tools/memory-model: Add more LKMM limitations

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>
Reviewed-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: akiyks@gmail.com
Cc: boqun.feng@gmail.com
Cc: dhowells@redhat.com
Cc: j.alglave@ucl.ac.uk
Cc: linux-arch@vger.kernel.org
Cc: luc.maranget@inria.fr
Cc: npiggin@gmail.com
Cc: parri.andrea@gmail.com
Cc: stern@rowland.harvard.edu
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/20180926182920.27644-4-paulmck@linux.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 39 insertions, 0 deletions

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.