linux-kernel - Re: [PATCH 1/2] kernel, timekeeping, add trylock option to ktime_get_with

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Date:	Wed, 06 Jan 2016 11:27:54 -0500
From:	Prarit Bhargava <prarit@...hat.com>
To:	Jiri Bohac <jbohac@...e.cz>
CC:	linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Xunlei Pang <pang.xunlei@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Baolin Wang <baolin.wang@...aro.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 1/2] kernel, timekeeping, add trylock option to ktime_get_with_offset()



On 01/06/2016 11:04 AM, Jiri Bohac wrote:
> 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?

Not exactly.  When I initially sent this as a RFE to jstultz he pointed out that
if CPU A had acquired the timekeeper_lock (and therefore incremented tk_core.seq
for a write), and CPU B panicked, no output would occur because the reads of
tk_core.seq would spin indefinitely.

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

Hmm ... John Stultz, any suggestions here?

P.

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