Re: [PATCH RFC LKMM 3/7] EXP tools/memory-model: Add more LKMM limitations
From: Andrea Parri <parri.andrea@gmail.com>
Date: 2018-08-30 09:17:26
Also in:
lkml
On Wed, Aug 29, 2018 at 02:10:49PM -0700, Paul E. McKenney wrote:
quoted hunk ↗ jump to hunk
This commit adds more detail about compiler optimizations and not-yet-modeled Linux-kernel APIs. Signed-off-by: Paul E. McKenney <redacted> --- 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
quoted hunk ↗ jump to hunk
+ + 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