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McKenney" To: Alan Stern Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, parri.andrea@gmail.com, will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com Subject: Re: [PATCH memory-model 5/8] tools/memory-model: Add a glossary of LKMM terms Message-ID: <20201106195912.GA3249@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20201105215953.GA15309@paulmck-ThinkPad-P72> <20201105220017.15410-5-paulmck@kernel.org> <20201106165930.GC47039@rowland.harvard.edu> <20201106180445.GX3249@paulmck-ThinkPad-P72> <20201106192351.GA53131@rowland.harvard.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201106192351.GA53131@rowland.harvard.edu> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 06, 2020 at 02:23:51PM -0500, Alan Stern wrote: > On Fri, Nov 06, 2020 at 10:04:46AM -0800, Paul E. McKenney wrote: > > On Fri, Nov 06, 2020 at 11:59:30AM -0500, Alan Stern wrote: > > > > + See also "Control Dependency". > > > > > > There should also be an entry for "Data Dependency", linked from here > > > and from Control Dependency. > > > > > > > +Marked Access: An access to a variable that uses an special function or > > > > + macro such as "r1 = READ_ONCE()" or "smp_store_release(&a, 1)". > > > > > > How about "r1 = READ_ONCE(x)"? > > > > Good catches! I am planning to squash the commit below into the > > original. Does that cover it? > > No, because you didn't add a glossary entry for "Data Dependency" and > there's no link from "Control Dependency" to "Data Dependency". Sigh. I was thinking "entry in the list", and didn't even thing to check for an entry in the glossary as a whole. With the patch below (on top of the one sent earlier), are we good? Thanx, Paul ------------------------------------------------------------------------ commit 5a49c32551e83d30e304d6c3fbb660737ba2654e Author: Paul E. McKenney Date: Fri Nov 6 11:57:25 2020 -0800 fixup! tools/memory-model: Add a glossary of LKMM terms Signed-off-by: Paul E. McKenney diff --git a/tools/memory-model/Documentation/glossary.txt b/tools/memory-model/Documentation/glossary.txt index 471bf13..b2da636 100644 --- a/tools/memory-model/Documentation/glossary.txt +++ b/tools/memory-model/Documentation/glossary.txt @@ -64,7 +64,7 @@ Control Dependency: When a later store's execution depends on a test fragile, and can be easily destroyed by optimizing compilers. Please see control-dependencies.txt for more information. - See also "Address Dependency". + See also "Address Dependency" and "Data Dependency". Cycle: Memory-barrier pairing is restricted to a pair of CPUs, as the name suggests. And in a great many cases, a pair of CPUs is all @@ -85,6 +85,23 @@ Cycle: Memory-barrier pairing is restricted to a pair of CPUs, as the See also "Pairing". +Data Dependency: When the data written by a later store is computed based + on the value returned by an earlier load, a "data dependency" + extends from that load to that later store. For example: + + 1 r1 = READ_ONCE(x); + 2 WRITE_ONCE(y, r1 + 1); + + In this case, the data dependency extends from the READ_ONCE() + on line 1 to the WRITE_ONCE() on line 2. Data dependencies are + fragile and can be easily destroyed by optimizing compilers. + Because optimizing compilers put a great deal of effort into + working out what values integer variables might have, this is + especially true in cases where the dependency is carried through + an integer. + + See also "Address Dependency" and "Control Dependency". + From-Reads (fr): When one CPU's store to a given variable happened too late to affect the value returned by another CPU's load from that same variable, there is said to be a from-reads