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diff --git a/Documentation/RCU/UP.txt b/Documentation/RCU/UP.txt new file mode 100644 index 000000000..90ec5341e --- /dev/null +++ b/Documentation/RCU/UP.txt @@ -0,0 +1,135 @@ +RCU on Uniprocessor Systems + + +A common misconception is that, on UP systems, the call_rcu() primitive +may immediately invoke its function. The basis of this misconception +is that since there is only one CPU, it should not be necessary to +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 three examples that demonstrate exactly how bad +an idea this is. + + +Example 1: softirq Suicide + +Suppose that an RCU-based algorithm scans a linked list containing +elements A, B, and C in process context, and can delete elements from +this same list in softirq context. Suppose that the process-context scan +is referencing element B when it is interrupted by softirq processing, +which deletes element B, and then invokes call_rcu() to free element B +after a grace period. + +Now, if call_rcu() were to directly invoke its arguments, then upon return +from softirq, the list scan would find itself referencing a newly freed +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 + +Of course, one could avert the suicide described in the preceding example +by having call_rcu() directly invoke its arguments only if it was called +from process context. However, this can fail in a similar manner. + +Suppose that an RCU-based algorithm again scans a linked list containing +elements A, B, and C in process contexts, but that it invokes a function +on each element as it is scanned. Suppose further that this function +deletes element B from the list, then passes it to call_rcu() for deferred +freeing. This may be a bit unconventional, but it is perfectly legal +RCU usage, since call_rcu() must wait for a grace period to elapse. +Therefore, in this case, allowing call_rcu() to immediately invoke +its arguments would cause it to fail to make the fundamental guarantee +underlying RCU, namely that call_rcu() defers invoking its arguments until +all RCU read-side critical sections currently executing have completed. + +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 + +Permitting call_rcu() to immediately invoke its arguments breaks RCU, +even on a UP system. So do not do it! Even on a UP system, the RCU +infrastructure -must- respect grace periods, and -must- invoke callbacks +from a known environment in which no locks are held. + +It -is- safe for synchronize_sched() and synchronize_rcu_bh() to return +immediately on an UP system. It is also safe for synchronize_rcu() +to return immediately on UP systems, except when running preemptable +RCU. + +Quick Quiz #3: Why can't synchronize_rcu() return immediately on + UP systems running preemptable RCU? + + +Answer to Quick Quiz #1: + Why is it -not- legal to invoke synchronize_rcu() in this case? + + 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. + +Answer to Quick Quiz #3: + Why can't synchronize_rcu() return immediately on UP systems + running preemptable RCU? + + Because some other task might have been preempted in the middle + of an RCU read-side critical section. If synchronize_rcu() + simply immediately returned, it would prematurely signal the + end of the grace period, which would come as a nasty shock to + that other thread when it started running again. |