From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f199.google.com (mail-pf1-f199.google.com [209.85.210.199]) by kanga.kvack.org (Postfix) with ESMTP id 9BA418E0001 for ; Fri, 11 Jan 2019 15:58:37 -0500 (EST) Received: by mail-pf1-f199.google.com with SMTP id u20so11245883pfa.1 for ; Fri, 11 Jan 2019 12:58:37 -0800 (PST) Received: from smtprelay.synopsys.com (smtprelay4.synopsys.com. [198.182.47.9]) by mx.google.com with ESMTPS id l7si15083723pgk.169.2019.01.11.12.58.36 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 11 Jan 2019 12:58:36 -0800 (PST) Subject: Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value References: <1547166387-19785-1-git-send-email-vgupta@synopsys.com> <1547166387-19785-4-git-send-email-vgupta@synopsys.com> <20190111092408.GM30894@hirez.programming.kicks-ass.net> From: Vineet Gupta Message-ID: Date: Fri, 11 Jan 2019 12:58:22 -0800 MIME-Version: 1.0 In-Reply-To: <20190111092408.GM30894@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra Cc: Mark Rutland , Miklos Szeredi , Jani Nikula , Will Deacon , linux-kernel@vger.kernel.org, Chris Wilson , linux-mm@kvack.org, Andrew Morton , linux-snps-arc@lists.infradead.org, Ingo Molnar On 1/11/19 1:24 AM, Peter Zijlstra wrote: > diff --git a/include/linux/bitops.h b/include/linux/bitops.h > index 705f7c442691..2060d26a35f5 100644 > --- a/include/linux/bitops.h > +++ b/include/linux/bitops.h > @@ -241,10 +241,10 @@ static __always_inline void __assign_bit(long nr, volatile unsigned long *addr, > const typeof(*(ptr)) mask__ = (mask), bits__ = (bits); \ > typeof(*(ptr)) old__, new__; \ > \ > + old__ = READ_ONCE(*(ptr)); \ > do { \ > - old__ = READ_ONCE(*(ptr)); \ > new__ = (old__ & ~mask__) | bits__; \ > - } while (cmpxchg(ptr, old__, new__) != old__); \ > + } while (!try_cmpxchg(ptr, &old__, new__)); \ > \ > new__; \ > }) > > > While there you probably want something like the above... As a separate change perhaps so that a revert (unlikely as it might be) could be done with less pain. > although, > looking at it now, we seem to have 'forgotten' to add try_cmpxchg to the > generic code :/ So it _has_ to be a separate change ;-) But can we even provide a sane generic try_cmpxchg. The asm-generic cmpxchg relies on local irq save etc so it is clearly only to prevent a new arch from failing to compile. atomic*_cmpxchg() is different story since atomics have to be provided by arch. Anyhow what is more interesting is the try_cmpxchg API itself. So commit a9ebf306f52c756 introduced/use of try_cmpxchg(), which indeed makes the looping "nicer" to read and obvious code gen improvements. So, for (;;) { new = val $op $imm; old = cmpxchg(ptr, val, new); if (old == val) break; val = old; } becomes do { } while (!try_cmpxchg(ptr, &val, val $op $imm)); But on pure LL/SC retry based arches, we still end up with generated code having 2 loops. We discussed something similar a while back: see [1] First loop is inside inline asm to retry LL/SC and the outer one due to code above. Explicit return of try_cmpxchg() means setting up a register with a boolean status of cmpxchg (AFAIKR ARMv7 already does that but ARC e.g. uses a CPU flag thus requires an additional insn or two). We could arguably remove the inline asm loop and retry LL/SC from the outer loop, but it seems cleaner to keep the retry where it belongs. Also under the hood, try_cmpxchg() would end up re-reading it for the issue fixed by commit 44fe84459faf1a. Heck, it would all be simpler if we could express this w/o use of cmpxchg. try_some_op(ptr, &val, val $op $imm); P.S. the horrible API name is for indicative purposes only This would remove the outer loop completely, also avoid any re-reads due to the semantics of cmpxchg etc. [1] https://www.spinics.net/lists/kernel/msg2029217.html From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AFAE8C43612 for ; Fri, 11 Jan 2019 20:58:38 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 608342183F for ; Fri, 11 Jan 2019 20:58:38 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=synopsys.com header.i=@synopsys.com header.b="ISh828SP" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 608342183F Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=synopsys.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 0098E8E0002; Fri, 11 Jan 2019 15:58:38 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id EFA448E0001; Fri, 11 Jan 2019 15:58:37 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id DE9DF8E0002; Fri, 11 Jan 2019 15:58:37 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from mail-pf1-f199.google.com (mail-pf1-f199.google.com [209.85.210.199]) by kanga.kvack.org (Postfix) with ESMTP id 9BA418E0001 for ; Fri, 11 Jan 2019 15:58:37 -0500 (EST) Received: by mail-pf1-f199.google.com with SMTP id u20so11245883pfa.1 for ; Fri, 11 Jan 2019 12:58:37 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:dkim-signature:subject:to:cc:newsgroups :references:from:openpgp:autocrypt:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=LzUuvRIYUaY7dDi9dT5pV4RhzRHJzG9HkZDrLehUx8E=; b=Wxtsy0GwFAAFE9jao5yGdYrq8Ooy2MtDLX8EXJxy3regn8sv03iv5Yq/skeN16+pmr Entk5UtXZrsBeDFeijzjJTxqfjqbvz9aDSE9xDczVEotGWaXpsqz/3PXYWTFXK5IGtx1 2MO8YZIfI6EYvrfWKTQXm7y6+FPHVSSSEJocX2YcVLIi7Z8Spn6bGbu1Yf76l219mUrl UnM9PN1OuIdHdi0Udf4EUehub8dCZgMyGRjw4yA/Vak5Jo6vMuVJZ414RgUAE4nAu6uW