Re: [PATCH] Fix data race in mark_rt_mutex_waiters

[Peter Zijlstra]

On Fri, Jan 20, 2023 at 02:55:25PM +0100, Hernan Ponce de Leon wrote:
> From: Hernan Ponce de Leon <hernanl.leon@xxxxxxxxxx>
> 
> Following the defition of data race in
> tools/memory-model/linux-kernel.cat the dartagnan tool
> https://github.com/hernanponcedeleon/Dat3M
> reported a race between mark_rt_mutex_waiters and rt_mutex_cmpxchg_release.
> 
> Commit 23f78d4a03c5 ("[PATCH] pi-futex: rt mutex core")
> later removed in commit d0aa7a70bf03 ("futex_requeue_pi optimization")
> and reverted in commit bd197234b0a6
> ("Revert "futex_requeue_pi optimization"")
> 
> The original commit introduced the data race.
> 
> Cc: stable@xxxxxxxxxxxxxxx # v2.6.18.x
> Fixes: 23f78d4a03c5 ("[PATCH] pi-futex: rt mutex core")
> Signed-off-by: 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,
>  				 owner | RT_MUTEX_HAS_WAITERS) != owner);
>  

Can't we replace the whole of that function with:

	set_bit(0, (unsigned long *)&lock->owner);

?