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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 17 Sep 2025 13:36:05.9921 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: a1ed192a-cbd7-49f6-a828-08ddf5ef273e 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: DB1PEPF000509F5.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AS2PR08MB10265 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250917_063617_914896_CDE900C4 X-CRM114-Status: GOOD ( 19.31 ) 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 Mark, > On Wed, Sep 17, 2025 at 12:08:38PM +0100, Yeoreum Yun wrote: > > +static __always_inline int > > +__lsui_cmpxchg64(u64 __user *uaddr, u64 *oldval, u64 newval) > > +{ > > + int ret = 0; > > + > > + asm volatile("// __lsui_cmpxchg64\n" > > + __LSUI_PREAMBLE > > +"1: casalt %x2, %x3, %1\n" > > +"2:\n" > > + _ASM_EXTABLE_UACCESS_ERR(1b, 2b, %w0) > > + : "+r" (ret), "+Q" (*uaddr), "+r" (*oldval) > > + : "r" (newval) > > + : "memory"); > > + > > + return ret; > > +} > > + > > +static __always_inline int > > +__lsui_cmpxchg32(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval) > > +{ > > + u64 __user *uaddr_al; > > Please use 'uaddr64' to match the other 64-bit variables. > > I assume that the '_al' suffix is meant to be short for 'aligned', but I > think using '64' is more consistent and clearer. > > > + u64 oval64, nval64, tmp; > > Likewise, 'orig64' would be clearer than 'tmp' here. Thanks for your suggestion. > > > + static const u64 hi_mask = IS_ENABLED(CONFIG_CPU_LITTLE_ENDIAN) ? > > + GENMASK_U64(63, 32): GENMASK_U64(31, 0); > > + static const u8 hi_shift = IS_ENABLED(CONFIG_CPU_LITTLE_ENDIAN) ? 32 : 0; > > + static const u8 lo_shift = IS_ENABLED(CONFIG_CPU_LITTLE_ENDIAN) ? 0 : 32; > > + > > + uaddr_al = (u64 __user *) PTR_ALIGN_DOWN(uaddr, sizeof(u64)); > > + if (get_user(oval64, uaddr_al)) > > + return -EFAULT; > > + > > + if ((u32 __user *)uaddr_al != uaddr) { > > + nval64 = ((oval64 & ~hi_mask) | ((u64)newval << hi_shift)); > > + oval64 = ((oval64 & ~hi_mask) | ((u64)oldval << hi_shift)); > > + } else { > > + nval64 = ((oval64 & hi_mask) | ((u64)newval << lo_shift)); > > + oval64 = ((oval64 & hi_mask) | ((u64)oldval << lo_shift)); > > + } > > + > > + tmp = oval64; > > + > > + if (__lsui_cmpxchg64(uaddr_al, &oval64, nval64)) > > + return -EFAULT; > > + > > + if (tmp != oval64) > > + return -EAGAIN; > > This means that we'll immediately return -EAGAIN upon a spurious failure > (where the adjacent 4 bytes have changed), whereas the LL/SC ops would > retry FUTEX_MAX_LOOPS before returning -EGAIN. > > I suspect we want to retry here (or in the immediate caller). Right. I've thought about it but at the time of writing, I return -EAGAIN immediately. Let's wait for other people's comments. > > > + > > + *oval = oldval; > > + > > + return 0; > > +} > > Aside from the retry issue, I *think* you can simplify this to something > like: > > static __always_inline int > __lsui_cmpxchg32(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval) > { > uaddr64 = (u64 __user *)PTR_ALIGN_DOWN(uaddr, sizeof(u64)); > u64 oval64, nval64, orig64; > > if (get_user(oval64, uaddr64) > return -EFAULT; > > if (IS_ALIGNED(addr, sizeof(u64)) == IS_ENABLED(CONFIG_CPU_LITTLE_ENDIAN)) { > FIELD_MODIFY(GENMASK_U64(31, 0), &oval64, oldval); > FIELD_MODIFY(GENMASK_U64(31, 0), &nval64, newval); > } else { > FIELD_MODIFY(GENMASK_U64(63, 32), &oval64, oldval); > FIELD_MODIFY(GENMASK_U64(63, 32), &nval64, newval); > } > orig64 = oval64; > > if (__lsui_cmpxchg64(uaddr_al, &oval64, nval64)) > return -EFAULT; > > if (oval64 != orig64) > return -EAGAIN; > > *oval = oldval; > return 0; > } Hmm I think this wouldn'b cover the case below when big-endianess used. struct { u32 others 0x55667788; u32 futex = 0x11223344; }; In this case, memory layout would be: 55 66 77 88 11 22 33 44 So, the value of fetched oval64 is 0x5566778811223344; So, it should modify the GENMASK_U64(31, 0) fields. But, it tries to modify GENMASK_U64(63, 32) fields. Thanks! [...] -- Sincerely, Yeoreum Yun