b5+PVy8Ca5qAmJm/tAegpAjtvuwOgrOBi0QuL6jJU6X4YjNGySq9+ijWGEa6ZGfMNfeB Z+1A== X-Gm-Message-State: AJcUukcUvmZmkM6dPvY5o1XB5pAVMOfH3ScFs14lu1eFNdbAbj6ZfZh+ UZUFDePucIfiRv07mQfnTJyUy/brCMryRfU+EheFVTTkraaXenh8prKomb3DOqjD1LeZMbW5z/3 j11YvTJjS/t9H/PEye6NyarkfYiPK5LjLsOYOGkRsNpP+a+mNl9uD6qg428I8WL/e0A== X-Received: by 2002:a62:b15:: with SMTP id t21mr16696520pfi.136.1547240317226; Fri, 11 Jan 2019 12:58:37 -0800 (PST) X-Google-Smtp-Source: ALg8bN5gCdMQ/2yZRJSA7bqj+1idQHpPRNZpA/FYPt5hid/HL8ZS+F0MBpuEEHQ9QgAM2uTo6BvC X-Received: by 2002:a62:b15:: with SMTP id t21mr16696476pfi.136.1547240316322; Fri, 11 Jan 2019 12:58:36 -0800 (PST) ARC-Seal: i=1; a=rsa-sha256; t=1547240316; cv=none; d=google.com; s=arc-20160816; b=bjt0Jlt9qA2DFUNHPuGjUBrxeb7G6rnAcc5Pg9bgcBzxBh7QQ9rhl4aCDK+6o1FTzV mvQHmYugk7MrM+TW+EcgLQRzHyy+wlftNB9b/AXH9RYEkqHbu2amZ+utgE8rXuf1NWU+ blgLcZVeziAQW9ihHuEBcJu+M1CQ0mq1+gy47gErSV2PDnkHYjFN1ow8aqgrZ/uM0XRh iDKr7SzumP5kOS8QZXoko/9+4tXD06Cl/JcXYQ8lh44kHWwxJbfRmZk8jRGruygDjwtm ZEpMxMAHcmKu1XHaHIk2VhiO55VLCI8Rot9wmH8xqj4Cz7Rgai3PB2McrHWPjVkRYe0c FwQg== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=content-transfer-encoding:content-language:in-reply-to:mime-version :user-agent:date:message-id:autocrypt:openpgp:from:references :newsgroups:cc:to:subject:dkim-signature; bh=LzUuvRIYUaY7dDi9dT5pV4RhzRHJzG9HkZDrLehUx8E=; b=FG790yQrIAPnEk39oLn2n+k8LiIduQLkxVm+F3sNcHRUN/mI0K/EIFFS4+Wwzhv24C Fwg3V0KA1CkL2uz9PQsIDJ4zgEfWihONvkipXhTDlPrgSUItwzsCGrUU3C9GisSRfyhf 9pAL+JgYEUXunOQMZ0SLnXELE8xMxxc6fyjoR5UbEFnJHWKLVIN6pFjsXZybMWXIOSEB 1z7zO20GAdT3NDoSN9URBdoDDqd0I/csZtiXOoNubrLo0GvqkGx++aH7l0DXQdKNIUAa Axkpfb+DoWY0jwXdaeQBY/MHMHoJuneqLqWovTSxjfdXiFmoBlsh4AEr8EugSUk2m/j1 /N+A== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@synopsys.com header.s=mail header.b=ISh828SP; spf=pass (google.com: domain of vineet.gupta1@synopsys.com designates 198.182.47.9 as permitted sender) smtp.mailfrom=vineet.gupta1@synopsys.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=synopsys.com Received: from smtprelay.synopsys.com (smtprelay4.synopsys.com. [198.182.47.9]) by mx.google.com with ESMTPS id l7si15083723pgk.169.2019.01.11.12.58.36 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 11 Jan 2019 12:58:36 -0800 (PST) Received-SPF: pass (google.com: domain of vineet.gupta1@synopsys.com designates 198.182.47.9 as permitted sender) client-ip=198.182.47.9; Authentication-Results: mx.google.com; dkim=pass header.i=@synopsys.com header.s=mail header.b=ISh828SP; spf=pass (google.com: domain of vineet.gupta1@synopsys.com designates 198.182.47.9 as permitted sender) smtp.mailfrom=vineet.gupta1@synopsys.