Re: [PATCH v3 next 5/5] Avoid writing to node->next in the osq_lock() fast path

[David Laight <david.laight.linux@gmail.com>]

On Fri,  6 Mar 2026 22:51:50 +0000
david.laight.linux@gmail.com wrote:

> From: David Laight <david.laight.linux@gmail.com>
> 
> When osq_lock() returns false or osq_unlock() returns static
> analysis shows that node->next should always be NULL.
> This means that it isn't necessary to explicitly set it to NULL
> prior to atomic_xchg(&lock->tail, curr) on entry to osq_lock().
> 
> Defer determining the address of the CPU's 'node' until after the
> atomic_exchange() so that it isn't done in the uncontented path.
> 
> Signed-off-by: David Laight <david.laight.linux@gmail.com>
> ---
>  kernel/locking/osq_lock.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 0619691e2756..3f0cfdf1cd0f 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -92,13 +92,10 @@ osq_wait_next(struct optimistic_spin_queue *lock,
>  
>  bool osq_lock(struct optimistic_spin_queue *lock)
>  {
> -	struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
> -	struct optimistic_spin_node *prev, *next;
> +	struct optimistic_spin_node *node, *prev, *next;
>  	unsigned int curr = encode_cpu(smp_processor_id());
>  	unsigned int prev_cpu;
>  
> -	node->next = NULL;
> -
>  	/*
>  	 * We need both ACQUIRE (pairs with corresponding RELEASE in
>  	 * unlock() uncontended, or fastpath) and RELEASE (to publish
> @@ -109,6 +106,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>  	if (prev_cpu == OSQ_UNLOCKED_VAL)
>  		return true;
>  
> +	node = this_cpu_ptr(&osq_node);
>  	WRITE_ONCE(node->prev_cpu, prev_cpu);
>  	prev = decode_cpu(prev_cpu);
>  	node->locked = 0;