Date:	Tue, 17 May 2016 12:46:38 -0700
From:	Peter Hurley <peter@...leysoftware.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <Waiman.Long@....com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>, kcc@...gle.com,
	dvyukov@...gle.com, dhowells@...hat.com
Subject: Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner
 field

On 05/16/2016 10:22 AM, Paul E. McKenney wrote:
> On Mon, May 16, 2016 at 07:17:42AM -0700, Peter Hurley wrote:
>> 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.
> 
> The point about eliding redundant loads is a good one, at least in those
> cases where it is a reasonable optimization.  Should we ever get to a
> point where we no longer use pre-C11 compilers, those use cases could
> potentially use memory_order_relaxed loads.  Preferably wrappered in
> something that can be typed with fewer characters.  And it could of course
> lead to an interesting discussion of what use cases would be required
> to justify this change, but what else is new?

I believe lockless access is quite widespread in the kernel, and this
use was based on the previous assumption that loads/stores to
char/short/int/long/void* are atomic, which is generally safe in the
absence of specific circumstances which may cause load- or store-tearing
(are there others besides immediate stores and packed structures?).

So I think it makes more sense to annotate usage that prevents load-
and store-tearing, separately from the forceably load/store READ_ONCE/
WRITE_ONCE macros.


>> 2. Makes a mess of otherwise readable code.
>>
>> 3. Error-prone; ie., easy to overlook in review.
> 
> But #2 and #3 are at odds with each other.  It is all too easy to miss a
> critically important load or store that has not been flagged in some way.
> So #2's readable code can easily be problematic, as the concurrency is
> hidden from both the compiler and the poor developer reading the code.

Not for the purpose of preventing load- and store-tearing; ie., the
vast majority of lockless use now.


>> There is no practical difference between _always_ using READ_ONCE()/WRITE_ONCE()
>> (to prevent tearing) and declaring the field volatile.
> 
> Actually, yes there is a difference.  If you hold the update-side lock,
> you don't have to use READ_ONCE() when reading the variable.  If you
> have further excluded readers (for example, at initialization time or
> at teardown time), then you don't have to use either READ_ONCE() or
> WRITE_ONCE().

This cuts both ways; on the one hand, you're saying using volatile modifier
doesn't let us control every use case, and on the other hand, we're adding
volatile access to list primitives that we _know_ are both frequently
used and in update-side locks. Where's the win?


>> So we've come full-circle from volatile-considered-harmful.
> 
> Not really.  We are (hopefully) using volatile for jobs that it can do.
> In contrast, in the past people were expecting it to do more than it
> reasonably can do.

Well, I wasn't referring to the never-did-work ideas and more about
the example I quoted from that document about cpu_relax() being a barrier.

Regards,
Peter Hurley

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