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Subject[PATCH RFC tip/core/rcu 03/11] rcu: add documentation saying which RCU flavor to choose
Reported-by: Paul Mackerras <>
Signed-off-by: Paul E. McKenney <>
Documentation/RCU/whatisRCU.txt | 31 +++++++++++++++++++++++++++++++
1 files changed, 31 insertions(+), 0 deletions(-)

diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index cfaac34..6ef6926 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -849,6 +849,37 @@ All: lockdep-checked RCU-protected pointer access
See the comment headers in the source code (or the docbook generated
from them) for more information.

+However, given that there are no fewer than four families of RCU APIs
+in the Linux kernel, how do you choose which one to use? The following
+list can be helpful:
+a. Will readers need to block? If so, you need SRCU.
+b. What about the -rt patchset? If readers would need to block
+ in an non-rt kernel, you need SRCU. If readers would block
+ in a -rt kernel, but not in a non-rt kernel, SRCU is not
+ necessary.
+c. Do you need to treat NMI handlers, hardirq handlers,
+ and code segments with preemption disabled (whether
+ via preempt_disable(), local_irq_save(), local_bh_disable(),
+ or some other mechanism) as if they were explicit RCU readers?
+ If so, you need RCU-sched.
+d. Do you need RCU grace periods to complete even in the face
+ of softirq monopolization of one or more of the CPUs? For
+ example, is your code subject to network-based denial-of-service
+ attacks? If so, you need RCU-bh.
+e. Is your workload too update-intensive for normal use of
+ RCU, but inappropriate for other synchronization mechanisms?
+ If so, consider SLAB_DESTROY_BY_RCU. But please be careful!
+f. Otherwise, use RCU.
+Of course, this all assumes that you have determined that RCU is in fact
+the right tool for your job.



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