Re: [dpdk-dev] [PATCH] ring: assert on zero objects dequeue/enqueue

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Mar 15, 2016 at 06:58:45PM +0200, Lazaros Koromilas wrote:
> Issuing a zero objects dequeue with a single consumer has no effect.
> Doing so with multiple consumers, can get more than one thread to succeed
> the compare-and-set operation and observe starvation or even deadlock in
> the while loop that checks for preceding dequeues.  The problematic piece
> of code when n = 0:
> 
>     cons_next = cons_head + n;
>     success = rte_atomic32_cmpset(&r->cons.head, cons_head, cons_next);
> 
> The same is possible on the enqueue path.
> 
> Signed-off-by: Lazaros Koromilas <l@nofutznetworks.com>

I'm not sure how serious a problem this really is, and I really suspect that
just calling rte_panic is rather an overreaction here. At worst, this should
be a check only when RTE_RING_DEBUG is on.

However, probably my preferred solution to this issue would be to just add
   if (n == 0)
	   return 0

to the mp and mc enqueue/dequeue functions. That way there is no performance
penalty for the higher-performing sp/sc paths, and you avoid and unnecessary
cmpset operations for the mp/mc cases.

/Bruce

> ---
>  lib/librte_ring/rte_ring.h | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h
> index 943c97c..2bf9ce3 100644
> --- a/lib/librte_ring/rte_ring.h
> +++ b/lib/librte_ring/rte_ring.h
> @@ -100,6 +100,7 @@ extern "C" {
>  #include <rte_lcore.h>
>  #include <rte_atomic.h>
>  #include <rte_branch_prediction.h>
> +#include <rte_debug.h>
>  
>  #define RTE_TAILQ_RING_NAME "RTE_RING"
>  
> @@ -211,6 +212,19 @@ struct rte_ring {
>  #endif
>  
>  /**
> + * @internal Assert macro.
> + * @param exp
> + *   The expression to evaluate.
> + */
> +#define RTE_RING_ASSERT(exp) do {                                 \
> +		if (!(exp)) {                                     \
> +			rte_panic("line%d\t"                      \
> +			          "assert \"" #exp "\" failed\n", \
> +			          __LINE__);                      \
> +		}                                                 \
> +	} while (0)
> +
> +/**
>   * Calculate the memory size needed for a ring
>   *
>   * This function returns the number of bytes needed for a ring, given
> @@ -406,6 +420,7 @@ void rte_ring_dump(FILE *f, const struct rte_ring *r);
>   *   A pointer to a table of void * pointers (objects).
>   * @param n
>   *   The number of objects to add in the ring from the obj_table.
> + *   Must be greater than zero.
>   * @param behavior
>   *   RTE_RING_QUEUE_FIXED:    Enqueue a fixed number of items from a ring
>   *   RTE_RING_QUEUE_VARIABLE: Enqueue as many items a possible from ring
> @@ -431,6 +446,8 @@ __rte_ring_mp_do_enqueue(struct rte_ring *r, void * const *obj_table,
>  	uint32_t mask = r->prod.mask;
>  	int ret;
>  
> +	RTE_RING_ASSERT(n > 0);
> +
>  	/* move prod.head atomically */
>  	do {
>  		/* Reset n to the initial burst count */
> @@ -510,6 +527,7 @@ __rte_ring_mp_do_enqueue(struct rte_ring *r, void * const *obj_table,
>   *   A pointer to a table of void * pointers (objects).
>   * @param n
>   *   The number of objects to add in the ring from the obj_table.
> + *   Must be greater than zero.
>   * @param behavior
>   *   RTE_RING_QUEUE_FIXED:    Enqueue a fixed number of items from a ring
>   *   RTE_RING_QUEUE_VARIABLE: Enqueue as many items a possible from ring
> @@ -533,6 +551,8 @@ __rte_ring_sp_do_enqueue(struct rte_ring *r, void * const *obj_table,
>  	uint32_t mask = r->prod.mask;
>  	int ret;
>  
> +	RTE_RING_ASSERT(n > 0);
> +
>  	prod_head = r->prod.head;
>  	cons_tail = r->cons.tail;
>  	/* The subtraction is done between two unsigned 32bits value
> @@ -594,6 +614,7 @@ __rte_ring_sp_do_enqueue(struct rte_ring *r, void * const *obj_table,
>   *   A pointer to a table of void * pointers (objects) that will be filled.
>   * @param n
>   *   The number of objects to dequeue from the ring to the obj_table.
> + *   Must be greater than zero.
>   * @param behavior
>   *   RTE_RING_QUEUE_FIXED:    Dequeue a fixed number of items from a ring
>   *   RTE_RING_QUEUE_VARIABLE: Dequeue as many items a possible from ring
> @@ -618,6 +639,8 @@ __rte_ring_mc_do_dequeue(struct rte_ring *r, void **obj_table,
>  	unsigned i, rep = 0;
>  	uint32_t mask = r->prod.mask;
>  
> +	RTE_RING_ASSERT(n > 0);
> +
>  	/* move cons.head atomically */
>  	do {
>  		/* Restore n as it may change every loop */
> @@ -689,6 +712,7 @@ __rte_ring_mc_do_dequeue(struct rte_ring *r, void **obj_table,
>   *   A pointer to a table of void * pointers (objects) that will be filled.
>   * @param n
>   *   The number of objects to dequeue from the ring to the obj_table.
> + *   Must be greater than zero.
>   * @param behavior
>   *   RTE_RING_QUEUE_FIXED:    Dequeue a fixed number of items from a ring
>   *   RTE_RING_QUEUE_VARIABLE: Dequeue as many items a possible from ring
> @@ -710,6 +734,8 @@ __rte_ring_sc_do_dequeue(struct rte_ring *r, void **obj_table,
>  	unsigned i;
>  	uint32_t mask = r->prod.mask;
>  
> +	RTE_RING_ASSERT(n > 0);
> +
>  	cons_head = r->cons.head;
>  	prod_tail = r->prod.tail;
>  	/* The subtraction is done between two unsigned 32bits value
> -- 
> 1.9.1
>