linux-kernel - [PATCH rcu 05/14] doc: Add another stall-warning root cause in stallwarn.rst

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Date:   Wed, 15 Sep 2021 16:33:34 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 05/14] 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@...nel.org>
---
 Documentation/RCU/stallwarn.rst | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst
index 5036df24ae61..28f8ad16db25 100644
--- a/Documentation/RCU/stallwarn.rst
+++ b/Documentation/RCU/stallwarn.rst
@@ -96,6 +96,16 @@ warnings:
 	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
-- 
2.31.1.189.g2e36527f23