Merge tag 'core-rcu-2020-12-14' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull RCU updates from Thomas Gleixner:
 "RCU, LKMM and KCSAN updates collected by Paul McKenney.

  RCU:
   - Avoid cpuinfo-induced IPI pileups and idle-CPU IPIs

   - Lockdep-RCU updates reducing the need for __maybe_unused

   - Tasks-RCU updates

   - Miscellaneous fixes

   - Documentation updates

   - Torture-test updates

  KCSAN:
   - updates for selftests, avoiding setting watchpoints on NULL pointers

   - fix to watchpoint encoding

  LKMM:
   - updates for documentation along with some updates to example-code
     litmus tests"

* tag 'core-rcu-2020-12-14' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (72 commits)
  srcu: Take early exit on memory-allocation failure
  rcu/tree: Defer kvfree_rcu() allocation to a clean context
  rcu: Do not report strict GPs for outgoing CPUs
  rcu: Fix a typo in rcu_blocking_is_gp() header comment
  rcu: Prevent lockdep-RCU splats on lock acquisition/release
  rcu/tree: nocb: Avoid raising softirq for offloaded ready-to-execute CBs
  rcu,ftrace: Fix ftrace recursion
  rcu/tree: Make struct kernel_param_ops definitions const
  rcu/tree: Add a warning if CPU being onlined did not report QS already
  rcu: Clarify nocb kthreads naming in RCU_NOCB_CPU config
  rcu: Fix single-CPU check in rcu_blocking_is_gp()
  rcu: Implement rcu_segcblist_is_offloaded() config dependent
  list.h: Update comment to explicitly note circular lists
  rcu: Panic after fixed number of stalls
  x86/smpboot:  Move rcu_cpu_starting() earlier
  rcu: Allow rcu_irq_enter_check_tick() from NMI
  tools/memory-model: Label MP tests' producers and consumers
  tools/memory-model: Use "buf" and "flag" for message-passing tests
  tools/memory-model: Add types to litmus tests
  tools/memory-model: Add a glossary of LKMM terms
  ...

1 files changed, 17 insertions, 0 deletions

diff --git a/tools/memory-model/Documentation/litmus-tests.txt b/tools/memory-model/Documentation/litmus-tests.txt
index 2f840dcd15cf..8a9d5d2787f9 100644
--- a/tools/memory-model/Documentation/litmus-tests.txt
+++ b/tools/memory-model/Documentation/litmus-tests.txt
@@ -946,6 +946,23 @@ Limitations of the Linux-kernel memory model (LKMM) include:
 	carrying a dependency, then the compiler can break that dependency
 	by substituting a constant of that value.
 
+	Conversely, LKMM sometimes doesn't recognize that a particular
+	optimization is not allowed, and as a result, thinks that a
+	dependency is not present (because the optimization would break it).
+	The memory model misses some pretty obvious control dependencies
+	because of this limitation.  A simple example is:
+
+		r1 = READ_ONCE(x);
+		if (r1 == 0)
+			smp_mb();
+		WRITE_ONCE(y, 1);
+
+	There is a control dependency from the READ_ONCE to the WRITE_ONCE,
+	even when r1 is nonzero, but LKMM doesn't realize this and thinks
+	that the write may execute before the read if r1 != 0.  (Yes, that
+	doesn't make sense if you think about it, but the memory model's
+	intelligence is limited.)
+
 2.	Multiple access sizes for a single variable are not supported,
 	and neither are misaligned or partially overlapping accesses.