220 30977 <5db98f8a-68ee-4753-b46a-3409b89862a4@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Proposed alternative approach to specifying required memory operation ordering Date: Fri, 17 Feb 2017 14:45:30 -0800 (PST) Lines: 102 Approved: news@gmane.org Message-ID: <5db98f8a-68ee-4753-b46a-3409b89862a4@isocpp.org> References: <20170216004134.4902991.82070.24891@gmail.com> <8575c41d-1c8e-4e6f-9f12-a07bb70a7703@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1474_903571175.1487371530887" X-Trace: blaine.gmane.org 1487371533 28623 195.159.176.226 (17 Feb 2017 22:45:33 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 17 Feb 2017 22:45:33 +0000 (UTC) Cc: inkwizytoryankes@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBC72TXCQKGQEACO7IVQ@isocpp.org Fri Feb 17 23:45:29 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oi0-f69.google.com ([209.85.218.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cerHK-0006mg-Fo for gclcip-std-proposals@m.gmane.org; Fri, 17 Feb 2017 23:45:26 +0100 Original-Received: by mail-oi0-f69.google.com with SMTP id a194sf64603485oib.5 for ; Fri, 17 Feb 2017 14:45:32 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=OP7xs8AYz2sOz7HlW9R6nMl4Jchh7v+m92TmQSJ0RZY=; b=EWHmqZ/DYL5NOpif9hud75eGR1k2+M4uykOt1OLAyahzjxHfpbVeAH04cvvKrrTLFP ysI5CYLdYLBQhxPlYRIKKcWGvme99J6/Q+DAE5O7o/RjBl/GoyJEv0D7WNQhR7dwzzcP mQvdjQdHIwK9uBCpzYiXrap6OjN5R8rW+ElIi+PnHUMfoK0foaSlFzAMqwKykogB79lG RIWAij3mAx+JpEtu44dfmwliyFqbWcLEI8tn8KJay3gvO/qhxSjpKP1frCd8aUoNmVut RPKQW0tXE+y/Lfr6MLI3dpsMRg+8g3S1XoCK7eF4ekLRtVzSAimZCnl/nzSk+vchxJyA I1Gg== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=OP7xs8AYz2sOz7HlW9R6nMl4Jchh7v+m92TmQSJ0RZY=; b=u73sdYaHyws/4tEeAJ8z9GkzGx9HseFXqVbPiMyux9LpUF55kAYveqrww0qEDnZEvF xYzpUQfR2KXSo/SXu30yagC79UlH1ggw6uPrZsI6YKtxMULl49y0aidHvbwTipM7hp9K RSC+LopV4rAXtCL77IQ3DrfwMuOlQ6q3m7yEFtcpUvGRxvmDpnWmWq5B0JcuKTUHDnNK BgF8mirlYemYivxL3qUEseubGT4n+HB3dXWLSjq3NhMCFk7TJHaYBbBrmRB5MDy23QAk n234gefdy8s26ArmGZ9u2qBXgCqVjFbHI7E8o3RaGTxd6HWooNWLTQEUcd4hdhw2ZeOY aPSQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:message-id:in-reply-to :references:subject:mime-version:x-original-sender:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=OP7xs8AYz2sOz7HlW9R6nMl4Jchh7v+m92TmQSJ0RZY=; b=BIR97Un74gZXFZZzTiQ4SdLpPEYoh1ivnFzz9h0BJJnN5rjwtaJpq478XLeyQtYJTO 7+qzfDJ/Qwnak5WoDk7NoKGxomZ/g1sNshXzcEav3UiW+eqYnPs9BZp4fKwcKM05HiHu FXpAwflj2JD94zdBygV/MAfAYQ+S9E3CQeVCTYRTPvpSW8LFygndmWKQa9ZkMMH7QS+l TqX0b31fUtHKTank5y2Z1Hk/FQMbga8hd8F8yqa2NwXfq+L/u22zdNs1DShiCiv7mUyc TANlx47fLgfrrT/NCHqhvpyrxrGhaAlfAxIQqH2DXnmaa1ofRSgROzDlpRZwSuloZq4D cMIw== X-Gm-Message-State: AMke39nS337sI3XdWVh+1bpIC4fRA0+pcnWIW8k8zCIxOWJj77C37qm/x9V4JAsrE/G2LQ== X-Received: by 10.157.48.23 with SMTP id d23mr3164658otc.51.1487371532153; Fri, 17 Feb 2017 14:45:32 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.56.105 with SMTP id r38ls5895447otd.15.gmail; Fri, 17 Feb 2017 14:45:31 -0800 (PST) X-Received: by 10.157.20.145 with SMTP id d17mr906835ote.18.1487371531454; Fri, 17 Feb 2017 14:45:31 -0800 (PST) In-Reply-To: X-Original-Sender: jmckesson@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: 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:30977 Archived-At: ------=_Part_1474_903571175.1487371530887 Content-Type: multipart/alternative; boundary="----=_Part_1475_762578859.1487371530887" ------=_Part_1475_762578859.1487371530887 Content-Type: text/plain; charset=UTF-8 On Friday, February 17, 2017 at 5:34:30 PM UTC-5, inkwizyt...@gmail.com wrote: > > > > On Friday, February 17, 2017 at 11:26:48 PM UTC+1, Nicol Bolas wrote: >> >> On Friday, February 17, 2017 at 5:14:55 PM UTC-5, Nicol Bolas wrote: >>> >>> Actually, it does use synchronization. `operator*` executes a `load()` >>> on the atomic object. That forces synchronization. >>> >> >> Sorry, I said that wrong. `operator unsigned()` performs a `load()`. The >> default load mechanism performs a full sync on the variable. So once the >> other thread is passed its `release` fence, then this thread will be able >> to see the new value. >> > > I was answering: > "When I don't enable optimization, this code works fine, whether access to > the thread-shared variables is atomic or non-atomic. When I enable > optimization. the threads get into infinite loops if the access to the > thread-shared variables is non-atomic. But atomic access, even with > relaxed order, causes the code to work fine with optimization." > This mean when `using Cardinal = unsigned;` and we don't have any load. > Hmm. I assumed that "but atomic access" was referring to using `Cardinal` class, not `using Cardinal = unsigned". But it's possible that he was referring to the use of `std::atomic_thread_fence`. If that's the case, then you're right; that particular fence is not a guarantee that `Cardinal` works. -- 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 email 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/5db98f8a-68ee-4753-b46a-3409b89862a4%40isocpp.org. ------=_Part_1475_762578859.1487371530887 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Friday, February 17, 2017 at 5:34:30 PM UTC-5, inkwizyt= ....@gmail.com wrote:


On Friday, February 17, 2017 at 11:26:48 PM UTC+1, Nicol B= olas wrote:
On Frid= ay, February 17, 2017 at 5:14:55 PM UTC-5, Nicol Bolas wrote:
Actually, it does use synch= ronization. `operator*` executes a `load()` on the atomic object. That forc= es synchronization.

Sorry, I said that= wrong. `operator unsigned()` performs a `load()`. The default load mechani= sm performs a full sync on the variable. So once the other thread is passed= its `release` fence, then this thread will be able to see the new value.

I was answering:
"When I don= 9;t enable optimization, this code works=20 fine, whether access to the thread-shared variables is atomic or=20 non-atomic. =C2=A0When I enable optimization. the threads get into infinite= =20 loops if the access to the thread-shared variables is non-atomic. =C2=A0But= =20 atomic access, even with relaxed order, causes the code to work fine=20 with optimization."
This mean when `using Cardinal =3D unsigned;` = and we don't have any load.

Hmm. I= assumed that "but atomic access" was referring to using `Cardina= l` class, not `using Cardinal =3D unsigned". But it's possible tha= t he was referring to the use of `std::atomic_thread_fence`.

If that= 's the case, then you're right; that particular fence is not a guar= antee that `Cardinal` works.

--
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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/5db98f8a-68ee-4753-b46a-3409b89862a4= %40isocpp.org.
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