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

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Date:   Fri, 20 Jan 2023 06:58:20 -0800
From:   Arjan van de Ven <arjan@...ux.intel.com>
To:     Hernan Ponce de Leon <hernan.poncedeleon@...weicloud.com>,
        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, paulmck@...nel.org,
        stern@...land.harvard.edu, diogo.behrens@...wei.com,
        jonas.oberhauser@...wei.com
Cc:     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 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