From: Mark Rutland <mark.rutland@arm.com>
To: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org,
Kees Cook <keescook@chromium.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Sami Tolvanen <samitolvanen@google.com>,
Masahiro Yamada <masahiroy@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 4/4] arm64: lto: Strengthen READ_ONCE() to acquire when CONFIG_LTO=y
Date: Tue, 3 Nov 2020 12:58:45 +0000 [thread overview]
Message-ID: <20201103125845.GD40454@C02TD0UTHF1T.local> (raw)
In-Reply-To: <20201103121721.5166-5-will@kernel.org>
On Tue, Nov 03, 2020 at 12:17:21PM +0000, Will Deacon wrote:
> When building with LTO, there is an increased risk of the compiler
> converting an address dependency headed by a READ_ONCE() invocation
> into a control dependency and consequently allowing for harmful
> reordering by the CPU.
>
> Ensure that such transformations are harmless by overriding the generic
> READ_ONCE() definition with one that provides acquire semantics when
> building with LTO.
>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Will Deacon <will@kernel.org>
[...]
Could we add a note above __READ_ONCE() along the lines of the commit
message, e.g.
/*
* With LTO a compiler might convert an address dependency headed by a
* READ_ONCE() into a control dependency, allowing for harmful
* reordering by the CPU.
*
* To prevent this, upgrade READ_OONCE() to provide acquire semantics
* when building with LTO.
*/
Either way:
Acked-by: Mark Rutland <mark.rutland@arm.com>
Mark
> +#define __READ_ONCE(x) \
> +({ \
> + typeof(&(x)) __x = &(x); \
> + int atomic = 1; \
> + union { __unqual_scalar_typeof(*__x) __val; char __c[1]; } __u; \
> + switch (sizeof(x)) { \
> + case 1: \
> + asm volatile(__LOAD_RCPC(b, %w0, %1) \
> + : "=r" (*(__u8 *)__u.__c) \
> + : "Q" (*__x) : "memory"); \
> + break; \
> + case 2: \
> + asm volatile(__LOAD_RCPC(h, %w0, %1) \
> + : "=r" (*(__u16 *)__u.__c) \
> + : "Q" (*__x) : "memory"); \
> + break; \
> + case 4: \
> + asm volatile(__LOAD_RCPC(, %w0, %1) \
> + : "=r" (*(__u32 *)__u.__c) \
> + : "Q" (*__x) : "memory"); \
> + break; \
> + case 8: \
> + asm volatile(__LOAD_RCPC(, %0, %1) \
> + : "=r" (*(__u64 *)__u.__c) \
> + : "Q" (*__x) : "memory"); \
> + break; \
> + default: \
> + atomic = 0; \
> + } \
> + atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(__x))__x);\
> +})
next prev parent reply other threads:[~2020-11-03 12:58 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-03 12:17 [PATCH v4 0/4] Upgrade READ_ONCE() to RCpc acquire on arm64 with LTO Will Deacon
2020-11-03 12:17 ` [PATCH v4 1/4] arm64: alternatives: Split up alternative.h Will Deacon
2020-11-03 12:40 ` Mark Rutland
2020-11-03 12:42 ` Will Deacon
2020-11-03 12:17 ` [PATCH v4 2/4] arm64: cpufeatures: Add capability for LDAPR instruction Will Deacon
2020-11-03 12:44 ` Mark Rutland
2020-11-03 12:17 ` [PATCH v4 3/4] arm64: alternatives: Remove READ_ONCE() usage during patch operation Will Deacon
2020-11-03 12:46 ` Mark Rutland
2020-11-03 12:17 ` [PATCH v4 4/4] arm64: lto: Strengthen READ_ONCE() to acquire when CONFIG_LTO=y Will Deacon
2020-11-03 12:58 ` Mark Rutland [this message]
2020-11-03 13:13 ` Will Deacon
2020-11-09 23:25 ` [PATCH v4 0/4] Upgrade READ_ONCE() to RCpc acquire on arm64 with LTO Will Deacon
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