220 26827 <2DCF8C2C-C662-49EE-B7E5-3FA11B47AB58@gmail.com> article Path: news.gmane.org!not-for-mail From: Howard Hinnant Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: std::lock and std::lock_guard for SharedLockable's lock_shared Date: Tue, 12 Jul 2016 15:34:28 -0400 Lines: 115 Approved: news@gmane.org Message-ID: <2DCF8C2C-C662-49EE-B7E5-3FA11B47AB58@gmail.com> References: <8d4cc002-f0f2-465f-b856-841adc1ecffd@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 9.3 \(3124\)) Content-Type: multipart/signed; boundary="Apple-Mail=_C393CB53-614F-4929-9F35-8246084FC829"; protocol="application/pgp-signature"; micalg=pgp-sha256 X-Trace: ger.gmane.org 1468352087 6462 80.91.229.3 (12 Jul 2016 19:34:47 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 12 Jul 2016 19:34:47 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCK2HM4L6YERBTMMSW6AKGQECGCF5TI@isocpp.org Tue Jul 12 21:34:41 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qk0-f199.google.com ([209.85.220.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bN3S3-0004tb-Bb for gclcip-std-proposals@m.gmane.org; Tue, 12 Jul 2016 21:34:39 +0200 Original-Received: by mail-qk0-f199.google.com with SMTP id a123sf54164945qkd.2 for ; Tue, 12 Jul 2016 12:34:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=mime-version:subject:from:in-reply-to:date:message-id:references:to :x-original-sender:x-original-authentication-results:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=ibEP027JSYDViRIpKgLYBMR9RYxRGFKkhNsGVeLrslc=; b=DupJZIIslq0b7pcgM9Mx5yOGgbzkMlxnrbS63N4lW39KQllbxoRDkrins8Av5yTMVv QEe26xHiURaKl0D3+PCVTJs8vSuzLipZ6NDAvjbiQnNCZ1lXsYuNPFhEd1wgsnf6Xzi0 BN7BSbiqlaieSatXt8L2kKdYVzz8QZl/E0h2fKGxiNdrkJ/hJ+Ygb+PpApYZlMB77ic/ nEnN+IvohnNYvhux91F25N6Pyme7YtiSGQxvWVpunKMcdRhEJeRa6Dy1lRthAqB2rdKy YtVxFwVT6dA+L1RjcE3VnbnWU3LJuT/WxsOMSuQbNNPAmWzGYmZBv3teVtcSl/FJpyro JCAg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:subject:from:in-reply-to:date :message-id:references:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=ibEP027JSYDViRIpKgLYBMR9RYxRGFKkhNsGVeLrslc=; b=aapzXnnXitFxv5DI2uN85FW0M69kdsA41xeqBqDOHyJOWPkWA7ifGG3I5AfZ78U0rJ sWPVD81guZXtgRyG7xOr9uDZ5ShppA4VFT+q0+YHd4SaUbE1c8Kck44sxuLADGr2/mAm OgAlkblbqjEr4H5d/JgHAVI1LCTVu8hoQRAMmARRijvNbXtXewYPRgu94s2NCNVon51O YIWyryE+UsACAUv25YJ0C5Q3tyRrItkW8IPcc+oOPrYIfHLMTj0MKvRD2DLxSRBdiZ0z Wlec1k4lphNt5irF/MfLuJ15oXx3ZqFCT5gVq3oT/6SMgOnEC6rCjlzul0BqfHupP1Wb 8CeQ== X-Gm-Message-State: ALyK8tKAttdV99U50eRQRtzUh9eU2tp6IDe3fivP9xrdsX8jEVYmgsLdKE+FR+zXRRQSkQ== X-Received: by 10.200.46.149 with SMTP id h21mr3141878qta.3.1468352078432; Tue, 12 Jul 2016 12:34:38 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.36.108.213 with SMTP id w204ls451720itb.1.gmail; Tue, 12 Jul 2016 12:34:37 -0700 (PDT) X-Received: by 10.55.217.89 with SMTP id u86mr5269743qki.114.1468352077539; Tue, 12 Jul 2016 12:34:37 -0700 (PDT) Original-Received: from mail-qt0-x232.google.com (mail-qt0-x232.google.com. [2607:f8b0:400d:c0d::232]) by mx.google.com with ESMTPS id c89si3322890qkj.30.2016.07.12.12.34.37 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 12 Jul 2016 12:34:37 -0700 (PDT) Received-SPF: pass (google.com: domain of howard.hinnant@gmail.com designates 2607:f8b0:400d:c0d::232 as permitted sender) client-ip=2607:f8b0:400d:c0d::232; Original-Received: by mail-qt0-x232.google.com with SMTP id 52so13908492qtq.3 for ; Tue, 12 Jul 2016 12:34:37 -0700 (PDT) X-Received: by 10.200.46.122 with SMTP id s55mr6209201qta.80.1468352077187; Tue, 12 Jul 2016 12:34:37 -0700 (PDT) Original-Received: from [10.0.1.2] (cpe-67-241-68-255.twcny.res.rr.com. [67.241.68.255]) by smtp.gmail.com with ESMTPSA id p39sm908710qtp.14.2016.07.12.12.34.35 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Tue, 12 Jul 2016 12:34:35 -0700 (PDT) X-Pgp-Agent: GPGMail In-Reply-To: <8d4cc002-f0f2-465f-b856-841adc1ecffd@isocpp.org> X-Mailer: Apple Mail (2.3124) X-Original-Sender: howard.hinnant@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of howard.hinnant@gmail.com designates 