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From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     linux-kernel@vger.kernel.org
Cc:     mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
        rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
        fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 1/6] doc: Add design documentation on interruption of NMI handlers
Date:   Wed, 29 Aug 2018 14:17:17 -0700
In-Reply-To: <20180829211637.GA20980@linux.vnet.ibm.com>
References: <20180829211637.GA20980@linux.vnet.ibm.com>
Message-Id: <20180829211722.21694-1-paulmck@linux.vnet.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
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Make Requirements.html talk about how NMI handlers can take what appear
to RCU to be normal interrupts.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 .../RCU/Design/Requirements/Requirements.html         | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 49690228b1c6..089a8e8faac1 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -2275,6 +2275,17 @@ he also kindly surprised me with
 <a href="https://lkml.kernel.org/g/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com">an algorithm</a>
 that meets this requirement.
 
+<p>
+Furthermore, NMI handlers can be interrupted by what appear to RCU
+to be normal interrupts.
+One way that this can happen is for code that directly invokes
+<tt>rcu_irq_enter()</tt> and </tt>rcu_irq_exit()</tt> to be called
+from an NMI handler.
+This astonishing fact of life prompted the current code structure,
+which has <tt>rcu_irq_enter()</tt> invoking <tt>rcu_nmi_enter()</tt>
+and <tt>rcu_irq_exit()</tt> invoking <tt>rcu_nmi_exit()</tt>.
+And yes, I also learned of this requirement the hard way.
+
 <h3><a name="Loadable Modules">Loadable Modules</a></h3>
 
 <p>
-- 
2.17.1