Pull Linux Kernel Memory Model scripting updates from Paul McKenney:
"This improves litmus-test documentation and improves the ability to do
before/after tests on the https://github.com/paulmckrcu/litmus repo"
* tag 'lkmm-scripting.2023.04.07a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu: (32 commits)
tools/memory-model: Remove out-of-date SRCU documentation
tools/memory-model: Document LKMM test procedure
tools/memory-model: Use "grep -E" instead of "egrep"
tools/memory-model: Use "-unroll 0" to keep --hw runs finite
tools/memory-model: Make judgelitmus.sh handle scripted Result: tag
tools/memory-model: Add data-race capabilities to judgelitmus.sh
tools/memory-model: Add checktheselitmus.sh to run specified litmus tests
tools/memory-model: Repair parseargs.sh header comment
tools/memory-model: Add "--" to parseargs.sh for additional arguments
tools/memory-model: Make history-check scripts use mselect7
tools/memory-model: Make checkghlitmus.sh use mselect7
tools/memory-model: Fix scripting --jobs argument
tools/memory-model: Implement --hw support for checkghlitmus.sh
tools/memory-model: Add -v flag to jingle7 runs
tools/memory-model: Make runlitmus.sh check for jingle errors
tools/memory-model: Allow herd to deduce CPU type
tools/memory-model: Keep assembly-language litmus tests
tools/memory-model: Move from .AArch64.litmus.out to .litmus.AArch.out
tools/memory-model: Make runlitmus.sh generate .litmus.out for --hw
tools/memory-model: Split runlitmus.sh out of checklitmus.sh
...
Commit 6cd244c87428 ("tools/memory-model: Provide exact SRCU semantics")
changed the semantics of partially overlapping SRCU read-side critical
sections (among other things), making such documentation out-of-date.
The new, semantic changes are discussed in explanation.txt. Remove the
out-of-date documentation.
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Most Linux-kernel uses of locking are straightforward, but there are
corner-case uses that rely on less well-known aspects of the lock and
unlock primitives. This commit therefore adds a locking.txt and litmus
tests in Documentation/litmus-tests/locking to explain these corner-case
uses.
[ paulmck: Apply Andrea Parri feedback for klitmus7. ]
[ paulmck: Apply Akira Yokosawa example-consistency feedback. ]
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
As reported by Viktor, plain accesses in LKMM are weaker than
accesses to registers: the latter carry dependencies but the former
do not. This is exemplified in the following snippet:
int r = READ_ONCE(*x);
WRITE_ONCE(*y, r);
Here a data dependency links the READ_ONCE() to the WRITE_ONCE(),
preserving their order, because the model treats r as a register.
If r is turned into a memory location accessed by plain accesses,
however, the link is broken and the order between READ_ONCE() and
WRITE_ONCE() is no longer preserved.
This is too conservative, since any optimizations on plain
accesses that might break dependencies are also possible on
registers; it also contradicts the intuitive notion of "dependency"
as the data stored by the WRITE_ONCE() does depend on the data read
by the READ_ONCE(), independently of whether r is a register or a
memory location.
This is resolved by redefining all dependencies to include
dependencies carried by memory accesses; a dependency is said to be
carried by memory accesses (in the model: carry-dep) from one load
to another load if the initial load is followed by an arbitrarily
long sequence alternating between stores and loads of the same
thread, where the data of each store depends on the previous load,
and is read by the next load.
Any dependency linking the final load in the sequence to another
access also links the initial load in the sequence to that access.
More deep details can be found in this LKML discussion:
https://lore.kernel.org/lkml/d86295788ad14a02874ab030ddb8a6f8@huawei.com/
Reported-by: Viktor Vafeiadis <viktor@mpi-sws.org>
Signed-off-by: Jonas Oberhauser <jonas.oberhauser@huawei.com>
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Viktor (as relayed by Jonas) has pointed out a weakness in the Linux
Kernel Memory Model. Namely, the memory ordering properties of atomic
operations are not monotonic: An atomic op with full-barrier semantics
does not always provide ordering as strong as one with release-barrier
semantics.
The following litmus test illustrates the problem:
--------------------------------------------------
C atomics-not-monotonic
{}
P0(int *x, atomic_t *y)
{
WRITE_ONCE(*x, 1);
smp_wmb();
atomic_set(y, 1);
}
P1(atomic_t *y)
{
int r1;
r1 = atomic_inc_return(y);
}
P2(int *x, atomic_t *y)
{
int r2;
int r3;
r2 = atomic_read(y);
smp_rmb();
r3 = READ_ONCE(*x);
}
exists (2:r2=2 /\ 2:r3=0)
--------------------------------------------------
The litmus test is allowed as shown with atomic_inc_return(), which
has full-barrier semantics. But if the operation is changed to
atomic_inc_return_release(), which only has release-barrier semantics,
the litmus test is forbidden. Clearly this violates monotonicity.
The reason is because the LKMM treats full-barrier atomic ops as if
they were written:
mb();
load();
store();
mb();
(where the load() and store() are the two parts of an atomic RMW op),
whereas it treats release-barrier atomic ops as if they were written:
load();
release_barrier();
store();
The difference is that here the release barrier orders the load part
of the atomic op before the store part with A-cumulativity, whereas
the mb()'s above do not. This means that release-barrier atomics can
effectively extend the cumul-fence relation but full-barrier atomics
cannot.
To resolve this problem we introduce the rmw-sequence relation,
representing an arbitrarily long sequence of atomic RMW operations in
which each operation reads from the previous one, and explicitly allow
it to extend cumul-fence. This modification of the memory model is
sound; it holds for PPC because of B-cumulativity, it holds for TSO
and ARM64 because of other-multicopy atomicity, and we can assume that
atomic ops on all other architectures will be implemented so as to
make it hold for them.
