65 lines
2.7 KiB
Plaintext
65 lines
2.7 KiB
Plaintext
RCU on Uniprocessor Systems
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A common misconception is that, on UP systems, the call_rcu() primitive
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may immediately invoke its function, and that the synchronize_rcu()
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primitive may return immediately. The basis of this misconception
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is that since there is only one CPU, it should not be necessary to
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wait for anything else to get done, since there are no other CPUs for
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anything else to be happening on. Although this approach will -sort- -of-
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work a surprising amount of the time, it is a very bad idea in general.
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This document presents two examples that demonstrate exactly how bad an
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idea this is.
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Example 1: softirq Suicide
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Suppose that an RCU-based algorithm scans a linked list containing
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elements A, B, and C in process context, and can delete elements from
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this same list in softirq context. Suppose that the process-context scan
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is referencing element B when it is interrupted by softirq processing,
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which deletes element B, and then invokes call_rcu() to free element B
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after a grace period.
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Now, if call_rcu() were to directly invoke its arguments, then upon return
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from softirq, the list scan would find itself referencing a newly freed
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element B. This situation can greatly decrease the life expectancy of
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your kernel.
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Example 2: Function-Call Fatality
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Of course, one could avert the suicide described in the preceding example
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by having call_rcu() directly invoke its arguments only if it was called
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from process context. However, this can fail in a similar manner.
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Suppose that an RCU-based algorithm again scans a linked list containing
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elements A, B, and C in process contexts, but that it invokes a function
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on each element as it is scanned. Suppose further that this function
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deletes element B from the list, then passes it to call_rcu() for deferred
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freeing. This may be a bit unconventional, but it is perfectly legal
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RCU usage, since call_rcu() must wait for a grace period to elapse.
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Therefore, in this case, allowing call_rcu() to immediately invoke
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its arguments would cause it to fail to make the fundamental guarantee
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underlying RCU, namely that call_rcu() defers invoking its arguments until
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all RCU read-side critical sections currently executing have completed.
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Quick Quiz: why is it -not- legal to invoke synchronize_rcu() in
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this case?
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Summary
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Permitting call_rcu() to immediately invoke its arguments or permitting
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synchronize_rcu() to immediately return breaks RCU, even on a UP system.
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So do not do it! Even on a UP system, the RCU infrastructure -must-
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respect grace periods.
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Answer to Quick Quiz
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The calling function is scanning an RCU-protected linked list, and
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is therefore within an RCU read-side critical section. Therefore,
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the called function has been invoked within an RCU read-side critical
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section, and is not permitted to block.
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