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