Based on kernel version 4.2. Page generated on 2015-09-09 12:14 EST.
1 RCU on Uniprocessor Systems 2 3 4 A common misconception is that, on UP systems, the call_rcu() primitive 5 may immediately invoke its function. The basis of this misconception 6 is that since there is only one CPU, it should not be necessary to 7 wait for anything else to get done, since there are no other CPUs for 8 anything else to be happening on. Although this approach will -sort- -of- 9 work a surprising amount of the time, it is a very bad idea in general. 10 This document presents three examples that demonstrate exactly how bad 11 an idea this is. 12 13 14 Example 1: softirq Suicide 15 16 Suppose that an RCU-based algorithm scans a linked list containing 17 elements A, B, and C in process context, and can delete elements from 18 this same list in softirq context. Suppose that the process-context scan 19 is referencing element B when it is interrupted by softirq processing, 20 which deletes element B, and then invokes call_rcu() to free element B 21 after a grace period. 22 23 Now, if call_rcu() were to directly invoke its arguments, then upon return 24 from softirq, the list scan would find itself referencing a newly freed 25 element B. This situation can greatly decrease the life expectancy of 26 your kernel. 27 28 This same problem can occur if call_rcu() is invoked from a hardware 29 interrupt handler. 30 31 32 Example 2: Function-Call Fatality 33 34 Of course, one could avert the suicide described in the preceding example 35 by having call_rcu() directly invoke its arguments only if it was called 36 from process context. However, this can fail in a similar manner. 37 38 Suppose that an RCU-based algorithm again scans a linked list containing 39 elements A, B, and C in process contexts, but that it invokes a function 40 on each element as it is scanned. Suppose further that this function 41 deletes element B from the list, then passes it to call_rcu() for deferred 42 freeing. This may be a bit unconventional, but it is perfectly legal 43 RCU usage, since call_rcu() must wait for a grace period to elapse. 44 Therefore, in this case, allowing call_rcu() to immediately invoke 45 its arguments would cause it to fail to make the fundamental guarantee 46 underlying RCU, namely that call_rcu() defers invoking its arguments until 47 all RCU read-side critical sections currently executing have completed. 48 49 Quick Quiz #1: why is it -not- legal to invoke synchronize_rcu() in 50 this case? 51 52 53 Example 3: Death by Deadlock 54 55 Suppose that call_rcu() is invoked while holding a lock, and that the 56 callback function must acquire this same lock. In this case, if 57 call_rcu() were to directly invoke the callback, the result would 58 be self-deadlock. 59 60 In some cases, it would possible to restructure to code so that 61 the call_rcu() is delayed until after the lock is released. However, 62 there are cases where this can be quite ugly: 63 64 1. 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 69 2. 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 76 If call_rcu() directly invokes the callback, painful locking restrictions 77 or API changes would be required. 78 79 Quick Quiz #2: What locking restriction must RCU callbacks respect? 80 81 82 Summary 83 84 Permitting call_rcu() to immediately invoke its arguments breaks RCU, 85 even on a UP system. So do not do it! Even on a UP system, the RCU 86 infrastructure -must- respect grace periods, and -must- invoke callbacks 87 from a known environment in which no locks are held. 88 89 It -is- safe for synchronize_sched() and synchronize_rcu_bh() to return 90 immediately on an UP system. It is also safe for synchronize_rcu() 91 to return immediately on UP systems, except when running preemptable 92 RCU. 93 94 Quick Quiz #3: Why can't synchronize_rcu() return immediately on 95 UP systems running preemptable RCU? 96 97 98 Answer 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. 105 106 Answer 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. 126 127 Answer 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.