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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 02 Jul 2025 16:35:39.3843 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 74a0b954-4642-418a-7f03-08ddb9867ae1 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DB5PEPF00014B9B.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DU0PR08MB8208 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250702_093545_779664_C5D5B5F9 X-CRM114-Status: GOOD ( 20.64 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi, > > On Wed, Jul 02, 2025 at 10:49:01AM +0100, Yeoreum Yun wrote: > > Hi Leo, > > > > > { > > > - return local_cmpxchg(&csdev->mode, CS_MODE_DISABLED, new_mode) == > > > - CS_MODE_DISABLED; > > > + int curr = CS_MODE_DISABLED; > > > + > > > + return atomic_try_cmpxchg_acquire(&csdev->mode, &curr, new_mode); > > > } > > > > Just question. why is acquire symentic enough in here? > > My understanding is that acquire semantics ensure ordering between > cmpxchg_acquire() and all memory accesses that follow it. However, it > does not guarantee that memory accesses appearing before the acquire > are ordered as well. > > This is exactly what we want in the driver. We must ensure to first grab > an active device mode, then it is safe to proceed later operations (e.g. > set configurations in driver data and access registers). > > > before this change, local_cmpxchg seems to use full_fenced. > > Not really. Arm64 has atomic instruction for cmpxchg, it does not use > full_fenced. It should run into the path of arch_cmpxchg(). No, It local_cmpxchg backend with atomic_cmpxchg() and it seems include full fenced. But, you've explained the what i concerned with below diagram clear CPU0 CPU1 // store csmode in store buffer atomic_cmpxchg_acquired(csmode) // couldn't see the contents in store buffer atomic_read_acquire(csmode); Send SMP call ,-------------------------------/ v atomic_cmpxchg_acquired(csmode) ==> Fail to grab another mode However, I think it would be good to change the atomic_try_cmpxchg() instead of atomic_cmpxchg_acquired() to prevent redundnat IPI trigger for observation failure. Thanks. -- Sincerely, Yeoreum Yun