Re: [PATCH 1/2] qspinlock: Ensure writes are pushed out of core write buffer

[Peter Zijlstra]

On Wed, Jan 27, 2021 at 09:01:08PM +0100, Alexander A Sverdlin wrote:
> From: Alexander Sverdlin <alexander.sverdlin@xxxxxxxxx>
> 
> Ensure writes are pushed out of core write buffer to prevent waiting code
> on another cores from spinning longer than necessary.

Our smp_wmb() as defined does not have that property. You're relying on
some arch specific details which do not belong in common code.


> diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
> index 5e10153..10e497a 100644
> --- a/kernel/locking/mcs_spinlock.h
> +++ b/kernel/locking/mcs_spinlock.h
> @@ -89,6 +89,11 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
>  		return;
>  	}
>  	WRITE_ONCE(prev->next, node);
> +	/*
> +	 * This is necessary to make sure that the corresponding "while" in the
> +	 * mcs_spin_unlock() doesn't loop forever
> +	 */

This comment is utterly inadequate, since it does not describe an
explicit ordering between two (or more) stores.

> +	smp_wmb();
>  
>  	/* Wait until the lock holder passes the lock down. */
>  	arch_mcs_spin_lock_contended(&node->locked);
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index cbff6ba..577fe01 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -469,6 +469,12 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  
>  		/* Link @node into the waitqueue. */
>  		WRITE_ONCE(prev->next, node);
> +		/*
> +		 * This is necessary to make sure that the corresponding
> +		 * smp_cond_load_relaxed() below (running on another core)
> +		 * doesn't spin forever.
> +		 */
> +		smp_wmb();

That's insane, cache coherency should not allow that to happen in the
first place. Our smp_wmb() cannot help with that.

>  		pv_wait_node(node, prev);
>  		arch_mcs_spin_lock_contended(&node->locked);