rcu: add comment saying why DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT.

The build will break if you change the Kconfig to allow
DEBUG_OBJECTS_RCU_HEAD and !PREEMPT, so document the reasoning
near where the breakage would occur.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index afd21d17c081..f3240e987928 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -214,6 +214,11 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
                  * Ensure that queued callbacks are all executed.
                  * If we detect that we are nested in a RCU read-side critical
                  * section, we should simply fail, otherwise we would deadlock.
+                 * Note that the machinery to reliably determine whether
+                 * or not we are in an RCU read-side critical section
+                 * exists only in the preemptible RCU implementations
+                 * (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why
+                 * DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT.
                  */
                 if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
                     irqs_disabled()) {