diff --git a/Documentation/RCU/UP.txt b/Documentation/RCU/UP.txt
index 3bfb84b..aab4a9e 100644
--- a/Documentation/RCU/UP.txt
+++ b/Documentation/RCU/UP.txt
@@ -8,7 +8,7 @@
 wait for anything else to get done, since there are no other CPUs for
 anything else to be happening on.  Although this approach will -sort- -of-
 work a surprising amount of the time, it is a very bad idea in general.
-This document presents two examples that demonstrate exactly how bad an
+This document presents three examples that demonstrate exactly how bad an
 idea this is.
 
 
@@ -26,6 +26,9 @@
 element B.  This situation can greatly decrease the life expectancy of
 your kernel.
 
+This same problem can occur if call_rcu() is invoked from a hardware
+interrupt handler.
+
 
 Example 2: Function-Call Fatality
 
@@ -44,8 +47,37 @@
 underlying RCU, namely that call_rcu() defers invoking its arguments until
 all RCU read-side critical sections currently executing have completed.
 
-Quick Quiz: why is it -not- legal to invoke synchronize_rcu() in
-this case?
+Quick Quiz #1: why is it -not- legal to invoke synchronize_rcu() in
+	this case?
+
+
+Example 3: Death by Deadlock
+
+Suppose that call_rcu() is invoked while holding a lock, and that the
+callback function must acquire this same lock.  In this case, if
+call_rcu() were to directly invoke the callback, the result would
+be self-deadlock.
+
+In some cases, it would possible to restructure to code so that
+the call_rcu() is delayed until after the lock is released.  However,
+there are cases where this can be quite ugly:
+
+1.	If a number of items need to be passed to call_rcu() within
+	the same critical section, then the code would need to create
+	a list of them, then traverse the list once the lock was
+	released.
+
+2.	In some cases, the lock will be held across some kernel API,
+	so that delaying the call_rcu() until the lock is released
+	requires that the data item be passed up via a common API.
+	It is far better to guarantee that callbacks are invoked
+	with no locks held than to have to modify such APIs to allow
+	arbitrary data items to be passed back up through them.
+
+If call_rcu() directly invokes the callback, painful locking restrictions
+or API changes would be required.
+
+Quick Quiz #2: What locking restriction must RCU callbacks respect?
 
 
 Summary
@@ -53,12 +85,35 @@
 Permitting call_rcu() to immediately invoke its arguments or permitting
 synchronize_rcu() to immediately return breaks RCU, even on a UP system.
 So do not do it!  Even on a UP system, the RCU infrastructure -must-
-respect grace periods.
+respect grace periods, and -must- invoke callbacks from a known environment
+in which no locks are held.
 
 
-Answer to Quick Quiz
+Answer to Quick Quiz #1:
+	Why is it -not- legal to invoke synchronize_rcu() in this case?
 
-The calling function is scanning an RCU-protected linked list, and
-is therefore within an RCU read-side critical section.  Therefore,
-the called function has been invoked within an RCU read-side critical
-section, and is not permitted to block.
+	Because the calling function is scanning an RCU-protected linked
+	list, and is therefore within an RCU read-side critical section.
+	Therefore, the called function has been invoked within an RCU
+	read-side critical section, and is not permitted to block.
+
+Answer to Quick Quiz #2:
+	What locking restriction must RCU callbacks respect?
+
+	Any lock that is acquired within an RCU callback must be
+	acquired elsewhere using an _irq variant of the spinlock
+	primitive.  For example, if "mylock" is acquired by an
+	RCU callback, then a process-context acquisition of this
+	lock must use something like spin_lock_irqsave() to
+	acquire the lock.
+
+	If the process-context code were to simply use spin_lock(),
+	then, since RCU callbacks can be invoked from softirq context,
+	the callback might be called from a softirq that interrupted
+	the process-context critical section.  This would result in
+	self-deadlock.
+
+	This restriction might seem gratuitous, since very few RCU
+	callbacks acquire locks directly.  However, a great many RCU
+	callbacks do acquire locks -indirectly-, for example, via
+	the kfree() primitive.