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| -rw-r--r-- | Documentation/RCU/checklist.txt | 44 | ||||
| -rw-r--r-- | Documentation/RCU/whatisRCU.txt | 12 |
2 files changed, 52 insertions, 4 deletions
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt index 49e27cc19385..1d50cf0c905e 100644 --- a/Documentation/RCU/checklist.txt +++ b/Documentation/RCU/checklist.txt @@ -144,9 +144,47 @@ over a rather long period of time, but improvements are always welcome! whether the increased speed is worth it. 8. Although synchronize_rcu() is a bit slower than is call_rcu(), - it usually results in simpler code. So, unless update performance - is important or the updaters cannot block, synchronize_rcu() - should be used in preference to call_rcu(). + it usually results in simpler code. So, unless update + performance is critically important or the updaters cannot block, + synchronize_rcu() should be used in preference to call_rcu(). + + An especially important property of the synchronize_rcu() + primitive is that it automatically self-limits: if grace periods + are delayed for whatever reason, then the synchronize_rcu() + primitive will correspondingly delay updates. In contrast, + code using call_rcu() should explicitly limit update rate in + cases where grace periods are delayed, as failing to do so can + result in excessive realtime latencies or even OOM conditions. + + Ways of gaining this self-limiting property when using call_rcu() + include: + + a. Keeping a count of the number of data-structure elements + used by the RCU-protected data structure, including those + waiting for a grace period to elapse. Enforce a limit + on this number, stalling updates as needed to allow + previously deferred frees to complete. + + Alternatively, limit only the number awaiting deferred + free rather than the total number of elements. + + b. Limiting update rate. For example, if updates occur only + once per hour, then no explicit rate limiting is required, + unless your system is already badly broken. The dcache + subsystem takes this approach -- updates are guarded + by a global lock, limiting their rate. + + c. Trusted update -- if updates can only be done manually by + superuser or some other trusted user, then it might not + be necessary to automatically limit them. The theory + here is that superuser already has lots of ways to crash + the machine. + + d. Use call_rcu_bh() rather than call_rcu(), in order to take + advantage of call_rcu_bh()'s faster grace periods. + + e. Periodically invoke synchronize_rcu(), permitting a limited + number of updates per grace period. 9. All RCU list-traversal primitives, which include list_for_each_rcu(), list_for_each_entry_rcu(), diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt index 6e459420ee9f..4f41a60e5111 100644 --- a/Documentation/RCU/whatisRCU.txt +++ b/Documentation/RCU/whatisRCU.txt @@ -184,7 +184,17 @@ synchronize_rcu() blocking, it registers a function and argument which are invoked after all ongoing RCU read-side critical sections have completed. This callback variant is particularly useful in situations where - it is illegal to block. + it is illegal to block or where update-side performance is + critically important. + + However, the call_rcu() API should not be used lightly, as use + of the synchronize_rcu() API generally results in simpler code. + In addition, the synchronize_rcu() API has the nice property + of automatically limiting update rate should grace periods + be delayed. This property results in system resilience in face + of denial-of-service attacks. Code using call_rcu() should limit + update rate in order to gain this same sort of resilience. See + checklist.txt for some approaches to limiting the update rate. rcu_assign_pointer() |