For similar reasons we also allow rmw-sequence to extend the
w-post-bounded relation, which is analogous to cumul-fence in some
ways.
Reported-by: Viktor Vafeiadis <viktor@mpi-sws.org>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reviewed-by: Jonas Oberhauser <jonas.oberhauser@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Paul Heidekrüger pointed out that the Linux Kernel Memory Model
documentation doesn't mention the distinction between syntactic and
semantic dependencies. This is an important difference, because the
compiler can easily break dependencies that are only syntactic, not
semantic.
This patch adds a few paragraphs to the LKMM documentation explaining
these issues and illustrating how they can matter.
Suggested-by: Paul Heidekrüger <paul.heidekrueger@in.tum.de>
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
A recent discussion[1] shows that we are in favor of strengthening the
ordering of unlock + lock on the same CPU: a unlock and a po-after lock
should provide the so-called RCtso ordering, that is a memory access S
po-before the unlock should be ordered against a memory access R
po-after the lock, unless S is a store and R is a load.
The strengthening meets programmers' expection that "sequence of two
locked regions to be ordered wrt each other" (from Linus), and can
reduce the mental burden when using locks. Therefore add it in LKMM.
[1]: https://lore.kernel.org/lkml/20210909185937.GA12379@rowland.harvard.edu/
Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Acked-by: Palmer Dabbelt <palmerdabbelt@google.com> (RISC-V)
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
It is possible to cause KCSAN to ignore marked accesses by applying
__no_kcsan to the function or applying data_race() to the marked accesses.
These approaches allow the developer to restrict compiler optimizations
while also causing KCSAN to ignore diagnostic accesses.
This commit therefore updates the documentation accordingly.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Data loaded for use by some sorts of heuristics can tolerate the
occasional erroneous value. In this case the loads may use data_race()
to give the compiler full freedom to optimize while also informing KCSAN
of the intent. However, for this to work, the heuristic needs to be
able to tolerate any erroneous value that could possibly arise. This
commit therefore adds a paragraph spelling this out.
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit adds example code for heuristic lockless reads, based loosely
on the sem_lock() and sem_unlock() functions.
[ paulmck: Apply Alan Stern and Manfred Spraul feedback. ]
Reported-by: Manfred Spraul <manfred@colorfullife.com>
[ paulmck: Update per Manfred Spraul and Hillf Danton feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
The current definition of read_foo_diagnostic() in the "Lock Protection
With Lockless Diagnostic Access" section returns a value, which could
be use for any purpose. This could mislead people into incorrectly
using data_race() in cases where READ_ONCE() is required. This commit
therefore makes read_foo_diagnostic() simply print the value read.
Reported-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
A misspelled git-grep regex revealed that smp_mb__after_spinlock()
was misspelled in explanation.txt. This commit adds the missing "_".
Fixes: 1c27b644c0 ("Automate memory-barriers.txt; provide Linux-kernel memory model")
[ paulmck: Apply Alan Stern commit-log feedback. ]
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit adapts the "Concurrency bugs should fear the big bad data-race
detector (part 2)" LWN article (https://lwn.net/Articles/816854/)
to kernel-documentation form. This allows more easily updating the
material as needed.
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
[ paulmck: Apply Marco Elver feedback. ]
[ paulmck: Update per Akira Yokosawa feedback. ]
Reviewed-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
atomic_ops.rst was removed by commit f0400a77eb ("atomic: Delete
obsolete documentation").
Remove the broken link in tools/memory-model/Documentation/simple.txt.
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Changeset b00aedf978 ("doc: Convert to rcu_dereference.txt to rcu_dereference.rst")
renamed: Documentation/RCU/rcu_dereference.txt
to: Documentation/RCU/rcu_dereference.rst.
Update its cross-reference accordingly.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit explicitly makes the connection between acquire loads and
the reads-from relation. It also adds an entry for happens-before,
and refers to the corresponding section of explanation.txt.
Reported-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
The Linux kernel has a number of categories of ordering primitives, which
are recorded in the LKMM implementation and hinted at by cheatsheet.txt.
But there is no overview of these categories, and such an overview
is needed in order to understand multithreaded LKMM litmus tests.
This commit therefore adds an ordering.txt as well as extracting a
control-dependencies.txt from memory-barriers.txt. It also updates the
README file.
[ paulmck: Apply Akira Yokosawa file-placement feedback. ]
[ paulmck: Apply Alan Stern feedback. ]
[ paulmck: Apply self-review feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit moves the descriptions of the files residing in
tools/memory-model/Documentation to a README file in that directory,
leaving behind the description of tools/memory-model/Documentation/README
itself. After this change, tools/memory-model/Documentation/README
provides a guide to the files in the tools/memory-model/Documentation
directory, guiding people with different skills and needs to the most
appropriate starting point.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Add a small section to the litmus-tests.txt documentation file for
the Linux Kernel Memory Model explaining that the memory model often
fails to recognize certain control dependencies.
Suggested-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit adds a key entry enumerating the various types of relaxed
operations. While in the area, it also renames the relaxed rows.
[ paulmck: Apply Boqun Feng feedback. ]
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Current LKMM documentation assumes that the reader already understands
concurrency in the Linux kernel, which won't necessarily always be the
case. This commit supplies a simple.txt file that provides a starting
point for someone who is new to concurrency in the Linux kernel.
That said, this file might also useful as a reminder to experienced
developers of simpler approaches to dealing with concurrency.
Link: Link: https://lwn.net/Articles/827180/
[ paulmck: Apply feedback from Joel Fernandes. ]
Co-developed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Co-developed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
