From: "Morten Brørup" <mb@smartsharesystems.com>
To: "Andrzej Ostruszka" <amo@semihalf.com>,
"Honnappa Nagarahalli" <honnappa.nagarahalli@arm.com>,
"Konstantin Ananyev" <konstantin.ananyev@intel.com>
Cc: <upstream@semihalf.com>, <dev@dpdk.org>
Subject: RE: [PATCH 1/1] ring: fix off by 1 mistake
Date: Mon, 3 Jan 2022 15:56:33 +0100 [thread overview]
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35D86DCF@smartserver.smartshare.dk> (raw)
In-Reply-To: <20220103142201.475552-2-amo@semihalf.com>
+Ring queue maintainers: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>, Konstantin Ananyev <konstantin.ananyev@intel.com>
> From: Andrzej Ostruszka [mailto:amo@semihalf.com]
> Sent: Monday, 3 January 2022 15.22
>
> When enqueueing/dequeueing to/from the ring we try to optimize by
> manual
> loop unrolling. The check for this optimization looks like:
>
> if (likely(idx + n < size)) {
>
> where 'idx' points to the first usable element (empty slot for enqueue,
> data for dequeue). The correct comparison here should be '<=' instead
> of '<'.
>
> This is not a functional error since we fall back to the loop with
> correct checks on indexes. Just a minor suboptimal behaviour for the
> case when we want to enqueue/dequeue exactly the number of elements
> that
> we have in the ring before wrapping to its beginning.
>
> Signed-off-by: Andrzej Ostruszka <amo@semihalf.com>
> ---
> lib/ring/rte_ring_elem_pvt.h | 12 ++++++------
> 1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/lib/ring/rte_ring_elem_pvt.h
> b/lib/ring/rte_ring_elem_pvt.h
> index 275ec55393..83788c56e6 100644
> --- a/lib/ring/rte_ring_elem_pvt.h
> +++ b/lib/ring/rte_ring_elem_pvt.h
> @@ -17,7 +17,7 @@ __rte_ring_enqueue_elems_32(struct rte_ring *r, const
> uint32_t size,
> unsigned int i;
> uint32_t *ring = (uint32_t *)&r[1];
> const uint32_t *obj = (const uint32_t *)obj_table;
> - if (likely(idx + n < size)) {
> + if (likely(idx + n <= size)) {
> for (i = 0; i < (n & ~0x7); i += 8, idx += 8) {
> ring[idx] = obj[i];
> ring[idx + 1] = obj[i + 1];
> @@ -62,7 +62,7 @@ __rte_ring_enqueue_elems_64(struct rte_ring *r,
> uint32_t prod_head,
> uint32_t idx = prod_head & r->mask;
> uint64_t *ring = (uint64_t *)&r[1];
> const unaligned_uint64_t *obj = (const unaligned_uint64_t
> *)obj_table;
> - if (likely(idx + n < size)) {
> + if (likely(idx + n <= size)) {
> for (i = 0; i < (n & ~0x3); i += 4, idx += 4) {
> ring[idx] = obj[i];
> ring[idx + 1] = obj[i + 1];
> @@ -95,7 +95,7 @@ __rte_ring_enqueue_elems_128(struct rte_ring *r,
> uint32_t prod_head,
> uint32_t idx = prod_head & r->mask;
> rte_int128_t *ring = (rte_int128_t *)&r[1];
> const rte_int128_t *obj = (const rte_int128_t *)obj_table;
> - if (likely(idx + n < size)) {
> + if (likely(idx + n <= size)) {
> for (i = 0; i < (n & ~0x1); i += 2, idx += 2)
> memcpy((void *)(ring + idx),
> (const void *)(obj + i), 32);
> @@ -151,7 +151,7 @@ __rte_ring_dequeue_elems_32(struct rte_ring *r,
> const uint32_t size,
> unsigned int i;
> uint32_t *ring = (uint32_t *)&r[1];
> uint32_t *obj = (uint32_t *)obj_table;
> - if (likely(idx + n < size)) {
> + if (likely(idx + n <= size)) {
> for (i = 0; i < (n & ~0x7); i += 8, idx += 8) {
> obj[i] = ring[idx];
> obj[i + 1] = ring[idx + 1];
> @@ -196,7 +196,7 @@ __rte_ring_dequeue_elems_64(struct rte_ring *r,
> uint32_t prod_head,
> uint32_t idx = prod_head & r->mask;
> uint64_t *ring = (uint64_t *)&r[1];
> unaligned_uint64_t *obj = (unaligned_uint64_t *)obj_table;
> - if (likely(idx + n < size)) {
> + if (likely(idx + n <= size)) {
> for (i = 0; i < (n & ~0x3); i += 4, idx += 4) {
> obj[i] = ring[idx];
> obj[i + 1] = ring[idx + 1];
> @@ -229,7 +229,7 @@ __rte_ring_dequeue_elems_128(struct rte_ring *r,
> uint32_t prod_head,
> uint32_t idx = prod_head & r->mask;
> rte_int128_t *ring = (rte_int128_t *)&r[1];
> rte_int128_t *obj = (rte_int128_t *)obj_table;
> - if (likely(idx + n < size)) {
> + if (likely(idx + n <= size)) {
> for (i = 0; i < (n & ~0x1); i += 2, idx += 2)
> memcpy((void *)(obj + i), (void *)(ring + idx), 32);
> switch (n & 0x1) {
> --
> 2.34.1.448.ga2b2bfdf31-goog
>
Well spotted! I took a very good look at it, and came to the same conclusion: It not a functional bug; the only consequence is that the optimized code path may not be taken in a situation where it could be taken. But it should be fixed as suggested in your patch.
Reviewed-by: Morten Brørup <mb@smartsharesystems.com>
next prev parent reply other threads:[~2022-01-03 14:56 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-03 14:22 [PATCH 0/1] Minor mistake in ring (en|de)queueing Andrzej Ostruszka
2022-01-03 14:22 ` [PATCH 1/1] ring: fix off by 1 mistake Andrzej Ostruszka
2022-01-03 14:56 ` Morten Brørup [this message]
2022-01-06 10:45 ` Olivier Matz
2022-01-10 15:09 ` Ananyev, Konstantin
2022-01-11 11:49 ` Andrzej Ostruszka
2022-01-11 11:37 ` [PATCH] ring: optimize corner case for enqueue/dequeue Andrzej Ostruszka
2022-01-11 12:00 ` Morten Brørup
2022-01-11 13:46 ` Andrzej Ostruszka
2022-01-11 13:53 ` Morten Brørup
2022-02-05 16:48 ` David Marchand
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