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=synopsys.com Received: from mailhost.synopsys.com (mailhost3.synopsys.com [10.12.238.238]) by smtprelay.synopsys.com (Postfix) with ESMTP id 34B0B24E1024; Fri, 11 Jan 2019 12:58:35 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=synopsys.com; s=mail; t=1547240315; bh=KjM2U8zzXjSYEEa9IM0sDfxFpgxQOltZA+m4yd6wiqk=; h=Subject:To:CC:References:From:Date:In-Reply-To:From; b=ISh828SPar9UKMdDEjE8Wdb2T3qkAGlNdSoyJP8tsP3sOULHigyLbanTW2Js9zoh4 YLhE5ar4nzjkb3n5vMEtLzHhGxKUDdQsRzXaX1NxjHNnw6Ytliday5/GVT9m1iLOQf E2HSA537Wxat4QRAYjs8IoYYM1YyTay7SSMwsKEY001A2I1NKpJO4KoUe9TkKJUzKq EY79+X7VqSD9iLlWMrVdBwD/8DIrlRtf5e9WlVrwrtsAJd6U1J6RNBk0/RIQ/kTh/Y QsmsbyiYYcWeCuoRD27/RyVyVKmZOfGOsihJPXap/ICvEi2ZQOPxTX5HNcRhX6D/r3 b31j2wSali7tw== Received: from US01WEHTC3.internal.synopsys.com (us01wehtc3.internal.synopsys.com [10.15.84.232]) by mailhost.synopsys.com (Postfix) with ESMTP id BDD2D37F9; Fri, 11 Jan 2019 12:58:34 -0800 (PST) Received: from IN01WEHTCA.internal.synopsys.com (10.144.199.104) by US01WEHTC3.internal.synopsys.com (10.15.84.232) with Microsoft SMTP Server (TLS) id 14.3.408.0; Fri, 11 Jan 2019 12:58:34 -0800 Received: from IN01WEHTCB.internal.synopsys.com (10.144.199.105) by IN01WEHTCA.internal.synopsys.com (10.144.199.103) with Microsoft SMTP Server (TLS) id 14.3.408.0; Sat, 12 Jan 2019 02:28:31 +0530 Received: from [10.10.161.70] (10.10.161.70) by IN01WEHTCB.internal.synopsys.com (10.144.199.243) with Microsoft SMTP Server (TLS) id 14.3.408.0; Sat, 12 Jan 2019 02:28:31 +0530 Subject: Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value To: Peter Zijlstra CC: Mark Rutland , Miklos Szeredi , Jani Nikula , Will Deacon , , Chris Wilson , , Andrew Morton , , Ingo Molnar Newsgroups: gmane.linux.kernel.arc,gmane.linux.kernel,gmane.linux.kernel.mm References: <1547166387-19785-1-git-send-email-vgupta@synopsys.com> <1547166387-19785-4-git-send-email-vgupta@synopsys.com> <20190111092408.GM30894@hirez.programming.kicks-ass.net> From: Vineet Gupta Openpgp: preference=signencrypt Autocrypt: addr=vgupta@synopsys.com; keydata= mQINBFEffBMBEADIXSn0fEQcM8GPYFZyvBrY8456hGplRnLLFimPi/BBGFA24IR+B/Vh/EFk B5LAyKuPEEbR3WSVB1x7TovwEErPWKmhHFbyugdCKDv7qWVj7pOB+vqycTG3i16eixB69row lDkZ2RQyy1i/wOtHt8Kr69V9aMOIVIlBNjx5vNOjxfOLux3C0SRl1veA8sdkoSACY3McOqJ8 zR8q1mZDRHCfz+aNxgmVIVFN2JY29zBNOeCzNL1b6ndjU73whH/1hd9YMx2Sp149T8MBpkuQ cFYUPYm8Mn0dQ5PHAide+D3iKCHMupX0ux1Y6g7Ym9jhVtxq3OdUI5I5vsED7NgV9c8++baM 7j7ext5v0l8UeulHfj4LglTaJIvwbUrCGgtyS9haKlUHbmey/af1j0sTrGxZs1ky1cTX7yeF nSYs12GRiVZkh/Pf3nRLkjV+kH++ZtR1GZLqwamiYZhAHjo1Vzyl50JT9EuX07/XTyq/Bx6E dcJWr79ZphJ+mR2HrMdvZo3VSpXEgjROpYlD4GKUApFxW6RrZkvMzuR2bqi48FThXKhFXJBd