An important note under the rcu_segcblist description could use a more detailed description. Especially explanation of the scenario where the ->head field may be temporarily NULL making it not wise to rely on it to determine if callbacks are associated with the rcu_segcblist. Thanks Paul for clarifying over email. Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> --- .../RCU/Design/Data-Structures/Data-Structures.html | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html index 3f9a8ae4287e..a45fc326d990 100644 --- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html +++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html @@ -928,10 +928,16 @@ this <tt>rcu_segcblist</tt> structure, <i>not</i> the <tt>->head</tt> pointer. The reason for this is that all the ready-to-invoke callbacks (that is, those in the <tt>RCU_DONE_TAIL</tt> segment) are extracted -all at once at callback-invocation time. +all at once at callback-invocation time (<tt>rcu_do_batch</tt>), due +to which <tt>->head</tt> may be set to NULL if there are no more +callbacks on the <tt>rcu_segcblist</tt>. If callback invocation must be postponed, for example, because a high-priority process just woke up on this CPU, then the remaining -callbacks are placed back on the <tt>RCU_DONE_TAIL</tt> segment. +callbacks are placed back on the <tt>RCU_DONE_TAIL</tt> segment and +<tt>->head</tt> would point to the start of the segment. So as you +can see, the head field is briefly NULL even though not all callbacks +could be invoked, so we can't rely on it to tell us if there are no +callbacks associated with the <tt>rcu_segcb_list</tt>. Either way, the <tt>->len</tt> and <tt>->len_lazy</tt> counts are adjusted after the corresponding callbacks have been invoked, and so again it is the <tt>->len</tt> count that accurately reflects whether -- 2.19.0.444.g18242da7ef-goog
