[dpdk-dev] [PATCH] ring: assert on zero objects dequeue/enqueue
Bruce Richardson
bruce.richardson at intel.com
Wed Mar 16 12:21:14 CET 2016
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 at 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
>
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