Messages in this thread |  | | Date | Sun, 22 Jan 2023 16:24:21 +0100 | Subject | Re: [PATCH] Fix data race in mark_rt_mutex_waiters | From | Hernan Ponce de Leon <> |
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On 1/20/2023 4:54 PM, Paul E. McKenney wrote: > On Fri, Jan 20, 2023 at 06:58:20AM -0800, Arjan van de Ven wrote: >> On 1/20/2023 5:55 AM, Hernan Ponce de Leon wrote: >>> From: Hernan Ponce de Leon <hernanl.leon@huawei.com> >>> >> >>> kernel/locking/rtmutex.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c >>> index 010cf4e6d0b8..7ed9472edd48 100644 >>> --- a/kernel/locking/rtmutex.c >>> +++ b/kernel/locking/rtmutex.c >>> @@ -235,7 +235,7 @@ static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock) >>> unsigned long owner, *p = (unsigned long *) &lock->owner; >>> do { >>> - owner = *p; >>> + owner = READ_ONCE(*p); >>> } while (cmpxchg_relaxed(p, owner, >> >> >> I don't see how this makes any difference at all. >> *p can be read a dozen times and it's fine; cmpxchg has barrier semantics for compilers afaics > > Doing so does suppress a KCSAN warning. You could also use data_race() > if it turns out that the volatile semantics would prevent a valuable > compiler optimization.
I think the import question is "is this a harmful data race (and needs to be fixed as proposed by the patch) or a harmless one (and we should use data_race() to silence tools)?".
In https://lkml.org/lkml/2023/1/22/160 I describe how this data race can affect important ordering guarantees for the rest of the code. For this reason I consider it a harmful one. If this is not the case, I would appreciate some feedback or pointer to resources about what races care to avoid spamming the mailing list in the future.
Hernan
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