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Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kZvto-0001D9-F5; Tue, 03 Nov 2020 12:58:56 +0000 Received: from foss.arm.com ([217.140.110.172]) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kZvtj-0001B7-KL for linux-arm-kernel@lists.infradead.org; Tue, 03 Nov 2020 12:58:52 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 44276106F; Tue, 3 Nov 2020 04:58:50 -0800 (PST) Received: from C02TD0UTHF1T.local (unknown [10.57.57.89]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6C3B13F718; Tue, 3 Nov 2020 04:58:48 -0800 (PST) Date: Tue, 3 Nov 2020 12:58:45 +0000 From: Mark Rutland To: Will Deacon Subject: Re: [PATCH v4 4/4] arm64: lto: Strengthen READ_ONCE() to acquire when CONFIG_LTO=y Message-ID: <20201103125845.GD40454@C02TD0UTHF1T.local> References: <20201103121721.5166-1-will@kernel.org> <20201103121721.5166-5-will@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20201103121721.5166-5-will@kernel.org> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201103_075851_791615_21635A61 X-CRM114-Status: GOOD ( 14.21 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kees Cook , Peter Zijlstra , Catalin Marinas , Masahiro Yamada , linux-kernel@vger.kernel.org, Sami Tolvanen , linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.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) > Signed-off-by: Will Deacon [...] 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 > +#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);\ > +}) _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel