Re: [PATCH] Fix data race in mark_rt_mutex_waiters
From: Hernan Ponce de Leon
Date: Sun Jan 22 2023 - 10:25:06 EST
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@xxxxxxxxxx>
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