cocci: codify authoring and reviewing practices

These practices largely reflect what we are already doing on the mailing
list, which should help new Coccinelle authors and reviewers get up to
speed.

Signed-off-by: Glen Choo <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: chooglen

contrib/coccinelle/README

@@ -92,3 +92,33 @@ that might be useful to developers.
 
    The absolute times will differ for you, but the relative speedup
    from caching should be on that order.
+
+== Authoring and reviewing coccinelle changes
+
+* When a .cocci is made, both the Git changes and .cocci file should be
+  reviewed. When reviewing such a change, do your best to understand the .cocci
+  changes (e.g. by asking the author to explain the change) and be explicit
+  about your understanding of the changes. This helps us decide whether input
+  from coccinelle experts is needed or not. If you aren't sure of the cocci
+  changes, indicate what changes you actively endorse and leave an Acked-by
+  (instead of Reviewed-by).
+
+* Authors should consider that reviewers may not be coccinelle experts, thus the
+  the .cocci changes may not be self-evident. A plain text description of the
+  changes is strongly encouraged, especially when using more esoteric features
+  of the language.
+
+* .cocci rules should target only the problem it is trying to solve; "collateral
+  damage" is not allowed. Reviewers should look out and flag overly-broad rules.
+
+* Consider the cost-benefit ratio of .cocci changes. In particular, consider the
+  effect on the runtime of "make coccicheck", and how often your .cocci check
+  will catch something valuable. As a rule of thumb, rules that can bail early
+  if a file doesn't have a particular token will have a small impact on runtime,
+  and vice-versa.
+
+* .cocci files used for refactoring should be temporarily kept in-tree to aid
+  the refactoring of out-of-tree code (e.g. in-flight topics). Periodically
+  evaluate the cost-benefit ratio to determine when the file should be removed.
+  For example, consider how many out-of-tree users are left and how much this
+  slows down "make coccicheck".