From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sat, 16 Apr 2005 15:20:36 -0700 Subject: Linux-2.6.12-rc2 Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip! --- Documentation/RCU/UP.txt | 64 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 64 insertions(+) create mode 100644 Documentation/RCU/UP.txt (limited to 'Documentation/RCU/UP.txt') diff --git a/Documentation/RCU/UP.txt b/Documentation/RCU/UP.txt new file mode 100644 index 0000000..551a803 --- /dev/null +++ b/Documentation/RCU/UP.txt @@ -0,0 +1,64 @@ +RCU on Uniprocessor Systems + + +A common misconception is that, on UP systems, the call_rcu() primitive +may immediately invoke its function, and that the synchronize_kernel +primitive may return immediately. 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 two 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. + + +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: why is it -not- legal to invoke synchronize_kernel() in +this case? + + +Summary + +Permitting call_rcu() to immediately invoke its arguments or permitting +synchronize_kernel() to immediately return breaks RCU, even on a UP system. +So do not do it! Even on a UP system, the RCU infrastructure -must- +respect grace periods. + + +Answer to Quick Quiz + +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. -- cgit v1.1