Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner field

[Peter Hurley]

On 05/16/2016 05:17 AM, Paul E. McKenney wrote:
> On Mon, May 16, 2016 at 01:09:48PM +0200, Peter Zijlstra wrote:
>> On Fri, May 13, 2016 at 10:58:05AM -0700, Peter Hurley wrote:
>>>> Note that barrier() and READ_ONCE() have overlapping but not identical
>>>> results and the combined use actually makes sense here.
>>>>
>>>> Yes, a barrier() anywhere in the loop will force a reload of the
>>>> variable, _however_ it doesn't force that reload to not suffer from
>>>> load tearing.
>>>>
>>>> Using volatile also forces a reload, but also ensures the load cannot
>>>> be torn IFF it is of machine word side and naturally aligned.
>>>>
>>>> So while the READ_ONCE() here is pointless for forcing the reload;
>>>> that's already ensured, we still need to make sure the load isn't torn.
>>>
>>> If load tearing a naturally aligned pointer is a real code generation
>>> possibility then the rcu list code is broken too (which loads ->next
>>> directly; cf. list_for_each_entry_rcu() & list_for_each_entry_lockless()).
>>>
>>> For 4.4, Paul added READ_ONCE() checks for list_empty() et al, but iirc
>>> that had to do with control dependencies and not load tearing.
>>
>> Well, Paul is the one who started the whole load/store tearing thing, so
>> I suppose he knows what he's doing.
> 
> That had to do with suppressing false positives for one of Dmitry
> Vjukov's concurrency checkers.  I suspect that Peter Hurley is right
> that continued use of that checker would identify other places needing
> READ_ONCE(), but from what I understand that is on hold pending a formal
> definition of the Linux-kernel memory model.  (KCC and Dmitry (CCed)
> can correct my if I am confused on this point.)
> 
>> That said; its a fairly recent as things go so lots of code hasn't been
>> updated yet, and its also a very unlikely thing for a compiler to do;
>> since it mostly doesn't make sense to emit multiple instructions where
>> one will do, so its not a very high priority thing either.
>>
>> But from what I understand, the compiler is free to emit all kinds of
>> nonsense for !volatile loads/stores.
> 
> That is quite true.  :-/
> 
>>> OTOH, this patch might actually produce store-tearing:
>>>
>>> +static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
>>> +{
>>> +	/*
>>> +	 * We check the owner value first to make sure that we will only
>>> +	 * do a write to the rwsem cacheline when it is really necessary
>>> +	 * to minimize cacheline contention.
>>> +	 */
>>> +	if (sem->owner != RWSEM_READER_OWNED)
>>> +		sem->owner = RWSEM_READER_OWNED;
>>> +}
>>
>> Correct; which is why we should always use {READ,WRITE}_ONCE() for
>> anything that is used locklessly.
> 
> Completely agreed.  Improve readability of code by flagging lockless
> shared-memory accesses, help checkers better find bugs, and prevent the
> occasional compiler mischief!

I think this would be a mistake for 3 reasons:

1. If READ_ONCE()/WRITE_ONCE() is necessary to prevent load/store tearing
   of any normally-atomic type (char/int/long/void*), then _every_ access
   would require READ_ONCE()/WRITE_ONCE(), thus eliminating any possibility
   of compiler optimization (eg. eliding redundant loads) where it would
   otherwise be possible.

2. Makes a mess of otherwise readable code.

3. Error-prone; ie., easy to overlook in review.

There is no practical difference between _always_ using READ_ONCE()/WRITE_ONCE()
(to prevent tearing) and declaring the field volatile.

So we've come full-circle from volatile-considered-harmful.

Regards,
Peter Hurley