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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 15 Sep 2025 09:15:54.2855 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: c0b5bbd3-a3f6-411c-5197-08ddf438791e 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: AM3PEPF00009B9D.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB4PR08MB9141 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250915_021603_939731_E91C4A71 X-CRM114-Status: GOOD ( 21.22 ) 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 Thu, Sep 11, 2025 at 04:22:24PM +0100, Will Deacon wrote: > > On Sat, Aug 16, 2025 at 04:19:29PM +0100, Yeoreum Yun wrote: > > > +static __always_inline int > > > +__lsui_futex_atomic_eor(int oparg, u32 __user *uaddr, int *oval) > > > +{ > > > + unsigned int loops = LLSC_MAX_LOOPS; > > > + int ret, oldval, tmp; > > > + > > > + uaccess_ttbr0_enable(); > > > + /* > > > + * there are no ldteor/stteor instructions... > > > + */ > > > > *sigh* > > > > Were these new instructions not added with futex in mind? > > I guess it was _most_ of the futex. > > > I wonder whether CAS would be better than exclusives for xor... > > I was first thinking we could share some of the code with > __futex_cmpxchg() but... > > > > +static __always_inline int > > > +__lsui_futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval) > > > +{ > > > + int ret = 0; > > > + unsigned int loops = LLSC_MAX_LOOPS; > > > + u32 val, tmp; > > > + > > > + uaccess_ttbr0_enable(); > > > + /* > > > + * cas{al}t doesn't support word size... > > > + */ > > > > What about just aligning down and doing a 64-bit cas in that case? > > I think it gets more complicated. Here we get the oldval from the > caller, so no need to do a read. With CAS, we'd need to read the full > 64-bit, replace half of it with oldval and newval just to be able to do > the operation. On top of this, we need to check which half of the 64-bit > value. I think it to hairy for little benefit. Agree. also the unrelated to change for other 32 bit can make a failure futex atomic operation. So, I'll keep the llsc method even using lsui for cmpxchg and eor. Thanks! -- Sincerely, Yeoreum Yun