* [PATCH v2 1/3] arm64: Fix non-atomic __READ_ONCE() with CONFIG_LTO=y [not found] <20260129005645.747680-1-elver@google.com> @ 2026-01-29 0:52 ` Marco Elver 2026-01-29 1:21 ` Boqun Feng 0 siblings, 1 reply; 3+ messages in thread From: Marco Elver @ 2026-01-29 0:52 UTC (permalink / raw) To: elver, Peter Zijlstra, Will Deacon Cc: Ingo Molnar, Thomas Gleixner, Boqun Feng, Waiman Long, Bart Van Assche, llvm, Catalin Marinas, Arnd Bergmann, linux-arm-kernel, linux-kernel, stable The implementation of __READ_ONCE() under CONFIG_LTO=y incorrectly qualified the fallback "once" access for types larger than 8 bytes, which are not atomic but should still happen "once" and suppress common compiler optimizations. The cast `volatile typeof(__x)` applied the volatile qualifier to the pointer type itself rather than the pointee. This created a volatile pointer to a non-volatile type, which violated __READ_ONCE() semantics. Fix this by casting to `volatile typeof(*__x) *`. With a defconfig + LTO + debug options build, we see the following functions to be affected: xen_manage_runstate_time (884 -> 944 bytes) xen_steal_clock (248 -> 340 bytes) ^-- use __READ_ONCE() to load vcpu_runstate_info structs Fixes: e35123d83ee3 ("arm64: lto: Strengthen READ_ONCE() to acquire when CONFIG_LTO=y") Cc: <stable@vger.kernel.org> Signed-off-by: Marco Elver <elver@google.com> --- arch/arm64/include/asm/rwonce.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h index 78beceec10cd..fc0fb42b0b64 100644 --- a/arch/arm64/include/asm/rwonce.h +++ b/arch/arm64/include/asm/rwonce.h @@ -58,7 +58,7 @@ default: \ atomic = 0; \ } \ - atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(__x))__x);\ + atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(*__x) *)__x);\ }) #endif /* !BUILD_VDSO */ -- 2.53.0.rc1.217.geba53bf80e-goog ^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH v2 1/3] arm64: Fix non-atomic __READ_ONCE() with CONFIG_LTO=y 2026-01-29 0:52 ` [PATCH v2 1/3] arm64: Fix non-atomic __READ_ONCE() with CONFIG_LTO=y Marco Elver @ 2026-01-29 1:21 ` Boqun Feng 2026-01-29 1:32 ` Marco Elver 0 siblings, 1 reply; 3+ messages in thread From: Boqun Feng @ 2026-01-29 1:21 UTC (permalink / raw) To: Marco Elver Cc: Peter Zijlstra, Will Deacon, Ingo Molnar, Thomas Gleixner, Boqun Feng, Waiman Long, Bart Van Assche, llvm, Catalin Marinas, Arnd Bergmann, linux-arm-kernel, linux-kernel, stable On Thu, Jan 29, 2026 at 01:52:32AM +0100, Marco Elver wrote: > The implementation of __READ_ONCE() under CONFIG_LTO=y incorrectly > qualified the fallback "once" access for types larger than 8 bytes, > which are not atomic but should still happen "once" and suppress common > compiler optimizations. > > The cast `volatile typeof(__x)` applied the volatile qualifier to the > pointer type itself rather than the pointee. This created a volatile > pointer to a non-volatile type, which violated __READ_ONCE() semantics. > > Fix this by casting to `volatile typeof(*__x) *`. I guess a `volatile typeof(x) *` also works. Either way, good catch! Reviewed-by: Boqun Feng <boqun@kernel.org> Regards, Boqun > > With a defconfig + LTO + debug options build, we see the following > functions to be affected: > > xen_manage_runstate_time (884 -> 944 bytes) > xen_steal_clock (248 -> 340 bytes) > ^-- use __READ_ONCE() to load vcpu_runstate_info structs > > Fixes: e35123d83ee3 ("arm64: lto: Strengthen READ_ONCE() to acquire when CONFIG_LTO=y") > Cc: <stable@vger.kernel.org> > Signed-off-by: Marco Elver <elver@google.com> > --- > arch/arm64/include/asm/rwonce.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h > index 78beceec10cd..fc0fb42b0b64 100644 > --- a/arch/arm64/include/asm/rwonce.h > +++ b/arch/arm64/include/asm/rwonce.h > @@ -58,7 +58,7 @@ > default: \ > atomic = 0; \ > } \ > - atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(__x))__x);\ > + atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(*__x) *)__x);\ > }) > > #endif /* !BUILD_VDSO */ > -- > 2.53.0.rc1.217.geba53bf80e-goog > ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH v2 1/3] arm64: Fix non-atomic __READ_ONCE() with CONFIG_LTO=y 2026-01-29 1:21 ` Boqun Feng @ 2026-01-29 1:32 ` Marco Elver 0 siblings, 0 replies; 3+ messages in thread From: Marco Elver @ 2026-01-29 1:32 UTC (permalink / raw) To: Boqun Feng Cc: Peter Zijlstra, Will Deacon, Ingo Molnar, Thomas Gleixner, Boqun Feng, Waiman Long, Bart Van Assche, llvm, Catalin Marinas, Arnd Bergmann, linux-arm-kernel, linux-kernel, stable On Thu, 29 Jan 2026 at 02:21, Boqun Feng <boqun@kernel.org> wrote: > > On Thu, Jan 29, 2026 at 01:52:32AM +0100, Marco Elver wrote: > > The implementation of __READ_ONCE() under CONFIG_LTO=y incorrectly > > qualified the fallback "once" access for types larger than 8 bytes, > > which are not atomic but should still happen "once" and suppress common > > compiler optimizations. > > > > The cast `volatile typeof(__x)` applied the volatile qualifier to the > > pointer type itself rather than the pointee. This created a volatile > > pointer to a non-volatile type, which violated __READ_ONCE() semantics. > > > > Fix this by casting to `volatile typeof(*__x) *`. > > I guess a `volatile typeof(x) *` also works. Either way, good catch! x might expand to some big expression, so better to refer to it only once, and then use the same-typed *__x as a proxy. Semantically the same, but compile-times ought to be better this way. > Reviewed-by: Boqun Feng <boqun@kernel.org> Thanks! ^ permalink raw reply [flat|nested] 3+ messages in thread
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[not found] <20260129005645.747680-1-elver@google.com>
2026-01-29 0:52 ` [PATCH v2 1/3] arm64: Fix non-atomic __READ_ONCE() with CONFIG_LTO=y Marco Elver
2026-01-29 1:21 ` Boqun Feng
2026-01-29 1:32 ` Marco Elver
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