linux-kernel - Re: [PATCH] Fix data race in mark_rt_mutex

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Date:   Sun, 22 Jan 2023 16:24:21 +0100
From:   Hernan Ponce de Leon <hernan.poncedeleon@...weicloud.com>
To:     paulmck@...nel.org, Arjan van de Ven <arjan@...ux.intel.com>
Cc:     peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        longman@...hat.com, boqun.feng@...il.com, akpm@...l.org,
        tglx@...utronix.de, joel@...lfernandes.org,
        stern@...land.harvard.edu, diogo.behrens@...wei.com,
        jonas.oberhauser@...wei.com, linux-kernel@...r.kernel.org,
        Hernan Ponce de Leon <hernanl.leon@...wei.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] Fix data race in mark_rt_mutex_waiters

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@...wei.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