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

[Prarit Bhargava]

On 01/06/2016 12:33 PM, John Stultz wrote:
> On Wed, Jan 6, 2016 at 9:28 AM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
>> On Wed, Jan 6, 2016 at 5:00 AM, Prarit Bhargava <prarit@xxxxxxxxxx> 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);
>>
>> Wait.. this doesn't make sense. The timekeeper lock is only for reading.
> 
> Only for writing.. sorry.. still drinking my coffee.
> 
>> What I was suggesting to you off line is to have something that avoids
>> spinning on the seqcounter should if a bug occurs and we IPI all the
>> cpus, that we don't deadlock or block any printk messages.
> 
> And more clearly here, if a cpu takes a write on the seqcounter in
> update_wall_time() and at that point another cpu hits a bug, and IPIs
> the cpus, the system would deadlock. That's really what I want to
> avoid.

Right -- but the only time that the seq_lock is taken for writing is when the
timekeeper_lock is acquired (including update_wall_time()).  This means that

if (!raw_spin_trylock_irqsave(&timekeeper_lock, flags))

is equivalent to

if (tk_core.seq & 1) // sequence_t is odd when writing

The problem with the latter is that it is possible that there is no
protection from a writer setting tk_core.seq odd AFTER I've read it,
and the protection for that AFAICT comes from the timekeeper_lock.

That means I need to check to see if the timekeeper_lock is locked.  And
the patch does exactly that -- checks to see if the lock is available, and
if not avoids spinning on the seq_lock.

P.



> 
> thanks
> -john
> 
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