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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 1 Jul 2019 22:39:54 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22036.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x61LdrWK13566520 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 1 Jul 2019 21:39:53 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 4D99AB2065; Mon, 1 Jul 2019 21:39:53 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 15994B205F; Mon, 1 Jul 2019 21:39:53 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.26]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 1 Jul 2019 21:39:53 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 4713A16C2BD7; Mon, 1 Jul 2019 14:39:56 -0700 (PDT) Date: Mon, 1 Jul 2019 14:39:56 -0700 From: "Paul E. McKenney" To: Alan Stern Cc: Andrea Parri , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, Will Deacon , Peter Zijlstra , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , Akira Yokosawa , Daniel Lustig Subject: Re: [PATCH] tools/memory-model: Update the informal documentation Reply-To: paulmck@linux.ibm.com References: <1561842644-5354-1-git-send-email-andrea.parri@amarulasolutions.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19070121-0072-0000-0000-0000044331C0 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011362; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01226073; UDB=6.00645444; IPR=6.01007285; MB=3.00027543; MTD=3.00000008; XFM=3.00000015; UTC=2019-07-01 21:39:57 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19070121-0073-0000-0000-00004CB368EC Message-Id: <20190701213956.GU26519@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-07-01_13:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1907010250 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 01, 2019 at 10:12:45AM -0400, Alan Stern wrote: > On Sat, 29 Jun 2019, Andrea Parri wrote: > > > The formal memory consistency model has added support for plain accesses > > (and data races). While updating the informal documentation to describe > > this addition to the model is highly desirable and important future work, > > update the informal documentation to at least acknowledge such addition. > > > > Signed-off-by: Andrea Parri > > Cc: Alan Stern > > Cc: Will Deacon > > Cc: Peter Zijlstra > > Cc: Boqun Feng > > Cc: Nicholas Piggin > > Cc: David Howells > > Cc: Jade Alglave > > Cc: Luc Maranget > > Cc: "Paul E. McKenney" > > Cc: Akira Yokosawa > > Cc: Daniel Lustig > > --- > > Acked-by: Alan Stern Applied, thank you both! Thanx, Paul > > tools/memory-model/Documentation/explanation.txt | 47 +++++++++++------------- > > tools/memory-model/README | 18 ++++----- > > 2 files changed, 30 insertions(+), 35 deletions(-) > > > > diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt > > index 68caa9a976d0c..b42f7cd718242 100644 > > --- a/tools/memory-model/Documentation/explanation.txt > > +++ b/tools/memory-model/Documentation/explanation.txt > > @@ -42,7 +42,8 @@ linux-kernel.bell and linux-kernel.cat files that make up the formal > > version of the model; they are extremely terse and their meanings are > > far from clear. > > > > -This document describes the ideas underlying the LKMM. It is meant > > +This document describes the ideas underlying the LKMM, but excluding > > +the modeling of bare C (or plain) shared memory accesses. It is meant > > for people who want to understand how the model was designed. It does > > not go into the details of the code in the .bell and .cat files; > > rather, it explains in English what the code expresses symbolically. > > @@ -354,31 +355,25 @@ be extremely complex. > > Optimizing compilers have great freedom in the way they translate > > source code to object code. They are allowed to apply transformations > > that add memory accesses, eliminate accesses, combine them, split them > > -into pieces, or move them around. Faced with all these possibilities, > > -the LKMM basically gives up. It insists that the code it analyzes > > -must contain no ordinary accesses to shared memory; all accesses must > > -be performed using READ_ONCE(), WRITE_ONCE(), or one of the other > > -atomic or synchronization primitives. These primitives prevent a > > -large number of compiler optimizations. In particular, it is > > -guaranteed that the compiler will not remove such accesses from the > > -generated code (unless it can prove the accesses will never be > > -executed), it will not change the order in which they occur in the > > -code (within limits imposed by the C standard), and it will not > > -introduce extraneous accesses. > > - > > -This explains why the MP and SB examples above used READ_ONCE() and > > -WRITE_ONCE() rather than ordinary memory accesses. Thanks to this > > -usage, we can be certain that in the MP example, P0's write event to > > -buf really is po-before its write event to flag, and similarly for the > > -other shared memory accesses in the examples. > > - > > -Private variables are not subject to this restriction. Since they are > > -not shared between CPUs, they can be accessed normally without > > -READ_ONCE() or WRITE_ONCE(), and there will be no ill effects. In > > -fact, they need not even be stored in normal memory at all -- in > > -principle a private variable could be stored in a CPU register (hence > > -the convention that these variables have names starting with the > > -letter 'r'). > > +into pieces, or move them around. The use of READ_ONCE(), WRITE_ONCE(), > > +or one of the other atomic or synchronization primitives prevents a > > +large number of compiler optimizations. In particular, it is guaranteed > > +that the compiler will not remove such accesses from the generated code > > +(unless it can prove the accesses will never be executed), it will not > > +change the order in which they occur in the code (within limits imposed > > +by the C standard), and it will not introduce extraneous accesses. > > + > > +The MP and SB examples above used READ_ONCE() and WRITE_ONCE() rather > > +than ordinary memory accesses. Thanks to this usage, we can be certain > > +that in the MP example, the compiler won't reorder P0's write event to > > +buf and P0's write event to flag, and similarly for the other shared > > +memory accesses in the examples. > > + > > +Since private variables are not shared between CPUs, they can be > > +accessed normally without READ_ONCE() or WRITE_ONCE(). In fact, they > > +need not even be stored in normal memory at all -- in principle a > > +private variable could be stored in a CPU register (hence the convention > > +that these variables have names starting with the letter 'r'). > > > > > > A WARNING > > diff --git a/tools/memory-model/README b/tools/memory-model/README > > index 2b87f3971548c..fc07b52f20286 100644 > > --- a/tools/memory-model/README > > +++ b/tools/memory-model/README > > @@ -167,15 +167,15 @@ scripts Various scripts, see scripts/README. > > LIMITATIONS > > =========== > > > > -The Linux-kernel memory model has the following limitations: > > - > > -1. Compiler optimizations are not modeled. Of course, the use > > - of READ_ONCE() and WRITE_ONCE() limits the compiler's ability > > - to optimize, but there is Linux-kernel code that uses bare C > > - memory accesses. Handling this code is on the to-do list. > > - For more information, see Documentation/explanation.txt (in > > - particular, the "THE PROGRAM ORDER RELATION: po AND po-loc" > > - and "A WARNING" sections). > > +The Linux-kernel memory model (LKMM) has the following limitations: > > + > > +1. Compiler optimizations are not accurately modeled. Of course, > > + the use of READ_ONCE() and WRITE_ONCE() limits the compiler's > > + ability to optimize, but under some circumstances it is possible > > + for the compiler to undermine the memory model. For more > > + information, see Documentation/explanation.txt (in particular, > > + the "THE PROGRAM ORDER RELATION: po AND po-loc" and "A WARNING" > > + sections). > > > > Note that this limitation in turn limits LKMM's ability to > > accurately model address, control, and data dependencies. > > >