Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Waiman Long <longman@redhat.com>]

On 1/24/23 09:57, Hernan Ponce de Leon wrote:
>> In the case, the value read is passed into cmpxchg_relaxed(), which
>> checks the value against memory.  In this case, as Arjan noted, the only
>> compiler-and-silicon difference between data_race() and READ_ONCE()
>> is that use of data_race() might allow the compiler to do things like
>> tear the load, thus forcing the occasional spurious cmpxchg_relaxed()
>> failure.  In contrast, LKMM (by design) throws up its hands when it sees
>> a data race.  Something about not being eager to track the 
>> idiosyncrasies
>> of many compiler versions.
>>
>> My approach in my own code is to use *_ONCE() unless it causes a visible
>> performance regression or if it confuses KCSAN.  An example of the 
>> latter
>> can be debug code, in which case use of data_race() avoids suppressing
>> KCSAN warnings (and also false positives, depending).
>
> I understand that *_ONCE() might avoid some compiler optimization and 
> reduce performance in the general case. However, if I understand your 
> first paragraph correctly, in this particular case data_race() could 
> allow the CAS to fail more often, resulting in more spinning 
> iterations and degraded performance. Am I right?
>
>>
>> Except that your other email seems to also be arguing that additional
>> ordering is required.  So is https://lkml.org/lkml/2023/1/20/702 really
>> sufficient just by itself, or is additional ordering required?
>
> I do not claim that we need to mark the read to add the ordering that 
> is needed for correctness (mutual exclusion). What I claim in this 
> patch is that there is a data race, and since it can affect ordering 
> constrains in subtle ways, I consider it harmful and thus I want to 
> fix it.
>
> What I explain in the other email is that if we fix the data race, 
> either the fence or the acquire store might be relaxed (because 
> marking the read gives us some extra ordering guarantees). If the race 
> is not fixed, both the fence and the acquire are needed according to 
> LKMM. The situation is different wrt hardware models. In that case the 
> tool cannot find any violation even if we don't fix the race and we 
> relax the store / remove the fence.

I would suggest to do it as suggested by PeterZ. Instead of set_bit(), 
however, it is probably better to use atomic_long_or() like

atomic_long_or_relaxed(RT_MUTEX_HAS_WAITERS, (atomic_long_t *)&lock->owner)

The mutex code stores the lock owner as atomic_long_t. So it is natural 
to treat &lock->owner as atomic_long_t here too.

Cheers,
Longman