Re: [PATCH 1/1] vmalloc: purge_fragmented_blocks: Acquire spinlock before reading vmap_block

[David Rientjes <rientjes@google.com>]

On Thu, 8 Dec 2011, Kautuk Consul wrote:

> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 3231bf3..2228971 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -855,11 +855,14 @@ static void purge_fragmented_blocks(int cpu)
>  
>  	rcu_read_lock();
>  	list_for_each_entry_rcu(vb, &vbq->free, free_list) {
> +		spin_lock(&vb->lock);
>  
> -		if (!(vb->free + vb->dirty == VMAP_BBMAP_BITS && vb->dirty != VMAP_BBMAP_BITS))
> +		if (!(vb->free + vb->dirty == VMAP_BBMAP_BITS &&
> +			  vb->dirty != VMAP_BBMAP_BITS)) {
> +			spin_unlock(&vb->lock);
>  			continue;
> +		}
>  
> -		spin_lock(&vb->lock);
>  		if (vb->free + vb->dirty == VMAP_BBMAP_BITS && vb->dirty != VMAP_BBMAP_BITS) {
>  			vb->free = 0; /* prevent further allocs after releasing lock */
>  			vb->dirty = VMAP_BBMAP_BITS; /* prevent purging it again */

Nack, this is wrong because the if-clause you're modifying isn't the 
criteria that is used to determine whether the purge occurs or not.  It's 
merely an optimization to prevent doing exactly what your patch is doing: 
taking vb->lock unnecessarily.

In the original code, if the if-clause fails, the lock is only then taken 
and the exact same test occurs again while protected.  If the test now 
fails, the lock is immediately dropped.  A branch here is faster than a 
contented spinlock.

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