JiTfiO8tpXaHg/yh/V9vNQqdu7KmZIuZ0EdeZHoXe+8lxoNyQPcPSj7LcmE6gONJR8ZqAzyk F5voeRIy005ZmJJ3VOH3Gw6Gz49LVy7Kz72yo1IPHZJNpSV5xwARAQABtCpWaW5lZXQgR3Vw dGEgKGFsaWFzKSA8dmd1cHRhQHN5bm9wc3lzLmNvbT6JAj4EEwECACgCGwMGCwkIBwMCBhUI AgkKCwQWAgMBAh4BAheABQJbBYpwBQkLx0HcAAoJEGnX8d3iisJeChAQAMR2UVbJyydOv3aV jmqP47gVFq4Qml1weP5z6czl1I8n37bIhdW0/lV2Zll+yU1YGpMgdDTHiDqnGWi4pJeu4+c5 xsI/VqkH6WWXpfruhDsbJ3IJQ46//jb79ogjm6VVeGlOOYxx/G/RUUXZ12+CMPQo7Bv+Jb+t NJnYXYMND2Dlr2TiRahFeeQo8uFbeEdJGDsSIbkOV0jzrYUAPeBwdN8N0eOB19KUgPqPAC4W HCg2LJ/o6/BImN7bhEFDFu7gTT0nqFVZNXlOw4UcGGpM3dq/qu8ZgRE0turY9SsjKsJYKvg4 djAaOh7H9NJK72JOjUhXY/sMBwW5vnNwFyXCB5t4ZcNxStoxrMtyf35synJVinFy6wCzH3eJ XYNfFsv4gjF3l9VYmGEJeI8JG/ljYQVjsQxcrU1lf8lfARuNkleUL8Y3rtxn6eZVtAlJE8q2 hBgu/RUj79BKnWEPFmxfKsaj8of+5wubTkP0I5tXh0akKZlVwQ3lbDdHxznejcVCwyjXBSny d0+qKIXX1eMh0/5sDYM06/B34rQyq9HZVVPRHdvsfwCU0s3G+5Fai02mK68okr8TECOzqZtG cuQmkAeegdY70Bpzfbwxo45WWQq8dSRURA7KDeY5LutMphQPIP2syqgIaiEatHgwetyVCOt6 tf3ClCidHNaGky9KcNSQuQINBFEffBMBEADXZ2pWw4Regpfw+V+Vr6tvZFRl245PV9rWFU72 xNuvZKq/WE3xMu+ZE7l2JKpSjrEoeOHejtT0cILeQ/Yhf2t2xAlrBLlGOMmMYKK/K0Dc2zf0 MiPRbW/NCivMbGRZdhAAMx1bpVhInKjU/6/4mT7gcE57Ep0tl3HBfpxCK8RRlZc3v8BHOaEf cWSQD7QNTZK/kYJo+Oyux+fzyM5TTuKAaVE63NHCgWtFglH2vt2IyJ1XoPkAMueLXay6enSK Nci7qAG2UwicyVDCK9AtEub+ps8NakkeqdSkDRp5tQldJbfDaMXuWxJuPjfSojHIAbFqP6Qa ANXvTCSuBgkmGZ58skeNopasrJA4z7OsKRUBvAnharU82HGemtIa4Z83zotOGNdaBBOHNN2M HyfGLm+kEoccQheH+my8GtbH1a8eRBtxlk4c02ONkq1Vg1EbIzvgi4a56SrENFx4+4sZcm8o ItShAoKGIE/UCkj/jPlWqOcM/QIqJ2bR8hjBny83ONRf2O9nJuEYw9vZAPFViPwWG8tZ7J+R euXKai4DDr+8oFOi/40mIDe/Bat3ftyd+94Z1RxDCngd3Q85bw13t2ttNLw5eHufLIpoEyAh TCLNQ58eT91YGVGvFs39IuH0b8ovVvdkKGInCT59Vr0MtfgcsqpDxWQXJXYZYTFHd3/RswAR AQABiQIlBBgBAgAPAhsMBQJbBYpwBQkLx0HdAAoJEGnX8d3iisJewe8P/36pkZrVTfO+U+Gl 1OQh4m6weozuI8Y98/DHLMxEujKAmRzy+zMHYlIl3WgSih1UMOZ7U84yVZQwXQkLItcwXoih ChKD5D2BKnZYEOLM+7f9DuJuWhXpee80aNPzEaubBYQ7dYt8rcmB7SdRz/yZq3lALOrF/zb6 SRleBh0DiBLP/jKUV74UAYV3OYEDHN9blvhWUEFFE0Z+j96M4/kuRdxvbDmp04Nfx79AmJEn fv1Vvc9CFiWVbBrNPKomIN+JV7a7m2lhbfhlLpUk0zGFDTWcWejl4qz/pCYSoIUU4r/VBsCV ZrOun4vd4cSi/yYJRY4kaAJGCL5k7qhflL2tgldUs+wERH8ZCzimWVDBzHTBojz0Ff3w2+gY 6FUbAJBrBZANkymPpdAB/lTsl8D2ZRWyy90f4VVc8LB/QIWY/GiS2towRXQBjHOfkUB1JiEX YH/i93k71mCaKfzKGXTVxObU2I441w7r4vtNlu0sADRHCMUqHmkpkjV1YbnYPvBPFrDBS1V9 OfD9SutXeDjJYe3N+WaLRp3T3x7fYVnkfjQIjDSOdyPWlTzqQv0I3YlUk7KjFrh1rxtrpoYS IQKf5HuMowUNtjyiK2VhA5V2XDqd+ZUT3RqfAPf3Y5HjkhKJRqoIDggUKMUKmXaxCkPGi91T hhqBJlyU6MVUa6vZNv8E Message-ID: Date: Fri, 11 Jan 2019 12:58:22 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20190111092408.GM30894@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.10.161.70] 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: