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-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.