linux-toradex.git/tools/memory-model, branch v6.1-rc1

tools/memory-model: Clarify LKMM's limitations in litmus-tests.txt

2022-08-31T12:15:31+00:00

As discussed, clarify LKMM not recognizing certain kinds of orderings.
In particular, highlight the fact that LKMM might deliberately make
weaker guarantees than compilers and architectures.

[ paulmck: Fix whitespace issue noted by checkpatch.pl. ]

Link: https://lore.kernel.org/all/YpoW1deb%2FQeeszO1@ethstick13.dse.in.tum.de/T/#u
Co-developed-by: Alan Stern 
Signed-off-by: Alan Stern 
Signed-off-by: Paul Heidekrüger 
Reviewed-by: Marco Elver 
Reviewed-by: Joel Fernandes (Google) 
Cc: Charalampos Mainas 
Cc: Pramod Bhatotia 
Cc: Soham Chakraborty 
Cc: Martin Fink 
Signed-off-by: Paul E. McKenney

tools/memory-model/README: Update klitmus7 compat table

2022-05-03T17:12:48+00:00

EXPORT_SYMBOL of do_exec() was removed in v5.17.  Unfortunately,
kernel modules from klitmus7 7.56 have do_exec() at the end of
each kthread.

herdtools7 7.56.1 has addressed the issue.

Update the compatibility table accordingly.

Signed-off-by: Akira Yokosawa 
Cc: Luc Maranget 
Cc: Jade Alglave 
Cc: stable@vger.kernel.org # v5.17+
Signed-off-by: Paul E. McKenney

tools/memory-model: Explain syntactic and semantic dependencies

2022-02-02T01:32:30+00:00

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 
Reviewed-by: Akira Yokosawa 
Signed-off-by: Alan Stern 
Signed-off-by: Paul E. McKenney

tools/memory-model: litmus: Add two tests for unlock(A)+lock(B) ordering

2021-12-01T01:47:08+00:00

The memory model has been updated to provide a stronger ordering
guarantee for unlock(A)+lock(B) on the same CPU/thread. Therefore add
two litmus tests describing this new guarantee, these tests are simple
yet can clearly show the usage of the new guarantee, also they can serve
as the self tests for the modification in the model.

Co-developed-by: Alan Stern 
Signed-off-by: Alan Stern 
Signed-off-by: Boqun Feng 
Acked-by: Peter Zijlstra (Intel) 
Signed-off-by: Paul E. McKenney

tools/memory-model: doc: Describe the requirement of the litmus-tests directory

2021-12-01T01:47:08+00:00

It's better that we have some "standard" about which test should be put
in the litmus-tests directory because it helps future contributors
understand whether they should work on litmus-tests in kernel or Paul's
GitHub repo. Therefore explain a little bit on what a "representative"
litmus test is.

Signed-off-by: Boqun Feng 
Acked-by: Peter Zijlstra (Intel) 
Signed-off-by: Paul E. McKenney

tools/memory-model: Provide extra ordering for unlock+lock pair on the same CPU

2021-12-01T01:47:08+00:00

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 
Signed-off-by: Alan Stern 
Signed-off-by: Boqun Feng 
Reviewed-by: Michael Ellerman  (powerpc)
Acked-by: Palmer Dabbelt  (RISC-V)
Acked-by: Peter Zijlstra (Intel) 
Signed-off-by: Paul E. McKenney

tools/memory-model: Document data_race(READ_ONCE())

2021-07-27T18:48:55+00:00

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

tools/memory-model: Heuristics using data_race() must handle all values

2021-07-27T18:48:55+00:00

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 
Signed-off-by: Paul E. McKenney

tools/memory-model: Add example for heuristic lockless reads

2021-07-27T18:47:34+00:00

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 
[ paulmck: Update per Manfred Spraul and Hillf Danton feedback. ]
Signed-off-by: Paul E. McKenney

tools/memory-model: Make read_foo_diagnostic() more clearly diagnostic

2021-07-20T20:52:03+00:00

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 
Signed-off-by: Paul E. McKenney