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1RCU on Uniprocessor Systems 2 3 4A common misconception is that, on UP systems, the call_rcu() primitive 5may immediately invoke its function. The basis of this misconception 6is that since there is only one CPU, it should not be necessary to 7wait for anything else to get done, since there are no other CPUs for 8anything else to be happening on. Although this approach will -sort- -of- 9work a surprising amount of the time, it is a very bad idea in general. 10This document presents three examples that demonstrate exactly how bad 11an idea this is. 12 13 14Example 1: softirq Suicide 15 16Suppose that an RCU-based algorithm scans a linked list containing 17elements A, B, and C in process context, and can delete elements from 18this same list in softirq context. Suppose that the process-context scan 19is referencing element B when it is interrupted by softirq processing, 20which deletes element B, and then invokes call_rcu() to free element B 21after a grace period. 22 23Now, if call_rcu() were to directly invoke its arguments, then upon return 24from softirq, the list scan would find itself referencing a newly freed 25element B. This situation can greatly decrease the life expectancy of 26your kernel. 27 28This same problem can occur if call_rcu() is invoked from a hardware 29interrupt handler. 30 31 32Example 2: Function-Call Fatality 33 34Of course, one could avert the suicide described in the preceding example 35by having call_rcu() directly invoke its arguments only if it was called 36from process context. However, this can fail in a similar manner. 37 38Suppose that an RCU-based algorithm again scans a linked list containing 39elements A, B, and C in process contexts, but that it invokes a function 40on each element as it is scanned. Suppose further that this function 41deletes element B from the list, then passes it to call_rcu() for deferred 42freeing. This may be a bit unconventional, but it is perfectly legal 43RCU usage, since call_rcu() must wait for a grace period to elapse. 44Therefore, in this case, allowing call_rcu() to immediately invoke 45its arguments would cause it to fail to make the fundamental guarantee 46underlying RCU, namely that call_rcu() defers invoking its arguments until 47all RCU read-side critical sections currently executing have completed. 48 49Quick Quiz #1: why is it -not- legal to invoke synchronize_rcu() in 50 this case? 51 52 53Example 3: Death by Deadlock 54 55Suppose that call_rcu() is invoked while holding a lock, and that the 56callback function must acquire this same lock. In this case, if 57call_rcu() were to directly invoke the callback, the result would 58be self-deadlock. 59 60In some cases, it would possible to restructure to code so that 61the call_rcu() is delayed until after the lock is released. However, 62there are cases where this can be quite ugly: 63 641. If a number of items need to be passed to call_rcu() within 65 the same critical section, then the code would need to create 66 a list of them, then traverse the list once the lock was 67 released. 68 692. In some cases, the lock will be held across some kernel API, 70 so that delaying the call_rcu() until the lock is released 71 requires that the data item be passed up via a common API. 72 It is far better to guarantee that callbacks are invoked 73 with no locks held than to have to modify such APIs to allow 74 arbitrary data items to be passed back up through them. 75 76If call_rcu() directly invokes the callback, painful locking restrictions 77or API changes would be required. 78 79Quick Quiz #2: What locking restriction must RCU callbacks respect? 80 81 82Summary 83 84Permitting call_rcu() to immediately invoke its arguments breaks RCU, 85even on a UP system. So do not do it! Even on a UP system, the RCU 86infrastructure -must- respect grace periods, and -must- invoke callbacks 87from a known environment in which no locks are held. 88 89It -is- safe for synchronize_sched() and synchronize_rcu_bh() to return 90immediately on an UP system. It is also safe for synchronize_rcu() 91to return immediately on UP systems, except when running preemptable 92RCU. 93 94Quick Quiz #3: Why can't synchronize_rcu() return immediately on 95 UP systems running preemptable RCU? 96 97 98Answer to Quick Quiz #1: 99 Why is it -not- legal to invoke synchronize_rcu() in this case? 100 101 Because the calling function is scanning an RCU-protected linked 102 list, and is therefore within an RCU read-side critical section. 103 Therefore, the called function has been invoked within an RCU 104 read-side critical section, and is not permitted to block. 105 106Answer to Quick Quiz #2: 107 What locking restriction must RCU callbacks respect? 108 109 Any lock that is acquired within an RCU callback must be 110 acquired elsewhere using an _irq variant of the spinlock 111 primitive. For example, if "mylock" is acquired by an 112 RCU callback, then a process-context acquisition of this 113 lock must use something like spin_lock_irqsave() to 114 acquire the lock. 115 116 If the process-context code were to simply use spin_lock(), 117 then, since RCU callbacks can be invoked from softirq context, 118 the callback might be called from a softirq that interrupted 119 the process-context critical section. This would result in 120 self-deadlock. 121 122 This restriction might seem gratuitous, since very few RCU 123 callbacks acquire locks directly. However, a great many RCU 124 callbacks do acquire locks -indirectly-, for example, via 125 the kfree() primitive. 126 127Answer to Quick Quiz #3: 128 Why can't synchronize_rcu() return immediately on UP systems 129 running preemptable RCU? 130 131 Because some other task might have been preempted in the middle 132 of an RCU read-side critical section. If synchronize_rcu() 133 simply immediately returned, it would prematurely signal the 134 end of the grace period, which would come as a nasty shock to 135 that other thread when it started running again.