ptr_ring_unconsume: memory corruption potential?

["Michael S. Tsirkin" <mst@redhat.com>]

Jason, guys,
reading ptr ring code, I noticed:


static inline void ptr_ring_unconsume(struct ptr_ring *r, void **batch, int n,
                                      void (*destroy)(void *))
{
        unsigned long flags;
        int head;
                
        spin_lock_irqsave(&r->consumer_lock, flags);
        spin_lock(&r->producer_lock);

        if (!r->size)
                goto done;
                
        /*
         * Clean out buffered entries (for simplicity). This way following code
         * can test entries for NULL and if not assume they are valid.
         */     
        head = r->consumer_head - 1;
        while (likely(head >= r->consumer_tail))
                r->queue[head--] = NULL;
                __ptr_ring_update(r, head);
        r->consumer_tail = r->consumer_head;


...



Does not look like this will DTRT if r->consumer_head == 0 .
In fact it looks like it will go off corrupting memory.

Why isn't this a concern?

-- 
MST