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McKenney" To: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, mingo@kernel.org Cc: stern@rowland.harvard.edu, parri.andrea@gmail.com, will.deacon@arm.com, 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, "Paul E. McKenney" Subject: [PATCH memory-model 4/5] tools/memory-model: Add more LKMM limitations Date: Wed, 26 Sep 2018 11:29:19 -0700 X-Mailer: git-send-email 2.17.1 In-Reply-To: <20180926182845.GA24839@linux.ibm.com> References: <20180926182845.GA24839@linux.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 18092618-2213-0000-0000-000002F74F05 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009776; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000267; SDB=6.01094026; UDB=6.00565497; IPR=6.00874064; MB=3.00023518; MTD=3.00000008; XFM=3.00000015; UTC=2018-09-26 18:29:27 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18092618-2214-0000-0000-00005BB05EA1 Message-Id: <20180926182920.27644-4-paulmck@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-09-26_08:,, 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=8.0.1-1807170000 definitions=main-1809260172 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Paul E. McKenney" This commit adds more detail about compiler optimizations and not-yet-modeled Linux-kernel APIs. Signed-off-by: Paul E. McKenney Reviewed-by: Andrea Parri --- tools/memory-model/README | 39 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 39 insertions(+) diff --git a/tools/memory-model/README b/tools/memory-model/README index ee987ce20aae..acf9077cffaa 100644 --- a/tools/memory-model/README +++ b/tools/memory-model/README @@ -171,6 +171,12 @@ The Linux-kernel memory model has the following limitations: 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. + For example, if the compiler can deduce the value of some variable + carrying a dependency, then the compiler can break that dependency + by substituting a constant of that value. + 2. Multiple access sizes for a single variable are not supported, and neither are misaligned or partially overlapping accesses. @@ -190,6 +196,36 @@ The Linux-kernel memory model has the following limitations: However, a substantial amount of support is provided for these operations, as shown in the linux-kernel.def file. + a. When rcu_assign_pointer() is passed NULL, the Linux + kernel provides no ordering, but LKMM models this + case as a store release. + + b. The "unless" RMW operations are not currently modeled: + atomic_long_add_unless(), atomic_add_unless(), + atomic_inc_unless_negative(), and + atomic_dec_unless_positive(). These can be emulated + in litmus tests, for example, by using atomic_cmpxchg(). + + c. The call_rcu() function is not modeled. It can be + emulated in litmus tests by adding another process that + invokes synchronize_rcu() and the body of the callback + function, with (for example) a release-acquire from + the site of the emulated call_rcu() to the beginning + of the additional process. + + d. The rcu_barrier() function is not modeled. It can be + emulated in litmus tests emulating call_rcu() via + (for example) a release-acquire from the end of each + additional call_rcu() process to the site of the + emulated rcu-barrier(). + + e. Sleepable RCU (SRCU) is not modeled. It can be + emulated, but perhaps not simply. + + f. Reader-writer locking is not modeled. It can be + emulated in litmus tests using atomic read-modify-write + operations. + The "herd7" tool has some additional limitations of its own, apart from the memory model: @@ -204,3 +240,6 @@ the memory model: Some of these limitations may be overcome in the future, but others are more likely to be addressed by incorporating the Linux-kernel memory model into other tools. + +Finally, please note that LKMM is subject to change as hardware, use cases, +and compilers evolve. -- 2.17.1