On Wed, Jan 16, 2019 at 06:48:41AM +0000, Cheng Fei Phung wrote:
> Hi,
>
> > https://gist.github.com/promach/65e9331d55a43a2815239430a28e29c6#file-circ_ring-c-L62
> > racy if there are multiple consumers.
> > just call ptr_ring_consume_any.
>
> If I modify pop_circ_queue() below to directly use ptr_ring_consume_any()
> without
> ptr_ring_empty_any() , I am afraid I am running into system crash due to NULL
> pointer
> deferencing for *item_pop:
>
> *item_pop = *((struct item *)ptr_ring_consume_any(buffer));
Yes - you want to check the returned pointer before dereferencing.
> Just one silly beginner question, why will ptr_ring_empty_any() lead to data
> race problem ?
because ring can become empty between the check and the call to consume?
>
> inline int pop_circ_queue(struct ptr_ring * buffer, struct item * item_pop)
> {
> DEBUG_MSG(KERN_INFO "Before pop, head = %u , tail = %u\n",
> buffer->consumer_head, buffer->consumer_tail);
>
> /* extract one item struct containing two unsigned integers from the
> buffer */
> *item_pop = *((struct item *)ptr_ring_consume_any(buffer));
>
> if((item_pop != NULL) && (item_pop->val1 > 0))
> {
> // val1 will never be zero since the event number starts from 1 (so,
> val1 in push_circ_queue() will not be zero, same case after
> pop_circ_queue()), and 0 is only possible during initialization, not during
> pop_circ_queue()
>
> DEBUG_MSG(KERN_INFO "val1 = %u , val2 = %u\n", item_pop->val1,
> item_pop->val2);
>
> DEBUG_MSG(KERN_INFO "After pop, head = %u , tail = %u\n",
> buffer->consumer_head, buffer->consumer_tail);
>
> return 0;
> }
>
> else {
> DEBUG_MSG(KERN_INFO "empty, nothing to pop from the ring\n");
>
> return 1;
> }
> }
>
>
>
> Regards,
> Phung
