- 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.