From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124]) by inbox.dpdk.org (Postfix) with ESMTP id C633E467B1; Wed, 21 May 2025 20:36:20 +0200 (CEST) Received: from mails.dpdk.org (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id 52BF742DDC; Wed, 21 May 2025 20:36:20 +0200 (CEST) Received: from dkmailrelay1.smartsharesystems.com (smartserver.smartsharesystems.com [77.243.40.215]) by mails.dpdk.org (Postfix) with ESMTP id B1C7C4065B for ; Wed, 21 May 2025 20:36:18 +0200 (CEST) Received: from smartserver.smartsharesystems.com (smartserver.smartsharesys.local [192.168.4.10]) by dkmailrelay1.smartsharesystems.com (Postfix) with ESMTP id 3F84B206C2; Wed, 21 May 2025 20:36:18 +0200 (CEST) Content-class: urn:content-classes:message MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Subject: RE: [PATCH v1 1/4] ring: introduce extra run-time checks X-MimeOLE: Produced By Microsoft Exchange V6.5 Date: Wed, 21 May 2025 20:36:15 +0200 Message-ID: <98CBD80474FA8B44BF855DF32C47DC35E9FC80@smartserver.smartshare.dk> In-Reply-To: <1e3bcd254b7d4aba8fced00d76b70cee@huawei.com> X-MS-Has-Attach: X-MS-TNEF-Correlator: Thread-Topic: [PATCH v1 1/4] ring: introduce extra run-time checks Thread-Index: AQHbykGpsQqK+1TaqEarZz25/dQkMLPc3a4AgAAkv/CAAGJtoA== References: <20250521111432.207936-1-konstantin.ananyev@huawei.com><20250521111432.207936-2-konstantin.ananyev@huawei.com> <98CBD80474FA8B44BF855DF32C47DC35E9FC7E@smartserver.smartshare.dk> <1e3bcd254b7d4aba8fced00d76b70cee@huawei.com> From: =?iso-8859-1?Q?Morten_Br=F8rup?= To: "Konstantin Ananyev" , Cc: , , , X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org > From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com] > Sent: Wednesday, 21 May 2025 14.35 >=20 > > > From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com] > > > Sent: Wednesday, 21 May 2025 13.14 > > > > > > Add RTE_ASSERT() to check that different move_tail() flavors > > > return meaningful *entries value. > > > It also helps to ensure that inside move_tail(), it uses correct > > > head/tail values. > > > > > > Signed-off-by: Konstantin Ananyev > > > --- > > > lib/ring/rte_ring_c11_pvt.h | 2 +- > > > lib/ring/rte_ring_elem_pvt.h | 8 ++++++-- > > > lib/ring/rte_ring_hts_elem_pvt.h | 8 ++++++-- > > > lib/ring/rte_ring_rts_elem_pvt.h | 8 ++++++-- > > > lib/ring/soring.c | 2 ++ > > > 5 files changed, 21 insertions(+), 7 deletions(-) > > > > > > diff --git a/lib/ring/rte_ring_c11_pvt.h > b/lib/ring/rte_ring_c11_pvt.h > > > index b9388af0da..0845cd6dcf 100644 > > > --- a/lib/ring/rte_ring_c11_pvt.h > > > +++ b/lib/ring/rte_ring_c11_pvt.h > > > @@ -104,10 +104,10 @@ __rte_ring_headtail_move_head(struct > > > rte_ring_headtail *d, > > > n =3D (behavior =3D=3D RTE_RING_QUEUE_FIXED) ? > > > 0 : *entries; > > > > > > + *new_head =3D *old_head + n; > > > if (n =3D=3D 0) > > > return 0; > > > > > > - *new_head =3D *old_head + n; > > > if (is_st) { > > > d->head =3D *new_head; > > > success =3D 1; > > > > Is there a need to assign a value to *new_head if n=3D=3D0? >=20 > Not really, main reason I just moved this line up - to keep compiler > happy. > Otherwise it complained that *new_head might be left uninitialized. Your change might give the impression that *new_head is used by a = caller. (Like I asked about.) To please the compiler, you could mark new_head __rte_unused, or: - if (n =3D=3D 0) + if (n =3D=3D 0) { + RTE_SET_USED(new_head); return 0; + } >=20 > > I don't think your suggestion is multi-thread safe. > > If d->head moves, the value in *new_head will be incorrect. >=20 > If d->head moves, then *old_head will also be incorrect. > For that case we do have CAS() below, it will return zero if (d->head > !=3D *old_head) > and we shall go to the next iteration (attempt). Exactly. And with my suggestion the same will happen if n=3D=3D0, and the next = attempt will update them both, until they are both correct. > Basically - if n =3D=3D 0, your *old_head and *new_head might be = invalid > and should not be used > (and they are not used). >=20 > > Instead, suggest: > > > > - if (n =3D=3D 0) > > - return 0; > > > > *new_head =3D *old_head + n; > > if (is_st) { > > d->head =3D *new_head; > > success =3D 1; > > } else > > /* on failure, *old_head is updated */ > > success =3D > rte_atomic_compare_exchange_strong_explicit( > > &d->head, old_head, *new_head, > > rte_memory_order_relaxed, > > rte_memory_order_relaxed); > > } while (unlikely(success =3D=3D 0)); >=20 > That's possible, but if (n =3D=3D0) we probably don't want to update = the > head - > as we are not moving head - it is pointless, while still expensive. Agree. Let's avoid unnecessary cost! My suggestion was only relevant if *new_head needed to be updated when = n=3D=3D0.