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rcu: Call out dangers of expedited RCU primitives

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The expedited RCU primitives can be quite useful, but they have some
high costs as well.  This commit updates and creates docbook comments
calling out the costs, and updates the RCU documentation as well.

Signed-off-by: Paul E. McKenney <[email protected]>
This commit is contained in:
Paul E. McKenney 2012-01-31 14:00:41 -08:00
commit 236fefafe5
5 changed files with 77 additions and 22 deletions
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14
Documentation/RCU/checklist.txt
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@ -180,6 +180,20 @@ over a rather long period of time, but improvements are always welcome!
operations that would not normally be undertaken while a real-time
workload is running.
In particular, if you find yourself invoking one of the expedited
primitives repeatedly in a loop, please do everyone a favor:
Restructure your code so that it batches the updates, allowing
a single non-expedited primitive to cover the entire batch.
This will very likely be faster than the loop containing the
expedited primitive, and will be much much easier on the rest
of the system, especially to real-time workloads running on
the rest of the system.
In addition, it is illegal to call the expedited forms from
a CPU-hotplug notifier, or while holding a lock that is acquired
by a CPU-hotplug notifier. Failing to observe this restriction
will result in deadlock.
7. If the updater uses call_rcu() or synchronize_rcu(), then the
corresponding readers must use rcu_read_lock() and
rcu_read_unlock(). If the updater uses call_rcu_bh() or