Messages in this thread | | | Date | Tue, 24 Jan 2023 10:42:24 -0500 | Subject | Re: [PATCH] Fix data race in mark_rt_mutex_waiters | From | Waiman Long <> |
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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
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