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Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1p7eFb-001u7k-I3; Tue, 20 Dec 2022 15:09:52 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 3BE3F300193; Tue, 20 Dec 2022 16:09:38 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 0457E223694AF; Tue, 20 Dec 2022 16:09:37 +0100 (CET) Date: Tue, 20 Dec 2022 16:09:37 +0100 From: Peter Zijlstra To: Linus Torvalds Cc: Boqun Feng , corbet@lwn.net, will@kernel.org, mark.rutland@arm.com, catalin.marinas@arm.com, dennis@kernel.org, tj@kernel.org, cl@linux.com, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, Herbert Xu , davem@davemloft.net, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, joro@8bytes.org, suravee.suthikulpanit@amd.com, robin.murphy@arm.com, dwmw2@infradead.org, baolu.lu@linux.intel.com, Arnd Bergmann , penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com, Andrew Morton , vbabka@suse.cz, roman.gushchin@linux.dev, 42.hyeyoo@gmail.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-s390@vger.kernel.org, linux-crypto@vger.kernel.org, iommu@lists.linux.dev, linux-arch@vger.kernel.org Subject: Re: [RFC][PATCH 05/12] arch: Introduce arch_{,try_}_cmpxchg128{,_local}() Message-ID: References: <20221219153525.632521981@infradead.org> <20221219154119.154045458@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: CC7E01C0024 X-Stat-Signature: jiwcysh67moxpa3ygj74scfp3pgcmcxx X-HE-Tag: 1671549004-393552 X-HE-Meta: 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 bgLK2b6t R1mFF X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Dec 20, 2022 at 08:31:19AM -0600, Linus Torvalds wrote: > On Tue, Dec 20, 2022 at 5:09 AM Peter Zijlstra wrote: > > > > On Mon, Dec 19, 2022 at 12:07:25PM -0800, Boqun Feng wrote: > > > > > > I wonder whether we should use "(*(u128 *)ptr)" instead of "(*(unsigned > > > long *) ptr)"? Because compilers may think only 64bit value pointed by > > > "ptr" gets modified, and they are allowed to do "useful" optimization. > > > > In this I've copied the existing cmpxchg_double() code; I'll have to let > > the arch folks speak here, I've no clue. > > It does sound like the right thing to do. I doubt it ends up making a > difference in practice, but yes, the asm doesn't have a memory > clobber, so the input/output types should be the right ones for the > compiler to not possibly do something odd and cache the part that it > doesn't see as being accessed. Right, and x86 does just *ptr, without trying to cast away the volatile even. I've pushed out a *(u128 *)ptr variant for arm64 and s390, then at least we'll know if the compiler objects.