Message-ID: <20190111205822.4-KS2-v8Zke1jCU4y2iftgr7vivlRj19Y9sCWnq9GOg@z> On 1/11/19 1:24 AM, Peter Zijlstra wrote: > diff --git a/include/linux/bitops.h b/include/linux/bitops.h > index 705f7c442691..2060d26a35f5 100644 > --- a/include/linux/bitops.h > +++ b/include/linux/bitops.h > @@ -241,10 +241,10 @@ static __always_inline void __assign_bit(long nr, volatile unsigned long *addr, > const typeof(*(ptr)) mask__ = (mask), bits__ = (bits); \ > typeof(*(ptr)) old__, new__; \ > \ > + old__ = READ_ONCE(*(ptr)); \ > do { \ > - old__ = READ_ONCE(*(ptr)); \ > new__ = (old__ & ~mask__) | bits__; \ > - } while (cmpxchg(ptr, old__, new__) != old__); \ > + } while (!try_cmpxchg(ptr, &old__, new__)); \ > \ > new__; \ > }) > > > While there you probably want something like the above... As a separate change perhaps so that a revert (unlikely as it might be) could be done with less pain. > although, > looking at it now, we seem to have 'forgotten' to add try_cmpxchg to the > generic code :/ So it _has_ to be a separate change ;-) But can we even provide a sane generic try_cmpxchg. The asm-generic cmpxchg relies on local irq save etc so it is clearly only to prevent a new arch from failing to compile. atomic*_cmpxchg() is different story since atomics have to be provided by arch. Anyhow what is more interesting is the try_cmpxchg API itself. So commit a9ebf306f52c756 introduced/use of try_cmpxchg(), which indeed makes the looping "nicer" to read and obvious code gen improvements. So, for (;;) { new = val $op $imm; old = cmpxchg(ptr, val, new); if (old == val) break; val = old; } becomes do { } while (!try_cmpxchg(ptr, &val, val $op $imm)); But on pure LL/SC retry based arches, we still end up with generated code having 2 loops. We discussed something similar a while back: see [1] First loop is inside inline asm to retry LL/SC and the outer one due to code above. Explicit return of try_cmpxchg() means setting up a register with a boolean status of cmpxchg (AFAIKR ARMv7 already does that but ARC e.g. uses a CPU flag thus requires an additional insn or two). We could arguably remove the inline asm loop and retry LL/SC from the outer loop, but it seems cleaner to keep the retry where it belongs. Also under the hood, try_cmpxchg() would end up re-reading it for the issue fixed by commit 44fe84459faf1a. Heck, it would all be simpler if we could express this w/o use of cmpxchg. try_some_op(ptr, &val, val $op $imm); P.S. the horrible API name is for indicative purposes only This would remove the outer loop completely, also avoid any re-reads due to the semantics of cmpxchg etc. [1] https://www.spinics.net/lists/kernel/msg2029217.html