Subject: [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath

From: [email-masked] (Peter Zijlstra)
Date: Thu, 26 Apr 2018 17:53:35 +0200

On Thu, Apr 26, 2018 at 11:34:19AM +0100, Will Deacon wrote:
> @@ -290,58 +312,50 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  	}
>  
>  	/*
> +	 * If we observe any contention; queue.
> +	 */
> +	if (val & ~_Q_LOCKED_MASK)
> +		goto queue;
> +
> +	/*
>  	 * trylock || pending
>  	 *
>  	 * 0,0,0 -> 0,0,1 ; trylock
>  	 * 0,0,1 -> 0,1,1 ; pending
>  	 */
> +	val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
> +	if (!(val & ~_Q_LOCKED_MASK)) {
>  		/*
> +		 * we're pending, wait for the owner to go away.
> +		 *
> +		 * *,1,1 -> *,1,0

Tail must be 0 here, right?

> +		 *
> +		 * this wait loop must be a load-acquire such that we match the
> +		 * store-release that clears the locked bit and create lock
> +		 * sequentiality; this is because not all
> +		 * clear_pending_set_locked() implementations imply full
> +		 * barriers.
>  		 */
> +		if (val & _Q_LOCKED_MASK) {
> +			smp_cond_load_acquire(&lock->val.counter,
> +					      !(VAL & _Q_LOCKED_MASK));
> +		}
>  
>  		/*
> +		 * take ownership and clear the pending bit.
> +		 *
> +		 * *,1,0 -> *,0,1
>  		 */

Idem.

> +		clear_pending_set_locked(lock);
>  		return;
> +	}
>  
>  	/*
> +	 * If pending was clear but there are waiters in the queue, then
> +	 * we need to undo our setting of pending before we queue ourselves.
>  	 */
> +	if (!(val & _Q_PENDING_MASK))
> +		clear_pending(lock);

This is the branch for when we have !0 tail.

>  
>  	/*
>  	 * End of pending bit optimistic spinning and beginning of MCS

> @@ -445,15 +459,15 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  	 * claim the lock:
>  	 *
>  	 * n,0,0 -> 0,0,1 : lock, uncontended
> +	 * *,*,0 -> *,*,1 : lock, contended
>  	 *
> +	 * If the queue head is the only one in the queue (lock value == tail)
> +	 * and nobody is pending, clear the tail code and grab the lock.
> +	 * Otherwise, we only need to grab the lock.
>  	 */
>  	for (;;) {
>  		/* In the PV case we might already have _Q_LOCKED_VAL set */
> +		if ((val & _Q_TAIL_MASK) != tail || (val & _Q_PENDING_MASK)) {
>  			set_locked(lock);
>  			break;
>  		}

This one hunk is terrible on the brain. I'm fairly sure I get it, but I
feel that comment can use help. Or at least, I need help reading it.

I'll try and cook up something when my brain starts working again.