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Replace _Generic-based __unqual_scalar_typeof() with more complete __rwonce_typeof_unqual(). This strips qualifiers from all types, not just integer types, which is required to be able to assign (must be non-const) to __u.__val in the non-atomic case (required for #2). One subtle point here is that non-integer types of __val could be const or volatile within the union with the old __unqual_scalar_typeof(), if the passed variable is const or volatile. This would then result in a forced load from the stack if __u.__val is volatile; in the case of const, it does look odd if the underlying storage changes, but the compiler is told said member is "const" -- it smells like UB. 2. Eliminate the atomic flag and ternary conditional expression. Move the fallback volatile load into the default case of the switch, ensuring __u is unconditionally initialized across all paths. The statement expression now unconditionally returns __u.__val. This refactoring appears to help the compiler improve (or fix) code generation. With a defconfig + LTO + debug options builds, we observe different codegen for the following functions: btrfs_reclaim_sweep (708 -> 1032 bytes) btrfs_sinfo_bg_reclaim_threshold_store (200 -> 204 bytes) check_mem_access (3652 -> 3692 bytes) [inlined bpf_map_is_rdonly] console_flush_all (1268 -> 1264 bytes) console_lock_spinning_disable_and_check (180 -> 176 bytes) igb_add_filter (640 -> 636 bytes) igb_config_tx_modes (2404 -> 2400 bytes) kvm_vcpu_on_spin (480 -> 476 bytes) map_freeze (376 -> 380 bytes) netlink_bind (1664 -> 1656 bytes) nmi_cpu_backtrace (404 -> 400 bytes) set_rps_cpu (516 -> 520 bytes) swap_cluster_readahead (944 -> 932 bytes) tcp_accecn_third_ack (328 -> 336 bytes) tcp_create_openreq_child (1764 -> 1772 bytes) tcp_data_queue (5784 -> 5892 bytes) tcp_ecn_rcv_synack (620 -> 628 bytes) xen_manage_runstate_time (944 -> 896 bytes) xen_steal_clock (340 -> 296 bytes) Increase of some functions are due to more aggressive inlining due to better codegen (in this build, e.g. bpf_map_is_rdonly is no longer present due to being inlined completely). NOTE: The return-value-of-function-drops-qualifiers hack was first suggested by Al Viro in [1], which notes some of its limitations which make it unsuitable for a general __unqual_scalar_typeof() replacement. Notably, array types are not supported, and GCC 8.1-8.3 still fail. Why should we use it here? READ_ONCE() does not support reading whole arrays, and the GCC version problem only affects 3 minor releases of a very ancient still-supported GCC version; not only that, this arm64 READ_ONCE() version is currently only activated by LTO builds, which to-date are *only supported by Clang*! Link: https://lore.kernel.org/all/20260111182010.GH3634291@ZenIV/ [1] Signed-off-by: Marco Elver --- v4: * Use the return-value-of-function-drops-qualifiers hack to paint the bikeshed. v3: * Comment. v2: * Add __rwonce_typeof_unqual() as fallback for old compilers. --- arch/arm64/include/asm/rwonce.h | 18 ++++++++++++++---- 1 file changed, 14 insertions(+), 4 deletions(-) diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h index fc0fb42b0b64..9fd24cef3376 100644 --- a/arch/arm64/include/asm/rwonce.h +++ b/arch/arm64/include/asm/rwonce.h @@ -19,6 +19,17 @@ "ldapr" #sfx "\t" #regs, \ ARM64_HAS_LDAPR) +/* + * Replace this with typeof_unqual() when minimum compiler versions are + * increased to GCC 14 and Clang 19. For the time being, we need this + * workaround, which relies on function return values dropping qualifiers. + */ +#define __rwonce_typeof_unqual(x) typeof(({ \ + __diag_push() \ + __diag_ignore_all("-Wignored-qualifiers", "") \ + ((typeof(x)(*)(void))0)(); \ + __diag_pop() })) + /* * When building with LTO, there is an increased risk of the compiler * converting an address dependency headed by a READ_ONCE() invocation @@ -32,8 +43,7 @@ #define __READ_ONCE(x) \ ({ \ typeof(&(x)) __x = &(x); \ - int atomic = 1; \ - union { __unqual_scalar_typeof(*__x) __val; char __c[1]; } __u; \ + union { __rwonce_typeof_unqual(*__x) __val; char __c[1]; } __u; \ switch (sizeof(x)) { \ case 1: \ asm volatile(__LOAD_RCPC(b, %w0, %1) \ @@ -56,9 +66,9 @@ : "Q" (*__x) : "memory"); \ break; \ default: \ - atomic = 0; \ + __u.__val = *(volatile typeof(*__x) *)__x; \ } \ - atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(*__x) *)__x);\ + __u.__val; \ }) #endif /* !BUILD_VDSO */ -- 2.53.0.335.g19a08e0c02-goog