel@vger.kernel.org>,
	Prarit Bhargava <prarit@redhat.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Richard Cochran <richardcochran@gmail.com>,
	Jan Kara <jack@suse.cz>, Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Shuah Khan <shuahkh@osg.samsung.com>
Subject: Re: [RFC][PATCH 3/4] ntp: Use printk_deferred in leapsecond path
Date: Tue, 2 Jun 2015 09:04:36 -0700	[thread overview]
Message-ID: <CALAqxLUCKW-mZEiA=vMrVx5xy4QesTG6AvTAWZbTdy1fct=eMg@mail.gmail.com> (raw)
In-Reply-To: <20150602103144.GB18714@midget.suse.cz>

On Tue, Jun 2, 2015 at 3:31 AM, Jiri Bohac <jbohac@suse.cz> wrote:
> Hi,
>
> On Fri, May 29, 2015 at 01:24:27PM -0700, John Stultz wrote:
>> Looking over the leapsecond code, I noticed the printk messages
>> reporting the leapsecond insertion in the second_overflow path
>> were not using the printk_deferred method. This was surprising
>> since the printk_deferred method was added in part to avoid
>> printk-ing while holding the timekeeping locks.
>>
>> See 6d9bcb621b0b (timekeeping: use printk_deferred when holding
>> timekeeping seqlock) for further rational.
>>
>> I can only guess that this omission was a git add -p oversight.
>
> second_overflow() is called from accumulate_nsecs_to_secs().
>
> accumulate_nsecs_to_secs() is called from update_wall_time()
> - once directly
> - once via logarithmic_accumulation()
> Both calls are before write_seqcount_begin(&tk_core.seq).
>
> So it looks safe to use printk there.

Good point. The update is being done to the shadow-timekeeper, so we
won't block readers.  This can probably be dropped then. Although I'm
almost consider keeping it for consistency so I don't forget this
again and worry about it in the future.

thanks
-john