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CTRY:IE; LANG:en; SCL:1; SRV:; IPV:CAL; SFV:NSPM; H:64aa7808-outbound-1.mta.getcheckrecipient.com; PTR:ec2-63-35-35-123.eu-west-1.compute.amazonaws.com; CAT:NONE; SFS:(13230040)(35042699022)(376014)(1800799024)(82310400026)(36860700013); DIR:OUT; SFP:1101; X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 09 Oct 2024 17:27:10.7906 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 49ac1c69-c5df-4a2b-1074-08dce8879b90 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d; Ip=[63.35.35.123]; Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-AuthSource: DU2PEPF00028D0C.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB4PR08MB9215 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 > > > > > > 1. rte_ring_generic_pvt.h: > > > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > > > > > > > > pseudo-c-code // relate= d armv8 instructions > > > > -------------------- = -------------------------------------- > > > > head.load() // ldr= [head] > > > > rte_smp_rmb() // dmb ish= ld > > > > opposite_tail.load() // ldr [opp= osite_tail] > > > > ... > > > > rte_atomic32_cmpset(head, ...) // ldrex[head];... stle= x[head] > > > > > > > > > > > > 2. rte_ring_c11_pvt.h > > > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > > > > > > > > pseudo-c-code // relat= ed armv8 instructions > > > > -------------------- = -------------------------------------- > > > > head.atomic_load(relaxed) // ldr[head] > > > > atomic_thread_fence(acquire) // dmb ish > > > > opposite_tail.atomic_load(acquire) // lda[opposite_tail] > > > > ... > > > > head.atomic_cas(..., relaxed) // ldrex[haed]; .= .. strex[head] > > > > > > > > > > > > 3. rte_ring_hts_elem_pvt.h > > > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D > > > > > > > > pseudo-c-code // relat= ed armv8 instructions > > > > -------------------- = -------------------------------------- > > > > head.atomic_load(acquire) // lda [head] > > > > opposite_tail.load() // ldr [opp= osite_tail] > > > > ... > > > > head.atomic_cas(..., acquire) // ldaex[head]; ..= . strex[head] > > > > > > > > The questions that arose from these observations: > > > > a) are all 3 approaches equivalent in terms of functionality? > > > Different, lda (Load with acquire semantics) and ldr (load) are diffe= rent. > > > > I understand that, my question was: > > lda {head]; ldr[tail] > > vs > > ldr [head]; dmb ishld; ldr [tail]; > > > > Is there any difference in terms of functionality (memory ops > ordering/observability)? >=20 > To be more precise: >=20 > lda {head]; ldr[tail] > vs > ldr [head]; dmb ishld; ldr [tail]; > vs > ldr [head]; dmb ishld; lda [tail]; >=20 > what would be the difference between these 3 cases? Case A: lda {head]; ldr[tail] load of the head will be observed by the memory subsystem before the load of the tail. Case B: ldr [head]; dmb ishld; ldr [tail]; load of the head will be observed by the memory subsystem Before the load of the tail. Case C: ldr [head]; dmb ishld; lda [tail]; load of the head will be observed by the memory subsystem before the load of the tail. In addition, any load or store program order after lda[tail] will not be observed by the memory subsystem before the load of the tail. Essentially both cases A and B are the same. They preserve following program orders. LOAD-LOAD LOAD-STORE