From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Andrea Parri <parri.andrea@gmail.com>
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
Date: Thu, 30 Aug 2018 15:18:14 -0700 [thread overview]
Message-ID: <20180830221814.GQ4225@linux.vnet.ibm.com> (raw)
Message-ID: <20180830221814.h1eAVJ2-sksoC-QKaxHG3wB6z7fth4Ds5E5DmIruGQc@z> (raw)
In-Reply-To: <20180830091713.GA4617@andrea>
On Thu, Aug 30, 2018 at 11:17:13AM +0200, Andrea Parri wrote:
> 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 <paulmck@linux.vnet.ibm.com>
> > ---
> > 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.)
Cool! It would be good to add this to the .def file once the underlying
herd7 machinery is ready. And then I would update the documentation
accordingly. Or happily accept a patch updating the documentation,
as the case might be. ;-)
Thanx, Paul
> 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
> >
>
next prev parent reply other threads:[~2018-08-31 2:22 UTC|newest]
Thread overview: 112+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-29 21:10 [PATCH RFC memory-model 0/7] Memory-model changes Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 1/7] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-30 12:50 ` Andrea Parri
2018-08-30 12:50 ` Andrea Parri
2018-08-30 21:31 ` Alan Stern
2018-08-30 21:31 ` Alan Stern
2018-08-31 9:17 ` Andrea Parri
2018-08-31 9:17 ` Andrea Parri
2018-08-31 14:52 ` Alan Stern
2018-08-31 14:52 ` Alan Stern
2018-08-31 16:06 ` Will Deacon
2018-08-31 16:06 ` Will Deacon
2018-08-31 18:28 ` Andrea Parri
2018-08-31 18:28 ` Andrea Parri
2018-09-03 9:01 ` Andrea Parri
2018-09-03 9:01 ` Andrea Parri
2018-09-03 17:04 ` Will Deacon
2018-09-03 17:04 ` Will Deacon
2018-09-04 8:11 ` Andrea Parri
2018-09-04 8:11 ` Andrea Parri
2018-09-04 19:09 ` Alan Stern
2018-09-04 19:09 ` Alan Stern
2018-09-05 7:21 ` Andrea Parri
2018-09-05 7:21 ` Andrea Parri
2018-09-05 14:33 ` Akira Yokosawa
2018-09-05 14:33 ` Akira Yokosawa
2018-09-05 14:53 ` Paul E. McKenney
2018-09-05 14:53 ` Paul E. McKenney
2018-09-05 15:00 ` Andrea Parri
2018-09-05 15:00 ` Andrea Parri
2018-09-05 15:04 ` Akira Yokosawa
2018-09-05 15:04 ` Akira Yokosawa
2018-09-05 15:24 ` Andrea Parri
2018-09-05 15:24 ` Andrea Parri
2018-09-03 17:52 ` Alan Stern
2018-09-03 17:52 ` Alan Stern
2018-09-03 18:28 ` Andrea Parri
2018-09-03 18:28 ` Andrea Parri
2018-09-06 1:25 ` Alan Stern
2018-09-06 1:25 ` Alan Stern
2018-09-06 9:36 ` Andrea Parri
2018-09-06 9:36 ` Andrea Parri
2018-09-07 16:00 ` Alan Stern
2018-09-07 16:00 ` Alan Stern
2018-09-07 16:09 ` Will Deacon
2018-09-07 16:09 ` Will Deacon
2018-09-07 16:39 ` Daniel Lustig
2018-09-07 16:39 ` Daniel Lustig
2018-09-07 17:38 ` Alan Stern
2018-09-07 17:38 ` Alan Stern
2018-09-08 0:04 ` Daniel Lustig
2018-09-08 0:04 ` Daniel Lustig
2018-09-08 9:58 ` Andrea Parri
2018-09-08 9:58 ` Andrea Parri
2018-09-11 19:31 ` Alan Stern
2018-09-11 19:31 ` Alan Stern
2018-09-11 20:03 ` Paul E. McKenney
2018-09-11 20:03 ` Paul E. McKenney
2018-09-12 14:24 ` Alan Stern
2018-09-12 14:24 ` Alan Stern
2018-09-13 17:07 ` Alan Stern
2018-09-13 17:07 ` Alan Stern
2018-09-14 14:37 ` Andrea Parri
2018-09-14 14:37 ` Andrea Parri
2018-09-14 16:29 ` Alan Stern
2018-09-14 16:29 ` Alan Stern
2018-09-14 19:44 ` Andrea Parri
2018-09-14 19:44 ` Andrea Parri
2018-09-14 21:08 ` [PATCH v5] " Alan Stern
2018-09-14 21:08 ` Alan Stern
2018-09-15 3:56 ` Paul E. McKenney
2018-09-15 3:56 ` Paul E. McKenney
2018-09-03 17:05 ` [PATCH RFC LKMM 1/7] " Will Deacon
2018-09-03 17:05 ` Will Deacon
2018-08-31 17:55 ` Andrea Parri
2018-08-31 17:55 ` Andrea Parri
2018-08-29 21:10 ` [PATCH RFC LKMM 2/7] doc: Replace smp_cond_acquire() with smp_cond_load_acquire() Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-09-14 16:59 ` Will Deacon
2018-09-14 16:59 ` Will Deacon
2018-09-14 18:20 ` Paul E. McKenney
2018-09-14 18:20 ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 3/7] EXP tools/memory-model: Add more LKMM limitations Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-30 9:17 ` Andrea Parri
2018-08-30 9:17 ` Andrea Parri
2018-08-30 22:18 ` Paul E. McKenney [this message]
2018-08-30 22:18 ` Paul E. McKenney
2018-08-31 9:43 ` Andrea Parri
2018-08-31 9:43 ` Andrea Parri
2018-09-06 18:34 ` Paul E. McKenney
2018-09-06 18:34 ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 4/7] tools/memory-model: Fix a README typo Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 5/7] EXP tools/memory-model: Add scripts to check github litmus tests Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 6/7] EXP tools/memory-model: Make scripts take "-j" abbreviation for "--jobs" Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 7/7] EXP tools/memory-model: Add .cfg and .cat files for s390 Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-31 16:06 ` Will Deacon
2018-08-31 16:06 ` Will Deacon
2018-09-01 17:08 ` Paul E. McKenney
2018-09-01 17:08 ` Paul E. McKenney
2018-09-14 16:36 ` [PATCH RFC memory-model 0/7] Memory-model changes Paul E. McKenney
2018-09-14 16:36 ` Paul E. McKenney
2018-09-14 17:19 ` Alan Stern
2018-09-14 17:19 ` Alan Stern
2018-09-14 18:29 ` Paul E. McKenney
2018-09-14 18:29 ` Paul E. McKenney
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