2607:f8b0:400d:c0d::232 as permitted sender) smtp.mailfrom=howard.hinnant@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:26827 Archived-At: --Apple-Mail=_C393CB53-614F-4929-9F35-8246084FC829 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 On Jul 12, 2016, at 3:09 PM, 'Bernd' via ISO C++ Standard - Future Proposal= s wrote: >=20 > In discussions on Andrei's NDC talk about folly/Synchronized [1] the ques= tion came up why his deadlock prevention [2] did not just use std::lock(). = It was pointed out that this is due to using SharedLockable::lock_shared(),= making std::lock() the wrong thing. There is the workaround of >=20 > std::shared_mutex a, b; > std::shared_lock<> lock_a (a, std::defer_lock); > std::shared_lock<> lock_b (b, std::defer_lock); > std::lock (lock_a, lock_b); >=20 > using the shared_lock adapter exposing SharedLockable as Lockable. Note t= hat the inverse order of std::lock and std::shared_lock (using std::adopt_l= ock) requires manual deadlock prevention. Note that the locks also have to = be non-const which is undesirable. >=20 > I propose adding std::lock_shared (SharedLockable&...) to allow said inve= rse order, for consistency: >=20 > std::shared_mutex a, b; > std::lock_shared (a, b); > std::shared_lock<> const lock_a (a, std::adopt_lock); > std::shared_lock<> const lock_b (b, std::adopt_lock); >=20 > On the same note, C++17 added std::lock_guard, which has no = SharedLockable variant either, which would make the above code even easier: >=20 > std::shared_lock_guard<> const lock_both (a, b); >=20 > The biggest issue I can see with it is that combining locking a Lockable = and a SharedLockable at once, which still requires the defer_lock approach = or manual deadlock prevention. It feels like this was an oversight when add= ing SharedLockable and an easy addition of basically "do as std::lock, but = use _shared methods=E2=80=9D. I can assure you it wasn=E2=80=99t an oversight. And your first =E2=80=9Cw= orkaround=E2=80=9D is the intended use. We don=E2=80=99t need or want a =E2=80=9Ckitchen sink=E2=80=9D API where th= ere are 3 three different ways to do everything. We already have that with= std::string, and we don=E2=80=99t need to repeat that design mistake. The =E2=80=9Cmixed case=E2=80=9D you speak of at the end is actually a very= prominent use case: class A { using ReadLock std::shared_lock; using WriteLock std::unique_lock; mutable std::shared_mutex mut_; public: // ... A& operator=3D(const A& a) { if (this !=3D &a) { WriteLock this_lock(mut_, std::defer_lock); ReadLock that_lock(a.mut_, std::defer_lock); std::lock(this_lock, that_lock); // mut_ is now unique-locked and a.mut_ is now share-locked // ... now safe to assign data ... } // mut_ and a.mut_ now unlocked, even if there was an excep= tion return *this; } // ... }; Howard --=20 You received this message because you are subscribed to the Google Groups "= ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/2DCF8C2C-C662-49EE-B7E5-3FA11B47AB58%40gmail.com= .. --Apple-Mail=_C393CB53-614F-4929-9F35-8246084FC829 Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc Content-Description: Message signed with OpenPGP using GPGMail -----BEGIN PGP SIGNATURE----- iQIcBAEBCAAGBQJXhUZKAAoJEGbcHCxKqhWCX2YP/0mqL7P/U1Ms2rqDm8Mwkvw+ vWsIHav10+U27M63jz9Az4xb3VBAKlmMGY/BFWozJookgVxH8LbZyWHoBRyOEsS6 C5w+QRJxNo1BF0UcbRXwHIG5pscSovPwlc1Y74mDCom9U+JA7gzJF9JF5keQCmwG lDUfcNRB+5huygMMk/8WSWNgdgPCCDEJu9NtMDer1fWT5M1bo3qGal/vPNmXySM5 B5LmD4GZBWpPpn2Q/YxYbIRtGQ6vUCDu+NbF6ou59pT6tkczk9GZNo8v+JmMWqPj 7ZdG1Kul7LTZgaNb0oV/p9l31lkGF5Vsz/pHc+KXLIM1L7e70UlnPxbUMPOoDDxV +gOnnWEsNFzSgztvKVSR2JAve9ZjnkVSP3vebO4Oaf/igYJ4SxTT0wMMZXFYZaSu RqarcSBGXi2/zpxLp5q8+NL1g0KWW6Wgf5uBEDrBOEd9WGuwUTB5GXmnokc7Q3vf ERPb47NrSRJZZjSvtP4/IzRrGz2DaaP2pMVwfuryzrr/FzTjG7G/C++TRcUYdcU6 ksib1Wm2omzPttSvT4k7ln7JhFwco1E/aS6z7GQoiPKbdDMljuHeIF5+4y7+OH9u g97zAYcOcGG/rmgQnSxI+2d/NNkwe/oSaMiVpa8eExV7/3+uXifXot9BjGaPiyxu 6sbRNTUQxZvCv7QEfzR4 =Rwpi -----END PGP SIGNATURE----- --Apple-Mail=_C393CB53-614F-4929-9F35-8246084FC829-- .