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[66.111.4.228]) by smtp.gmail.com with ESMTPSA id c192sm2923711qkg.125.2020.02.14.02.40.08 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 14 Feb 2020 02:40:09 -0800 (PST) Received: from compute6.internal (compute6.nyi.internal [10.202.2.46]) by mailauth.nyi.internal (Postfix) with ESMTP id 1C90B2220E; Fri, 14 Feb 2020 05:40:08 -0500 (EST) Received: from mailfrontend1 ([10.202.2.162]) by compute6.internal (MEProxy); Fri, 14 Feb 2020 05:40:08 -0500 X-ME-Sender: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgedugedrjedtgddukecutefuodetggdotefrodftvf curfhrohhfihhlvgemucfhrghsthforghilhdpqfgfvfdpuffrtefokffrpgfnqfghnecu uegrihhlohhuthemuceftddtnecusecvtfgvtghiphhivghnthhsucdlqddutddtmdenuc fjughrpeffhffvuffkfhggtggujgesthdtredttddtvdenucfhrhhomhepuehoqhhunhcu hfgvnhhguceosghoqhhunhdrfhgvnhhgsehgmhgrihhlrdgtohhmqeenucffohhmrghinh epkhgvrhhnvghlrdhorhhgnecukfhppeehvddrudehhedrudduuddrjedunecuvehluhhs thgvrhfuihiivgeptdenucfrrghrrghmpehmrghilhhfrhhomhepsghoqhhunhdomhgvsh hmthhprghuthhhphgvrhhsohhnrghlihhthidqieelvdeghedtieegqddujeejkeehheeh vddqsghoqhhunhdrfhgvnhhgpeepghhmrghilhdrtghomhesfhhigihmvgdrnhgrmhgv X-ME-Proxy: Received: from localhost (unknown [52.155.111.71]) by mail.messagingengine.com (Postfix) with ESMTPA id 1F083328005A; Fri, 14 Feb 2020 05:40:04 -0500 (EST) Date: Fri, 14 Feb 2020 18:40:03 +0800 From: Boqun Feng To: Andrea Parri Cc: linux-kernel@vger.kernel.org, Alan Stern , Will Deacon , Peter Zijlstra , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , "Paul E. McKenney" , Akira Yokosawa , Daniel Lustig , Jonathan Corbet , linux-arch@vger.kernel.org, linux-doc@vger.kernel.org Subject: Re: [RFC 2/3] tools/memory-model: Add a litmus test for atomic_set() Message-ID: <20200214104003.GC20408@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net> References: <20200214040132.91934-1-boqun.feng@gmail.com> <20200214040132.91934-3-boqun.feng@gmail.com> <20200214081213.GA17708@andrea> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200214081213.GA17708@andrea> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 14, 2020 at 09:12:13AM +0100, Andrea Parri wrote: > > @@ -0,0 +1,24 @@ > > +C Atomic-set-observable-to-RMW > > + > > +(* > > + * Result: Never > > + * > > + * Test of the result of atomic_set() must be observable to atomic RMWs. > > + *) > > + > > +{ > > + atomic_t v = ATOMIC_INIT(1); > > +} > > + > > +P0(atomic_t *v) > > +{ > > + (void)atomic_add_unless(v,1,0); > > We blacklisted this primitive some time ago, cf. section "LIMITATIONS", > entry (6b) in tools/memory-model/README; the discussion was here: > > https://lkml.kernel.org/r/20180829211053.20531-3-paulmck@linux.vnet.ibm.com > And in an email replying to that email, you just tried and seemed atomic_add_unless() works ;-) > but unfortunately I can't remember other details at the moment: maybe > it is just a matter of or the proper time to update that section. > I spend a few time looking into the changes in herd, the dependency problem seems to be as follow: For atomic_add_unless(ptr, a, u), the return value (true or false) depends on both *ptr and u, this is different than other atomic RMW, whose return value only depends on *ptr. Considering the following litmus test: C atomic_add_unless-dependency { int y = 1; } P0(int *x, int *y, int *z) { int r0; int r1; int r2; r0 = READ_ONCE(*x); if (atomic_add_unless(y, 2, r0)) WRITE_ONCE(*z, 42); else WRITE_ONCE(*z, 1); } P1(int *x, int *y, int *z) { int r0; r0 = smp_load_acquire(z); WRITE_ONCE(*x, 1); } exists (1:r0 = 1 /\ 0:r0 = 1) , the exist-clause will never trigger, however if we replace "atomic_add_unless(y, 2, r0)" with "atomic_add_unless(y, 2, 1)", the write on *z and the read from *x on CPU 0 are not ordered, so we could observe the exist-clause triggered. I just tried with the latest herd, and herd can work out this dependency. So I think we are good now and can change the limitation section in the document. But I will wait for Luc's input for this. Luc, did I get this correct? Is there any other limitation on atomic_add_unless() now? Regards, Boqun > Thanks, > Andrea