Subject: Re: [PATCH 02/12] powerpc: qspinlock: Mark accesses to qnode lock checks

From: "Nicholas Piggin" <[email-masked]>
Date: Tue, 09 May 2023 12:02:57 +1000

On Mon May 8, 2023 at 12:01 PM AEST, Rohan McLure wrote:
> The powerpc implemenation of qspinlocks will both poll and spin on the
> bitlock guarding a qnode. Mark these accesses with READ_ONCE to convey
> to KCSAN that polling is intentional here.

Yeah, and obviously pairs with the WRITE_ONCE so comment isn't really
necessary.

Reviewed-by: Nicholas Piggin <[email-masked]>

>
> Signed-off-by: Rohan McLure <[email-masked]>
> ---
>  arch/powerpc/lib/qspinlock.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
> index 9cf93963772b..579290d55abf 100644
> --- a/arch/powerpc/lib/qspinlock.c
> +++ b/arch/powerpc/lib/qspinlock.c
> @@ -435,7 +435,7 @@ static __always_inline bool yield_to_prev(struct qspinlock *lock, struct qnode *
>  
>  	smp_rmb(); /* See __yield_to_locked_owner comment */
>  
> -	if (node->locked) {
> +	if (READ_ONCE(node->locked)) {
>  		yield_to_preempted(prev_cpu, yield_count);
>  		spin_begin();
>  		return preempted;
> @@ -584,7 +584,7 @@ static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, b
>  
>  		/* Wait for mcs node lock to be released */
>  		spin_begin();
> -		while (node->locked) {
> +		while (READ_ONCE(node->locked)) {
>  			spec_barrier();
>  
>  			if (yield_to_prev(lock, node, old, paravirt))
> -- 
> 2.37.2