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rcu: Fix rcu_barrier() documentation

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There was a time when rcu_barrier() was guaranteed to wait for at least
a grace period, but that time ended due to energy-efficiency concerns.
So now rcu_barrier() is a no-op if there are no RCU callbacks queued in
the system.  This commit updates the documentation to reflect this change.

Now, rcu_barrier() often does wait for a grace period, so, one could
imagine some modification to rcu_barrier() to more efficiently handle
cases where both rcu_barrier() and a grace period are needed.  But this
must wait until someone shows a real-world need for a change.

Reported-by: Bob Copeland <bob@cozybit.com>
Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit is contained in:
Paul E. McKenney 2013-04-18 14:25:03 -07:00
parent f7f7bac9cb
commit d84297c99b
1 changed files with 8 additions and 4 deletions
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12
Documentation/RCU/rcubarrier.txt
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@ -70,10 +70,14 @@ in realtime kernels in order to avoid excessive scheduling latencies.
rcu_barrier() rcu_barrier()
We instead need the rcu_barrier() primitive. This primitive is similar We instead need the rcu_barrier() primitive. Rather than waiting for
to synchronize_rcu(), but instead of waiting solely for a grace a grace period to elapse, rcu_barrier() waits for all outstanding RCU
period to elapse, it also waits for all outstanding RCU callbacks to callbacks to complete. Please note that rcu_barrier() does -not- imply
complete. Pseudo-code using rcu_barrier() is as follows: synchronize_rcu(), in particular, if there are no RCU callbacks queued
anywhere, rcu_barrier() is within its rights to return immediately,
without waiting for a grace period to elapse.
Pseudo-code using rcu_barrier() is as follows:
1. Prevent any new RCU callbacks from being posted. 1. Prevent any new RCU callbacks from being posted.
2. Execute rcu_barrier(). 2. Execute rcu_barrier().