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[109.80.100.85]) by smtp.gmail.com with ESMTPSA id x7-v6sm2852193edx.65.2018.08.30.02.17.20 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Thu, 30 Aug 2018 02:17:21 -0700 (PDT) Date: Thu, 30 Aug 2018 11:17:13 +0200 From: Andrea Parri To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, mingo@kernel.org, stern@rowland.harvard.edu, 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 Subject: Re: [PATCH RFC LKMM 3/7] EXP tools/memory-model: Add more LKMM limitations Message-ID: <20180830091713.GA4617@andrea> References: <20180829211018.GA19646@linux.vnet.ibm.com> <20180829211053.20531-3-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180829211053.20531-3-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 29, 2018 at 02:10:49PM -0700, Paul E. McKenney wrote: > This commit adds more detail about compiler optimizations and > not-yet-modeled Linux-kernel APIs. > > Signed-off-by: Paul E. McKenney > --- > 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(). There is a prototype atomic_add_unless(): with current herd7, $ cat atomic_add_unless.litmus C atomic_add_unless {} P0(atomic_t *u, atomic_t *v) { int r0; int r1; r0 = atomic_add_unless(u, 1, 2); r1 = atomic_read(v); } P1(atomic_t *u, atomic_t *v) { int r0; int r1; r0 = atomic_add_unless(v, 1, 2); r1 = atomic_read(u); } exists (0:r1=0 /\ 1:r1=0) $ herd7 -conf linux-kernel.cfg atomic_add_unless.litmus Test atomic_add_unless Allowed States 3 0:r1=0; 1:r1=1; 0:r1=1; 1:r1=0; 0:r1=1; 1:r1=1; No Witnesses Positive: 0 Negative: 3 Condition exists (0:r1=0 /\ 1:r1=0) Observation atomic_add_unless Never 0 3 Time atomic_add_unless 0.00 Hash=fa37a2359831690299e4cc394e45d966 The last commit in the herdtools7 repo. related to this implementation (AFAICT) is: 9523c340917b6a ("herd/linux: make atomic_add_unless a primitive, so as to yield more precise dependencies for the returned boolean.") but I can only vaguely remember those dependencies issues now :/ ...; maybe we can now solve these issues? or should we change herd7 to re- turn a warning? (Notice that this primitive is currently not exported to the linux-kernel.def file.) Andrea > + > + 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 >