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Subject[PATCH RFC LKMM 3/7] EXP tools/memory-model: Add more LKMM limitations
Date
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().
+
+ 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
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