1.. _up_doc:
2
3RCU on Uniprocessor Systems
4===========================
5
6A common misconception is that, on UP systems, the call_rcu() primitive
7may immediately invoke its function.  The basis of this misconception
8is that since there is only one CPU, it should not be necessary to
9wait for anything else to get done, since there are no other CPUs for
10anything else to be happening on.  Although this approach will *sort of*
11work a surprising amount of the time, it is a very bad idea in general.
12This document presents three examples that demonstrate exactly how bad
13an idea this is.
14
15Example 1: softirq Suicide
16--------------------------
17
18Suppose that an RCU-based algorithm scans a linked list containing
19elements A, B, and C in process context, and can delete elements from
20this same list in softirq context.  Suppose that the process-context scan
21is referencing element B when it is interrupted by softirq processing,
22which deletes element B, and then invokes call_rcu() to free element B
23after a grace period.
24
25Now, if call_rcu() were to directly invoke its arguments, then upon return
26from softirq, the list scan would find itself referencing a newly freed
27element B.  This situation can greatly decrease the life expectancy of
28your kernel.
29
30This same problem can occur if call_rcu() is invoked from a hardware
31interrupt handler.
32
33Example 2: Function-Call Fatality
34---------------------------------
35
36Of course, one could avert the suicide described in the preceding example
37by having call_rcu() directly invoke its arguments only if it was called
38from process context.  However, this can fail in a similar manner.
39
40Suppose that an RCU-based algorithm again scans a linked list containing
41elements A, B, and C in process context, but that it invokes a function
42on each element as it is scanned.  Suppose further that this function
43deletes element B from the list, then passes it to call_rcu() for deferred
44freeing.  This may be a bit unconventional, but it is perfectly legal
45RCU usage, since call_rcu() must wait for a grace period to elapse.
46Therefore, in this case, allowing call_rcu() to immediately invoke
47its arguments would cause it to fail to make the fundamental guarantee
48underlying RCU, namely that call_rcu() defers invoking its arguments until
49all RCU read-side critical sections currently executing have completed.
50
51Quick Quiz #1:
52	Why is it *not* legal to invoke synchronize_rcu() in this case?
53
54:ref:`Answers to Quick Quiz <answer_quick_quiz_up>`
55
56Example 3: Death by Deadlock
57----------------------------
58
59Suppose that call_rcu() is invoked while holding a lock, and that the
60callback function must acquire this same lock.  In this case, if
61call_rcu() were to directly invoke the callback, the result would
62be self-deadlock *even if* this invocation occurred from a later
63call_rcu() invocation a full grace period later.
64
65In some cases, it would possible to restructure to code so that
66the call_rcu() is delayed until after the lock is released.  However,
67there are cases where this can be quite ugly:
68
691.	If a number of items need to be passed to call_rcu() within
70	the same critical section, then the code would need to create
71	a list of them, then traverse the list once the lock was
72	released.
73
742.	In some cases, the lock will be held across some kernel API,
75	so that delaying the call_rcu() until the lock is released
76	requires that the data item be passed up via a common API.
77	It is far better to guarantee that callbacks are invoked
78	with no locks held than to have to modify such APIs to allow
79	arbitrary data items to be passed back up through them.
80
81If call_rcu() directly invokes the callback, painful locking restrictions
82or API changes would be required.
83
84Quick Quiz #2:
85	What locking restriction must RCU callbacks respect?
86
87:ref:`Answers to Quick Quiz <answer_quick_quiz_up>`
88
89It is important to note that userspace RCU implementations *do*
90permit call_rcu() to directly invoke callbacks, but only if a full
91grace period has elapsed since those callbacks were queued.  This is
92the case because some userspace environments are extremely constrained.
93Nevertheless, people writing userspace RCU implementations are strongly
94encouraged to avoid invoking callbacks from call_rcu(), thus obtaining
95the deadlock-avoidance benefits called out above.
96
97Summary
98-------
99
100Permitting call_rcu() to immediately invoke its arguments breaks RCU,
101even on a UP system.  So do not do it!  Even on a UP system, the RCU
102infrastructure *must* respect grace periods, and *must* invoke callbacks
103from a known environment in which no locks are held.
104
105Note that it *is* safe for synchronize_rcu() to return immediately on
106UP systems, including PREEMPT SMP builds running on UP systems.
107
108Quick Quiz #3:
109	Why can't synchronize_rcu() return immediately on UP systems running
110	preemptable RCU?
111
112.. _answer_quick_quiz_up:
113
114Answer to Quick Quiz #1:
115	Why is it *not* legal to invoke synchronize_rcu() in this case?
116
117	Because the calling function is scanning an RCU-protected linked
118	list, and is therefore within an RCU read-side critical section.
119	Therefore, the called function has been invoked within an RCU
120	read-side critical section, and is not permitted to block.
121
122Answer to Quick Quiz #2:
123	What locking restriction must RCU callbacks respect?
124
125	Any lock that is acquired within an RCU callback must be acquired
126	elsewhere using an _bh variant of the spinlock primitive.
127	For example, if "mylock" is acquired by an RCU callback, then
128	a process-context acquisition of this lock must use something
129	like spin_lock_bh() to acquire the lock.  Please note that
130	it is also OK to use _irq variants of spinlocks, for example,
131	spin_lock_irqsave().
132
133	If the process-context code were to simply use spin_lock(),
134	then, since RCU callbacks can be invoked from softirq context,
135	the callback might be called from a softirq that interrupted
136	the process-context critical section.  This would result in
137	self-deadlock.
138
139	This restriction might seem gratuitous, since very few RCU
140	callbacks acquire locks directly.  However, a great many RCU
141	callbacks do acquire locks *indirectly*, for example, via
142	the kfree() primitive.
143
144Answer to Quick Quiz #3:
145	Why can't synchronize_rcu() return immediately on UP systems
146	running preemptable RCU?
147
148	Because some other task might have been preempted in the middle
149	of an RCU read-side critical section.  If synchronize_rcu()
150	simply immediately returned, it would prematurely signal the
151	end of the grace period, which would come as a nasty shock to
152	that other thread when it started running again.
153