doc: Add another stall-warning root cause in stallwarn.rst
This commit adds a bullet item noting that both deficiencies and surpluses
of calls to rcu_*_enter() and rcu_*_exit() can result in RCU CPU stall
warnings.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst
index 5036df2..28f8ad1 100644
--- a/Documentation/RCU/stallwarn.rst
+++ b/Documentation/RCU/stallwarn.rst
@@ -96,6 +96,16 @@
the ``rcu_.*timer wakeup didn't happen for`` console-log message,
which will include additional debugging information.
+- A low-level kernel issue that either fails to invoke one of the
+ variants of rcu_user_enter(), rcu_user_exit(), rcu_idle_enter(),
+ rcu_idle_exit(), rcu_irq_enter(), or rcu_irq_exit() on the one
+ hand, or that invokes one of them too many times on the other.
+ Historically, the most frequent issue has been an omission
+ of either irq_enter() or irq_exit(), which in turn invoke
+ rcu_irq_enter() or rcu_irq_exit(), respectively. Building your
+ kernel with CONFIG_RCU_EQS_DEBUG=y can help track down these types
+ of issues, which sometimes arise in architecture-specific code.
+
- A bug in the RCU implementation.
- A hardware failure. This is quite unlikely, but has occurred